INTRODUCTION.
Meteorology is that part of natural science which treats of the changes that take place in our atmosphere, as they are perceptible to our senses, or as they are indicated by certain instruments which the ingenuity of man or accident has discovered to answer that purpose. In as far as it describes the phenomena produced by such changes, meteorology is a department of natural history; but in its attempts to account for the appearances, it is almost entirely dependent on Natural Philosophy and Chemistry.
The connection of Meteorology with Chemistry is sufficiently evident to those who take only a superficial view of the subject, though it has only of late attracted the notice of philosophers. That the air is sometimes hotter and sometimes colder than usual; that it is at one time much rarefied, and at another greatly condensed; now uncommonly dry, and now furnished with moisture—are circumstances that daily meet the senses of the most casual observer, as they are circumstances that powerfully, and often unpleasantly, arrest his attention. That these changes are the result of decompositions and combinations that are continually going on in the atmosphere, and of new modifications of its component principles, is manifest to him who is acquainted merely with the first elements of modern chemistry.
Indeed to modern chemistry this science is indebted for the progress it has made within the last 50 years; a period which may be considered as the second epoch of meteorology. In fact, this science is still in its infancy; but from the ardour with which it is now cultivated, from the abilities of the philosophers who are engaged in the study, and from the progress that is daily making in the kindred sciences, we may reasonably look forward to a period, at no great distance, when it shall please the great Author of nature to unveil many of those wonders which are now involved in darkness and obscurity, and permit us to control the jarring elements, as he has allowed us to exercise dominion over the beasts of the earth, the fowls of the air, and the fishes of the sea.
A late ingenious writer on the climate of Britain has suggested some useful hints for the improvement of meteorology, which we shall here extract. "With this view, our first step must be that recommended by Mr Kirwan and others, to establish corresponding societies in different parts of the world; these societies must be furnished with similar apparatus, equally adjusted, and graduated in their construction, for making observations on the weather. In our own island it will be necessary to procure registers, carefully kept, from the different parts of the sea coast, and from those parts of the country situated in the interior. The various states of the barometer, thermometer, hygrometer, and electroscope, should be carefully noted; with the variations and the degrees of wind, as well as the diurnal and nocturnal aspect of the heavens discriminately marked; the appearance of the sky; and in familiar language, such as might be understood by the respective and distant observers; for instance, whether the sun is totally or partially obscured by vapour;—whether the clouds are mottled, or fleaky;—whether they assume the appearance of horizontal streaks, or appear in radii apparently from a centre—or in masses of dense vapour—or loose and fleecy—or those familiarly known by the name of mare-tail clouds—with any other new or accustomed phenomena. The common terms fair, cloudy, or wet, are insufficient for forming a judgement of the weather; as the term fair is generally at present expressed only in opposition to rain, without distinguishing whether the atmosphere is obscure, dull, or bright. The appearance of the stratum of air on the earth's surface, that is, the space between the clouds and the earth, should be always accurately described. Is there a blue haze, white mist, and dense fog? or is the air transparent? which is the case when distant objects appear more than commonly distinct and near to the eye of the observer: the temperature of the ocean at full tide should be frequently ascertained, as it will be found to have considerable influence in these respects on an insular country. By the remarks of observers, stationed in various parts of our coasts, we should soon be enabled to discover when vapour is wafted in from the sea, or generated by the aqueous and vegetable surface of our island. During a north-west wind, which is frequently attended with storms of hail and rain, and usually experienced in the spring, an observer stationed on the coast of Sligo in Ireland, or Denbighshire in Wales, might ascertain whether the disposition of the atmosphere to storm and cloud came in with the air from the Atlantic ocean, or was generated by the vapours of our own island. It would be desirable also again, that the temperature and blue hazy appearance of the atmosphere during the north-east winds, so common in May and June, should be noticed by observers on the north-east coast, in the counties of York, Lincoln, Essex and Kent; and by others, on the opposite western coasts of Pembroke, Devon, and Cornwall, so as to determine what changes in temperature this wind undergoes in its passage over the island; and whether or not the degree of haze increases or diminishes by its progress from either quarter; and whether the vapour is more or less disposed to produce storms?
By such comparative observations on the coast, conjoined with those made by others in the central parts of the kingdom, we might rapidly proceed in meteorological science, or, as it is commonly called, a knowledge of the weather. The observations made in the interior of the country would enable us at all times to trace the origin and progress of storms; in situations where tillage or pasturage is most attended to, the effects of spring frosts and blights should be particularly noticed, as well as the first appearance of the aphid and coccus, the caterpillar and larvae of other insects, on fruit trees, and particularly those peculiar to the hop plantations. The first opening of the vernal foliage on trees... trees and hedges in the spring, should likewise be remarked, and compared with the starting up grass on the highly manured pastures in the neighbourhood of towns, and on those also affixed with manure, as well as the natural herbage on the commons and wastes. Some attention should be paid to the effects of thunder storms, in destroying the aphis and other destructive insects, the peat of fruit and hop plantations; and the first appearance of the mildew or rust on wheat should be particularly observed, and remarks made to ascertain, whether or not the moisture, which occasions the disease in its commencement was attended with wind and rain, or a clove damp state of the air. The different kinds of soil, where the crops, from the disease, suffered most, should be noticed, and the situation of the land for ventilation, with the height of the fences, size of inclosures, and vicinity to coppices, trees, or hedge-rows.
The importance of the study of meteorology requires little elucidation. In climates where the succession of seasons is nearly stated and regular, where the periods of parching drought or deluging torrents, of the tempestuous hurricane or the refreshing breeze, are fixed and ascertained, mankind has little to do, but expect the dreaded changes, and provide against their devastations; but in countries like our own, where all the vicissitudes of seasons may take place in the course of a few hours, it is of the highest consequence to investigate the nature of the change, and the circumstances that precede or accompany it. To the farmer, the mariner, the traveller, the physician, meteorology is in some measure a study of necessity; to the philosopher it is a study of interest and delight; and to the observer of nature it affords objects of grandeur and sublimity not to be found in any other department of his favourite science. Surely nothing can contribute more to elevate the mind of man, to raise it "from nature up to nature's God," than the contemplation of the sweeping whirlwind, the dazzling lightning, or the awful thunder.
Our limits will not admit of our entering into a historical detail of the progress of meteorology; but it may be proper in this place to enumerate the principal writers on this science both in our own country and on the continent.
In this country, we may reckon Dr Kirwan, (in his "Estimate of the Temperature of different Climates,"
his "Essay on the Variations of the Atmosphere," and Introduction in the "Irish Transactions"), Mr John Dalton (chiefly in the "Manchester Memoirs"), Col. Capper (in his "Observations on the Winds and Monsoons"), Mr Writers on Williams (in his "Climate of Great Britain"), and meteorologist Luke Howard (in the Philosophical Magazine), as the principal cultivators of meteorological knowledge; and on the continent, the names of Cotte ("Traite de Meteorologie," and Journal de Physique), Saussure ("Essai sur l'Hygrometrie," and Voyage aux Alpes"), De Luc (a) ("Recherches sur les Modifications de l'Atmosphere," Idees sur la Meteorologie," and other works), and Lamarck (See Journ. de Phys. pafifim) stand most conspicuous in this branch of natural science.
In considering the subject of meteorology, we may properly divide it into seven general heads: 1. of the changes which take place in the gravity of the air; 2. of the changes of the temperature of the air; 3. of the changes produced by evaporation and rain; 4. of the changes produced by winds; 5. of atmospherical electricity; 6. of meteors or those visible phenomena accompanied with light, which take place in the atmosphere or near the surface of the earth; and 7. the application of the principles of meteorology to the useful purposes of life. Of these heads, the fifth has been already fully considered under Electricity, and much of the sixth has been exhausted under Meteorolite. The remaining circumstances will form the subjects of the following chapters.
CHAP. I. Of the Changes which take place in the Gravity of the Air.
Many of the facts relating to this part of our subject have been already anticipated under the article Barometer, and several circumstances fall to be considered more properly under Pneumatics than in this place. We shall here confine ourselves to a general view of the changes in the gravity of the atmosphere, as indicated by the barometer, in various situations on or near the surface of the earth, and briefly examine the conclusions that may be drawn from them.
The most general fact indicated by the barometer is, that this instrument shows us the weight of a column of air high at the level of the sea,
(A) In again mentioning the name of a philosopher so respectable as M. de Luc, we embrace the first opportunity of doing him justice, and of vindicating his character against an unfortunate misconception of the late Professor Robison, a mistake which we have inadvertently contributed to disseminate, by quoting Dr Robison's statement in our account of Dr Black, where M. de Luc is accused of having arrogated to himself Dr Black's discovery of latent heat.
M. de Luc's vindication of himself (as printed in the 12th number of the Edinburgh Review) is before the public. We owe it to candour and justice to acknowledge our conviction that Dr Robison was too hasty in his assertion, and that M. de Luc, so far from arrogating to himself the doctrine of latent heat, has, in various parts of his numerous writings, expressly mentioned Dr Black as the author of that doctrine. This will appear from the following citations. In his "Introduction à la Physique terrestre," p. 102, M. de Luc thus expresses himself.
"Ne connoissant point le feu latent, dans la vapeur à toute température, dont la première découverte est due au Dr Black, &c. Again, p. 232 of the same work. "Ce qui développe l'idée de chaleur latente par laquelle le Dr Black avoue dégager ce phénomène,"—and at p. 385, "Le Dr Black ayant découvert qu'une certaine quantité de chaleur disparaît quand la vapeur de l'eau bouillante se forme, nomma ce phénomène chaleur latente dans la vapeur."
We trust that these quotations, with M. de Luc's own justification of himself above referred to, will be sufficient to exculpate him from the charge of literary felony so warmly brought against him by Professor Robison; and we have no doubt the Professor himself, were he still alive, would under such evidence retract his accusation. Gravity of air whose base is equal to the diameter of the mercury in the tube, and whose height is equal to the extent of the atmosphere above the place of observation. As the height of this column must vary in different situations, and must, ceteris paribus, be greatest, at the level of the sea, the mercury in the tube will, under the same circumstances, stand highest in such a situation.
The medium height of the barometer at the level of the sea is 30 inches, as has been found by observations in the British channel, and in the Mediterranean sea, at the temperatures of 54° and 60°; on the coast of Peru at the temperature of 84°, and in latitude 89°.
As we ascend above the surface of the earth, the medium height of the mercury diminishes; and some late observations made in balloons at a considerable distance above the tops of the highest mountains, have shewn that in the higher regions of the air, the column of mercury is very considerably shortened. This fact, as we have seen (see Barometer), has been usefully applied to the measuring of heights and depths that cannot be ascertained by the usual geometrical methods.
As the absolute gravity of the atmosphere is constantly varying, even in the same place, the column of air pressing on the surface of the mercury without the tube, must press with more or less force, in proportion as these changes are greater; and hence the barometer points out these variations, falling when the atmosphere is lighter, and rising when it is heavier than usual.
For an account of the observations that were made on the rise and fall of the barometer by the earlier philosophers, and the attempts which were made by them to explain these phenomena, see Barometer.
It will be of advantage here to consider the variations of the barometer, as they take place in different situations, in order, if possible, to point out the cause by which these variations are produced, as this cause must have considerable influence on the changes of the weather.
It is found, that between the tropics the variations of the barometer are exceedingly small, and it is remarkable, that in that part of the world it does not descend above half as much for every 200 feet of elevation as it does beyond the tropics*. In the torrid zone, too, the barometer is elevated about \( \frac{1}{2} \) of a line twice every day; and this elevation happens at the same time with the tides of the sea†.
As the latitude advances towards the poles, the range of the barometer gradually increases, till at last it amounts to two or three inches. This gradual increase will appear from the following table.
| Latitude | Places | Range of the Barometer | |----------|------------|------------------------| | 6° | Peru | Greatest: 0.20 | | 22° | Calcutta | 0.77 | | 40° | Naples | 1.00 | | 51° | Dover | 2.47 | | 53° | Middlewick | 3.00 | | 53° | Liverpool | 2.89 | | 59° | Pittsburgh | 3.43 |
There is, however, some exception to this general rule, as in North America the range of the barometer is much less than in the corresponding European latitudes.
The range of the barometer is greater at the level of the sea than on mountains, and in the same degree of latitude the extent of the range is in the inverse ratio of the height of the place above the level of the sea.
It appears probable that the barometer has a tendency to rise during the day from morning to evening, and that this tendency is greatest between 2 and 9 P.M., the greatest elevation being at this last period. The elevation at 2 differs from that at 9 by \( \frac{1}{2} \), while that at 2 differs from the morning elevation only by \( \frac{1}{4} \); and that in certain climates the greatest elevation takes place at 2 o'clock*.
The range of the barometer is greater in winter than in summer, as appears from some observations made at Kendal during five years; the mean range from October to March being 7.982, and that from April to September being only 5.447†.
When the atmosphere is serene and settled the mercury is generally high; and in calm weather, when it is inclined to rain, the mercury is low. On the approach of high winds it sinks, as it does with a southerly wind, but rises very high on the approach of easterly and northerly winds. It is found, however, that at Calcutta the mercury is highest with north-westerly and northerly, and lowest with south-easterly winds.
The mercury suddenly falls on the approach of tempests, and during their continuance undergoes great oscillations.
To these general facts that have been observed on the rise and fall of the barometer, we shall annex the following axioms by M. Cotte:
1. The greatest changes of the barometer commonly take place during clear weather, with a north wind, and the small risings during cloudy, rainy, or windy weather, with a south, or nearly south wind.
2. The state of the mercury changes more in the winter than in the summer months; so that its greatest rising and falling takes place in winter; but its mean elevation is greater in summer than in winter.
3. The changes of the state of the barometer are nearly null at the equator, and become greater the more one removes from it towards the poles.
4. They are more considerable in valleys than on mountains.
5. The more variable the wind, the more changeable the state of the barometer.
6. It is lower at midnight and noon than at other periods of the day; its greatest daily height is towards evening.
7. Between 10 at night and 2 in the morning, and also in the day, the rising and falling of the mercury are less; the contrary is the case between 6 and 10 in the morning and evening.
8. Between 2 and 6 in the morning and evening it rises as often as it falls; but in such a manner that it oftener rises about that time in the winter months, and falls oftener in the summer months.
9. The oscillations are less in summer, greater in winter, and very great at the equinoxes.
10. They They are greater also in the daytime than during the night.
11. The higher the sun rises above the horizon, the less are the oscillations; they increase as he approaches the western side of the horizon, and are exceedingly great when he comes opposite to the eastern part of the horizon.
12. They are, to a certain degree, independent of the changes of temperature.
13. The mercury generally rises between the new and the full moon, and falls between the latter and the new moon.
14. It rises more in the apogee than the perigee; it usually rises between the northern lunifice and the southern, and falls between the southern lunifice and the northern.
15. In general, a comparison of the variations of the mercury with the positions of the moon gives nothing certain; the results of No. 13. and 14. are the most constant.
16. In the neighbourhood of Paris the barometer never continues 24 hours without changing.
17. The barometers in the western districts rise and fall sooner than those in the more eastern.
18. When the sun passes the meridian, the mercury, if falling, continues to fall, and its fall is often hastened.
19. When the mercury at the same period is rising, it falls, remains stationary, or rises more slowly.
20. When the mercury, under the same circumstances, is stationary, it falls, unless before or after it becomes stationary, it has been in the act of rising.
21. The above changes commonly take place between 11 in the morning and 1 in the afternoon, but oftener before than after noon.
22. Before high tides there is almost always a great fall of the mercury; this takes place oftener at the full than the new moon.
Such is a general view of the variations in the gravity of the air, as far as they have been observed by the barometer; and we shall now endeavour to give some plausible theory of them.
It is evident that the density of the atmosphere is least at the equator, and greatest at the poles; for at the equator the centrifugal force, the distance from the centre of the earth, and the heat (all of which tend to diminish the density of the air), are at their maximum, while at the poles they are at their minimum.
The mean height of the barometer at the level of the sea, all over the globe, is 30 inches; the weight of the atmosphere, therefore, is the same all over the globe. This weight depends on the density and height of the air; where the density is greatest, its height must be least; and on the contrary, where its density is least, its height must be greatest. The height of the atmosphere, therefore, must be greatest at the equator, and least at the poles; and it must decrease gradually between the equator and the poles, so that its upper surface will resemble two inclined planes, meeting above the equator their highest part.*
During summer, when the sun is in our hemisphere, the mean heat between the equator and the pole does not differ so much as in winter. Hence the rarity of the atmosphere at the pole, and consequently its height, will be increased. The upper surface of the atmosphere, therefore, in the northern hemisphere, will be less inclined; while that of the southern hemisphere, from contrary causes, will be much more inclined. The reverse will take place during our winter.
The density of the atmosphere depends in a great measure on the pressure of the superincumbent column, and therefore decreases according to the height, as the pressure of the superincumbent column constantly decreases. But the density of the atmosphere in the torrid zone will not decrease so fast as in the temperate and frigid zones, because its column is larger, and because there is a greater proportion of air in the higher part of this column. This accounts for the observation of Mr. Caffon, that the barometer sinks only half as much for every 200 feet of elevation in the torrid as in the temperate zones. The density of the atmosphere at the equator, therefore, though at the surface of the earth it is least, must at a certain height equal, and at a still greater must exceed, the density of the atmosphere in the temperate zones and at the poles.
We shall presently endeavour to prove, that a quantity of air is constantly ascending at the equator, and mercury is that part of it at least reaches and continues in the highest in er parts of the atmosphere. From the fluidity of air, northern it is evident that it cannot accumulate above the equa-titudes, but must roll down the inclined plane which the upper surface of the atmosphere assumes towards the poles. As the surface of the atmosphere of the northern hemisphere is more inclined during our winter than that of the southern hemisphere, a greater quantity of the equatorial current of air must flow over upon the northern than upon the southern hemisphere; so that the quantity of our atmosphere will be greater during winter than that of the southern hemisphere; but during summer the reverse will take place. Hence the greatest mercurial heights take place during winter, and the range of the barometer is less in summer than in winter.
The density of the atmosphere is in a great measure regulated by the heat of the place; wherever the cold is greatest, there the density of the atmosphere will be greatest, and its column thickest. High countries, and ranges of lofty mountains, the tops of which are covered with snow the greatest part of the year, must be much colder than other places situated in the same degree of latitude, and consequently the column of air over them much shorter. The current of superior air will linger and accumulate over these places in its passage towards the poles, and thus occasion an irregularity in its motion, which will produce a similar irregularity in the barometer. Such accumulations will be formed over the north-western parts of Asia, and over North America; hence the barometer usually stands higher, and varies less there, than in Europe. Accumulations also are formed upon the Pyrenees, the Alps, the mountains of Africa, Turkey in Europe, Tartary, and Tibet. When these accumulations have gone on for some time, the density of the air becomes too great to be balanced by the surrounding atmosphere; it rushes down on the neighbouring countries, and produces cold winds which raise the barometer. Hence the rise of the barometer which generally attends north-east winds in Europe, as they proceed from accumulations in the north-west of Asia, or about the pole; hence, too, the north-west wind from the mountains of Tibet raises the barometer at Calcutta.
* Irisf. Trans. vol. ii. p. 43; &c. We shall presently endeavour to show, that considerable quantities of air are occasionally destroyed in the north polar regions. When this happens, the atmosphere to the south rushes in to supply the deficiency. Hence south-west winds take place, and the barometer falls.
As the mean heat of our hemisphere differs in different years, the density of the atmosphere, and consequently the quantity of equatorial air which flows towards the poles, must also be variable. Does this range correspond to the mean annual heat; that is to say, is the range greatest when the heat is least, and least when the heat is greatest? In some years greater accumulations than usual take place in the mountainous parts of the south of Europe and Asia, owing, perhaps, to earlier falls of snow, or to the rays of the sun having been excluded by long-continued fogs. When this takes place, the atmosphere in the polar regions will be proportionally lighter. Hence the prevalence of southerly winds during some winters more than others.
As the heat in the torrid zone never differs much, the density, and consequently the height, of the atmosphere, will not vary much. Hence the range of the barometer within the tropics is comparatively small; and it increases gradually as we approach the poles, because the difference of the temperature, and consequently of the density, of the atmosphere, increases with the latitude.
The diurnal elevation of the barometer in the torrid zone corresponding to the tides, observed by Mr Caffon and others, must be owing to the influence of the moon on the atmosphere. This influence, notwithstanding the ingenious attempts of D'Alembert and several other philosophers, seems altogether inadequate to account for the various phenomena of the winds. It is not so easy to account for the tendency which the barometer has to rise as the day advances. Perhaps it may be accounted for by the additional quantity of vapour added to the atmosphere, which, by increasing the quantity of the atmosphere, may possibly be adequate to produce the effect.
The falls of the barometer which precede, and the oscillations which accompany, violent storms and hurricanes, show us that these phenomena are produced by very great rarefactions, or perhaps deflections of air, in particular parts of the atmosphere. The falls of the barometer, too, that accompany winds proceed from the same cause. The observation made by Mr Copeland, that a high barometer is accompanied by a temperature above the mean, will be easily accounted for by every one acquainted with Dr Black's theory of latent heat. The higher the mercury stands, the denser the atmosphere must be; and the denser it becomes, the more latent heat it must give out. It is well known that air evolves heat when condensed artificially. The falling of the barometer, which generally precedes rain, remains still to be accounted for; but we know too little about the causes by which rain is produced, to be able to account for it in a satisfactory manner.
It has been for some time suspected that the variation of the barometer is affected by the changes of the moon. The theory of lunar influence has been discussed on the continent chiefly by Lamarck and Cotte, (see Journal de Physique, p. 179); and in this country by Mr Luke Howard. Mr Howard's suspicions of this influence on the barometer were first conceived, in consequence of the printed charts, of which he made use in keeping a register of the barometer, having the phases of the moon marked on them, and of his observing a remarkable coincidence between these and certain states of the mercury. This coincidence consists in the depression of the barometrical line on the approach of the new and full moon, and its elevation on that of the quarters. In above 30 out of the 50 lunar weeks in the year 1798, the barometer was found to have changed its general direction once in each week, in such a manner as to be either rising or at its maximum, for the week preceding and following, about the time of each quarter; and to be either falling or at its minimum, for the two weeks, about the new and full. It is remarkable, that the point of greatest depression during the year, viz. to 28.67, was found about 12 hours after the new moon on the 8th of November; and that at its greatest and extraordinary elevation to 30.89, on the 7th of February, at the time of the last quarter. Moreover, this coincidence appeared to take place most regularly in fair and moderate weather; and, in general, when the barometer fell, during the interval between the new or full moon and the quarters, an evident perturbation in the atmosphere accompanied it; which may be inferred February 15 to 23, when the barometer, after an uncommon rise, continued to fall rapidly after the new moon, with severe cold, which ended suddenly in stormy and wet weather; again, June 14 to 20, when two weeks of fair weather ended in a thunderstorm. In the greater part of December the usual coincidence disappeared, and the converse took place; the barometer being low at the quarter and high at the full, amidst continual alternations of rain, frost, and snow, and, for part of the time, high winds. On the two days preceding the last quarter, the barometer rose rapidly, and rain followed.
On the whole, Mr Howard thought there appeared sufficient ground, on the evidence of the year 1798, to suppose that the gravity of our atmosphere, as indicated by the barometer, may be subject to certain periodical changes, effected by a cause more steady and regular than either change of temperature, currents, or solution and precipitation of water, to which he believes the whole variation has been heretofore attributed.
The mean of the register at large appeared to be 29.89, whence it appears that the depression at the new and full moon either amounted to more, on the whole, than the elevations at the quarters, or that they fell out nearer to the time. He was quite satisfied, in passing through this register, that if he had allowed himself to choose the higher rotations about the quarters, and the lower about the new and full, with a latitude of 24 or 36 hours, it would have made the results as much more favourable to his conclusions as in her former case.
Now, to omit the consideration of other proofs for the present, it appeared to him evident, that the atmosphere is subject to a periodical change of gravity, whereby the barometer, on a mean of ten years, is depressed at least one-tenth of an inch while the moon is passing from the quarters to the full and new; and elevated, in the same proportion, during the return to the quarter. To what causes shall we attribute this periodical change, other than the attraction of the sun and moon for the matter composing the atmosphere?
The atmosphere is a gravitating fluid, differing, in a physical Though there is a considerable difference in every part of the world between the temperature of the atmosphere in summer and in winter; though in the same season the temperature of almost every day, and even every hour, differs from that which precedes and follows it; though the heat varies continually in the most irregular and seemingly capricious manner—still there is a certain mean temperature in every climate, which the atmosphere has always a tendency to observe, and which it neither exceeds nor comes short of beyond a certain number of degrees. What this temperature is, may be known by taking the mean of tables of observations kept for a number of years; and our knowledge of it must be the more accurate the greater the number of observations is.
The mean annual temperature is greatest at the equator (or at least a degree or two on the north side of it), and it diminishes gradually towards the poles, where it is least. This diminution takes place in arithmetical progression, or, to speak more properly, the annual temperatures of all the latitudes are arithmetical means between the mean annual temperature of the equator and that of the pole. This was first ascertained by Mr Meyer; and Dr Kirwan improving on Meyer's hint, has calculated in the following table the mean annual temperature of every latitude between the equator and the pole. It must be remarked, however, that this table is calculated only for a particular part of the earth's surface, viz. that part of the Atlantic ocean which lies between the 80° of northern, and the 45° of southern latitude, extending westward as far as the Gulf stream, and to within a few leagues of the coast of America, and for all that part of the Pacific ocean that reaches from 45° of north latitude to 45° of south latitude, and extending between the 20th and 275th degree of longitude east from London. This part of the ocean is called by Dr Kirwan the standard, and was best suited to his purpose, as the rest of the ocean is subject to irregularities, which will be noticed presently (d).
| Lat. | Temper. | Lat. | Temper. | Lat. | Temper. | Lat. | Temper. | Lat. | Temper. | Lat. | Temper. | |------|---------|------|---------|------|---------|------|---------|------|---------|------|---------| | 90 | 31 | 77 | 33.7 | 64 | 41.2 | 51 | 52.4 | 38 | 63.9 | 25 | 74.5 | | 89 | 31.24 | 76 | 34.1 | 63 | 41.9 | 50 | 52.9 | 37 | 64.8 | 24 | 75.4 | | 88 | 31.10 | 75 | 34.5 | 62 | 42.7 | 49 | 53.8 | 36 | 65.7 | 23 | 75.9 | | 87 | 31.14 | 74 | 35.0 | 61 | 43.5 | 48 | 54.7 | 35 | 66.6 | 22 | 76.5 | | 86 | 31.2 | 73 | 35.5 | 60 | 44.3 | 47 | 55.6 | 34 | 67.4 | 21 | 77.2 | | 85 | 31.4 | 72 | 36.0 | 59 | 45.9 | 46 | 56.4 | 33 | 68.3 | 20 | 77.8 | | 84 | 31.5 | 71 | 36.6 | 58 | 45.8 | 45 | 57.5 | 32 | 69.1 | 19 | 78.3 | | 83 | 31.7 | 70 | 37.2 | 57 | 46.7 | 44 | 58.4 | 31 | 69.9 | 18 | 78.9 | | 82 | 32.0 | 69 | 37.8 | 56 | 47.5 | 43 | 59.4 | 30 | 70.7 | 17 | 79.4 | | 81 | 32.2 | 68 | 38.4 | 55 | 48.4 | 42 | 60.3 | 29 | 71.5 | 16 | 80.4 | | 80 | 32.6 | 67 | 39.1 | 54 | 49.2 | 41 | 61.2 | 28 | 72.3 | 15 | 80.8 | | 79 | 32.9 | 66 | 39.7 | 53 | 50.2 | 40 | 62.0 | 27 | 72.8 | 14 | 81.3 | | 78 | 33.2 | 65 | 40.4 | 52 | 51.1 | 39 | 63.0 | 26 | 73.8 | 13 | 81.7 |
(d) In calculating this table, Dr Kirwan proceeded on the following principle. Let the mean annual heat at the equator be $m$ and at the pole $m-n$; put $\phi$ for any other latitude; the mean annual temperature of that latitude will be $m-n\times\sin.\phi$. If, therefore, the temperature of any two latitudes be known, the value of $m$ and $n$ may be found. Now, the temperature of north latitude 45° has been found by the best observations to be 62.1°, and Dr Kirwan has also calculated in the following table the mean monthly temperature of the same standard (K).
| Latitude | 80° | 79° | 78° | 77° | 76° | 75° | 74° | 73° | 72° | 71° | 70° | 69° | 68° | 67° | 66° | 65° | 64° | 63° | 62° | |----------|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----| | Jan. | 22.5| 23. | 23.5| 24. | 24.5| 25. | 25.5| 26. | 26.5| 27. | 27.5| 28. | 28. | 28. | 29. | 30. | 31. | | Feb. | 23. | 23.5| 24. | 24.5| 25. | 25.5| 26. | 26.5| 27. | 27.5| 28. | 28.5| 29. | 30. | 31. | 32. | 33. | | March | 27. | 27.5| 28. | 28.5| 29. | 29.5| 30. | 30.5| 31. | 31.5| 32. | 32.5| 33. | 33.5| 34. | 35. | 36. | 37. | | April | 32.6| 32.9| 33.7| 34.1| 34.5| 35. | 35.5| 36. | 36.5| 37.2| 37.8| 38.4| 39.1| 39.7| 40.4| 41.2| 41.9| 42.7| | May | 36.5| 36.5| 37. | 37.5| 38. | 38.5| 39. | 39.5| 40. | 40.5| 41. | 41.5| 42. | 42.5| 43. | 44. | 45. | 46. | | June | 51. | 51.5| 52. | 52. | 52.5| 53. | 53.5| 54. | 54. | 54.5| 54.5| 55. | 55.5| 55.5| 55.5| 55.5| 56. | | July | 50. | 50.5| 51. | 51. | 51.5| 52. | 52.5| 53. | 53.5| 54. | 54.5| 54.5| 55. | 55.5| 55.5| 55.5| 55.5| 56. | | Aug. | 39.5| 40. | 41. | 41.5| 42. | 42.5| 43. | 43.5| 44. | 44.5| 45. | 45.5| 46. | 46. | 47. | 48. | 48.5| 49. | | Sept. | 33.5| 34. | 34.5| 35. | 35.5| 36. | 36.5| 37. | 38. | 38.5| 39. | 39.5| 40. | 40. | 41. | 42. | 43. | 44. | | Oct. | 28.5| 29. | 29.5| 30. | 30.5| 31. | 31.5| 32. | 32.5| 33. | 33.5| 34. | 34. | 35. | 36. | 37. | 37.5| 38. | | Nov. | 23. | 23.5| 24. | 24.5| 25. | 25.5| 26. | 26.5| 27. | 27.5| 28. | 28.5| 29. | 30. | 31. | 32. | 32.5| 33. | | Dec. | 22.5| 23. | 23.5| 24. | 24.5| 25. | 25.5| 26. | 26.5| 27. | 27.5| 28. | 28. | 29. | 30. | 30.5| 31. | 32. |
and that of latitude 50°, 52.9°. The square of the sine of 40° is nearly 0.419, and the square of the sine of 50° is nearly 0.586. Therefore,
\[ m = 0.41 \quad n = 62.1, \text{ and} \]
\[ m = 0.58 \quad n = 52.9; \text{ therefore,} \]
\[ 62.1 + 0.41n = 52.9 + 0.58n, \]
as each of them, from the two first equations, is equal to \( m \). From this last equation the value of \( n \) is found to be nearly 53; and \( m \) is nearly equal to 84. The mean temperature of the equator, therefore, is 84°, and that of the pole 31°. To find the mean temperature for every other latitude we have only to find 88 arithmetical means between 84 and 31.
(E.) In calculating the table of mean monthly temperature, Dr Kirwan proceeded on the following principles. The mean temperature of April seems to approach very nearly to the mean temperature of the whole year, and as far as heat depends on the action of the solar rays, the mean heat of each month may be considered as proportional... It appears from the above table that January is the coldest month in every latitude; that July is the warmest month in all latitudes above 48°; that in lower latitudes August is generally the warmest month; that the difference between the hottest and coldest months increases according to the distance of the place from the equator. All habitable latitudes are found to enjoy a medium heat of 60° for at least 2 months, which is a very favourable circumstance, as probably no corn could be produced under a lower medium temperature. The temperatures within 10° of the poles differ very little, nor do they differ much within 10° of the equator. Hence it was unnecessary to note these latitudes in the table. The temperatures of different years vary but little near the equator, but this difference increases more and more as the latitudes approach the poles.
It is well known that the temperature of the atmosphere gradually diminishes according to the height of the place above the level of the sea. It was found by Dr Hutton of Edinburgh, that a thermometer kept on the top of Arthur's seat, a height of about 800 feet, usually stood 3° lower than one kept at the foot of this hill; and Bouguer observed that on the top of Pinchincha, a height of about 15,564 feet, a thermometer stood 54° lower than it did at the level of the sea in the same latitude.
We are indebted to Dr Kirwan for a very ingenious method of determining the rate of the diminution in the temperature in particular cases, having the temperature of the surface of the earth given. The temperature of the atmosphere constantly diminishing as we rise above the level of the sea, we must at a certain height arrive at a point where a perpetual congelation takes place. This point must vary in height according to the latitude, being highest at the equator, and coming gradually nearer the earth as we approach the poles; it must vary also with the season, being highest in summer, and lowest in winter. The cold on the top of Pinchincha was found by M. Bouguer to extend from 7° below the freezing point every morning just before sunrise; hence he concluded that between the tropics the medium height of the term of congelation (where it freezes at some part of the day all the year round) should be fixed at 15,577 feet above the level of the sea; but in latitude 28°, and during the summer, at 13,440 feet. If we take the difference between the temperature at the equator, and the freezing point, this difference will bear the same proportion to the term of congelation at the equator, that the difference between the medium temperature at any other latitude and the freezing point bears to the term of congelation at that latitude. Suppose the medium heat at the equator to be 84°, the difference between which and 32° is 52°; and suppose the medium heat of latitude 28° to be 72°, the difference between which and 32° is 40°. Then by the following proportion, \( \frac{52}{15577} = \frac{40}{13440} : \frac{12}{72} \), gives us the term of congelation at 28°. In this way Dr Kirwan proceeded in calculating the following table.
| Lat. | 23° | 22° | 21° | 20° | 19° | 18° | 17° | 16° | 15° | 14° | 13° | 12° | 11° | 10° | |------|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----| | Jan. | 68 | 69 | 71 | 72 | 72.5| 73 | 73.5| 74 | 74.5| 75 | 76 | 76.5| 77 | 77.5| | Feb. | 72 | 72.5| 74 | 75 | 76 | 76.5| 77 | 77.5| 78 | 78.5| 79 | 79.5| 79.5| 79.5| | March| 75 | 75.5| 76 | 77 | 77.5| 78 | 78.5| 79 | 79.5| 80 | 80.5| 81 | 81.5| 81.8| | April| 78.5| 79 | 79.5| 80 | 80.5| 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| | May | 78.5| 79 | 79.5| 80 | 80.5| 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| | June | 79 | 79.5| 80 | 80.5| 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| 84.8| | July | 79 | 79.5| 80 | 80.5| 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| 84.8| | Aug. | 79 | 79.5| 80 | 80.5| 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| 84.8| | Sept.| 78.5| 79 | 79.5| 80 | 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.5| 84.8| 84.8| | Oct. | 75 | 75.5| 77 | 78 | 79 | 80 | 81 | 81.5| 82 | 82.5| 83 | 83.5| 84 | 84.8| | Nov. | 74 | 74.5| 75 | 75.5| 76 | 77 | 78 | 78.5| 79 | 79.5| 80 | 80.5| 80.8| 81 | | Dec. | 71 | 71.5| 72 | 72.5| 73 | 74 | 75 | 75.5| 76 | 76.5| 77 | 77.5| 78 | 78.5|
As the fine of the sun’s mean altitude in April : the mean heat of April = the fine of the sun’s mean altitude in May : mean heat of May.
In the same manner the mean heat of June, July, and August may be found; but for the temperature of the succeeding months we must take into consideration another circumstance, since the above rule would make the temperature of these months too low, as it does not take in the heat derived from the earth, which is nearly equal to the mean annual temperature. The real mean heats of these months must be considered as an arithmetical mean between the astronomical and terrestrial heats. Thus, for latitude 51°, the astronomical heat of September being 44.6°, and the mean annual heat 52.4°, the real heat of September ought to be \( \frac{44.6 + 52.4}{2} = 48.5 \). Dr Kirwan, however, after going through a tedious calculation, found the results to correspond so little with actual observation, that he drew up the table partly from calculating from principles, and partly from an examination of several sea journals. This last height of 120 feet M. Bouguer called the lower term of congelation. He also distinguished another term of congelation above which no visible vapour rises, and this he called the upper term of congelation. This line is considered by Kirwan as much less variable during the summer months than the lower line, and it has therefore been adopted by him to determine the rate of diminution in the temperature as we ascend into the atmosphere. He has calculated its height for every degree of north latitude in the following table.
| N. Lat. | Feet | N. Lat. | Feet | N. Lat. | Feet | N. Lat. | Feet | |---------|------|---------|------|---------|------|---------|------| | 0 | 28000| 26 | 22906| 48 | 12245| 70 | 4413 | | 5 | 27784| 27 | 22389| 49 | 11750| 71 | 4354 | | 6 | 27644| 28 | 21872| 50 | 11253| 72 | 4295 | | 7 | 27504| 29 | 21355| 51 | 10124| 73 | 4236 | | 8 | 27364| 30 | 20838| 52 | 8065 | 74 | 4177 | | 9 | 27224| 31 | 20492| 53 | 7806 | 75 | 4119 | | 10 | 27084| 32 | 20146| 54 | 6647 | 76 | 4067 | | 11 | 26880| 33 | 19800| 55 | 5617 | 77 | 4015 | | 12 | 26676| 34 | 19454| 56 | 5333 | 78 | 3963 | | 13 | 26472| 35 | 19169| 57 | 5439 | 79 | 3911 | | 14 | 26268| 36 | 18777| 58 | 5345 | 80 | 3861 | | 15 | 26061| 37 | 17983| 59 | 5251 | 81 | 3815 | | 16 | 25781| 38 | 17393| 60 | 5148 | 82 | 3769 | | 17 | 25501| 39 | 16801| 61 | 5068 | 83 | 3723 | | 18 | 25221| 40 | 16207| 62 | 4989 | 84 | 3677 | | 19 | 24941| 41 | 15712| 63 | 4910 | 85 | 3631 | | 20 | 24661| 42 | 15217| 64 | 4831 | 86 | 3592 | | 21 | 24404| 43 | 14722| 65 | 4752 | 87 | 3553 | | 22 | 24147| 44 | 14227| 66 | 4684 | 88 | 3514 | | 23 | 23800| 45 | 13730| 67 | 4616 | 89 | 3475 | | 24 | 23633| 46 | 13235| 68 | 4548 | 90 | 3432 | | 25 | 23423| 47 | 12740| 69 | 4480 | (f) |
Vol. XIII. Part II.
(f) Dr Kirwan has given us the following rule for ascertaining the temperature at any required height, supposing we know the temperature of the surface of the earth.
For the temperature observed at the surface of the earth, put \( m \); for the given height \( h \), and \( t \) for the height of the upper term of congelation at the given latitude; then
\[ \frac{m - 32}{t} = \text{the diminution of temperature for every 100 feet of elevation; or it is the common difference of the terms of the progression required.} \]
Let this common difference thus found be denoted by \( c \); then \( c \times \frac{h}{100} \) gives us the whole diminution of temperature from the surface of the earth to the given height. Let this diminution be denoted by \( d \), then \( m - d \) is obviously the M E T E O R O L O G Y.
7. Moisture has a peculiar influence on it, if followed by a wind which disperses it.
8. The greatest heat, and the greatest cold, take place about six weeks after the northern or southern solstice.
9. The thermometer changes more in summer than in winter.
10. The coldest period of the day is before sunrise.
11. The greatest heat in the sun and the shade seldom takes place on the same day.
12. The heat decreases with far more rapidity from September and October, than it increased from July to September.
13. It is not true, that a very cold winter is the prognostic of a very hot summer.
CHAP. III. Of the Changes which take place in the Air with respect to Evaporation and Rain.
There seems no reason to doubt that water exists in Qualities of the atmosphere in an intermediate state between that of vapour, a fluid and that of absolute steam. This is the state of vapour, of the qualities of which it is proper that we should here take a general view.
We are indebted to the experiments of Saussure and de Luc for much of our knowledge of the qualities of vapour. It is an elastic invisible fluid like common air, but lighter; being to common air, according to Saussure, as 10 to 14, or, according to Kirwan, as 10 to 12; it cannot pass beyond a certain maximum of density, otherwise the particles of water which compose it unite together, and form small, hollow, visible vehicles, called vesicular vapour; which is of the same specific gravity with atmospheric air. It is of this vapour that clouds and fogs are composed. This maximum increases with the temperature; and at the heat of boiling water is so great, that steam can resist the whole pressure of the air, and exist in the atmosphere in any quantity.
After what has been stated under Chemistry with respect to the nature and properties of vapour, we have nothing here to add on that subject, except to give the result of observations that have been made on the state of vapour in the atmosphere.
It is found that the evaporation of water into the air Evaporation is confined entirely to the surface, and hence it is always proportional to the surface exposed to the action of the air. Accordingly, observation shows that in maritime countries, and in marshy situations, in the neighbourhood of lakes, rivers, &c., the evaporation is much greater than in inland countries, and dry situations.
It is found that evaporation is greatest in hot weather; Proportionally to the temperature of the air, whence it must depend, in some degree, on the temperature of the air.
the temperature required. An example will make this rule sufficiently obvious. In latitude 56° the heat below being 54°; required the temperature of the air at the height of 803 feet?
Here \( m = 54, t = 5533, \frac{m - 32}{t} = \frac{22}{54.33} = 0.404 = c \), and \( c \times \frac{h}{100} = 0.404 \times 8.03 = 3.24 = d \), and \( m - d = 54 - 3.24 = 50.75 \). Hence we see that the temperature of the air at the height of 803 feet above the surface is 50°.75. Evaporation and Rain.
This was ascertained by Mr Dalton from actual experiments, the result of which was, that the quantity evaporated per minute from a given surface of water at a given temperature, is to the quantity evaporated from the surface at $212^\circ$, as the force of vapour at the given temperature is to the force of vapour at $212^\circ$. By means of the table expressing the force of vapour at various temperatures given under Chemistry, p. 468, we may discover by the above rule the quantity of water at a given temperature lost by evaporation.
There are several circumstances that affect the quantity of vapour rising from water, even at the same temperature. Thus, we find that evaporation is least in calm weather, increases when there is wind, and is greater in proportion as the wind is stronger. This evidently arises from the agitation of the water, by which a new surface is perpetually exposed to the action of the air.
We shall here insert a table by Mr Dalton, expressing the quantity of vapour raised in various atmospheric temperatures, from a circular surface six inches in diameter.
| Temperature | Force of vapour in inches | Evaporating force in grains | |-------------|--------------------------|----------------------------| | $212^\circ$ | | | | 30 | .129 | .52 | | 120 | .67 | .82 | | 154 | .85 | | | 189 | .54 | | | 20 | .134 | .56 | | 21 | .71 | .88 | | 22 | .91 | | | 23 | .73 | | | 24 | .94 | | | 25 | .79 | | | 26 | .97 | | | 27 | .82 | | | 28 | .86 | | | 29 | .90 | | | 30 | .93 | | | 31 | .99 | | | 32 | .93 | | | 33 | .97 | | | 34 | .91 | | | 35 | .95 | | | 36 | .98 | | | 37 | .92 | | | 38 | .95 | | | 39 | .98 | | | 40 | .92 | | | 41 | .95 | | | 42 | .98 | | | 43 | .92 | | | 44 | .95 | | | 45 | .98 | | | 46 | .92 | | | 47 | .95 | | | 48 | .98 | | | 49 | .92 | | | 50 | .95 | | | 51 | .98 | | | 52 | .92 | |
The first column of the above table expresses the temperature; the second, the corresponding force of vapour; the other three columns give the number of grains of water that would be evaporated from a surface of six inches in diameter in the respective temperatures, on the supposition of there being previously no aqueous vapour in the atmosphere. These columns present the extremes and the mean of evaporation likely to be noticed, or nearly such; for the first is calculated upon the supposition of 35 grains lost per minute from the vessel of three inches and a quarter in diameter; the second 45, and the third 55 grains per minute.
As yet we have stated only the degree of evaporation that would take place under various circumstances, provided that the atmosphere were, at the time, entirely free from moisture; but as this can scarcely happen, it becomes necessary to ascertain the rate of evaporation when qualified by the vapour already existing in the atmosphere. This is readily done by first finding the force of the vapour already in the atmosphere, as above directed, and subtracting it from the force of vapour at the given temperature. The remainder is the actual force of evaporation, from which, by the last table, we find the required rate of evaporation. Evaporation and Rain.
Suppose, for instance, it be required to know the rate of evaporation at the temperature of 59°. From the last table we see that the force of vapour at 59° is about 0.5 or \( \frac{1}{2} \) its force at 212°. Now, suppose that by trials we find the force of the vapour which already exists in the atmosphere to be 0.25 or \( \frac{1}{4} \) of \( \frac{1}{2} \). Subtracting the latter from the former, we have for a remainder 0.25—the force of evaporation required, which is therefore just the half of what it would be if the atmosphere were entirely free from vapour.
The force of vapour existing in the atmosphere is scarcely ever equal to the force of vapour of the temperature of the atmosphere. Hence evaporation may, with a few exceptions, be considered as going on without intermission. Attempts have been made to ascertain the quantity of evaporation that takes place in the course of a year; but the investigation of this problem is so difficult, that these attempts have succeeded only in obtaining approximations towards the truth. Mr Dobson of Liverpool, from a course of experiments made in 1772, 1773, 1774, and 1775, concludes that the mean annual evaporation from the surface of water, amounted to 36.78 inches. The proportions for each month are as follows:
| Month | Inches | |---------|--------| | January | 1.50 | | February| 1.77 | | March | 2.04 | | April | 3.30 | | May | 4.34 | | June | 4.41 | | July | 5.11 | | August | 5.01 | | September| 3.18 | | October | 2.51 | | November| 1.51 | | December| 1.49 |
The experiments of Mr Dalton shew that the evaporation from the surface of water in a very dry and hot summer day, was rather more than two tenths of an inch.
Several experiments have been made on the quantity of evaporation from land, especially by Mr Williams in America, and Dr Watson, Mr Dalton and Mr Hoyle in Britain.
Mr Williams's experiments appear to show that the evaporation from the surface of such land as is covered with trees and other vegetables is about one third greater than the evaporation from the surface of water, though much reliance is not laid on these experiments.
From an experiment made by Dr Watson during the summer, when the earth had been parched by a month's drought, it appeared that 1600 gallons of water were evaporated from a single acre in 12 hours. Dr Watson's experiment, however, was of a nature that did not admit of great precision.
The experiments made by Mr Dalton and Mr Hoyle in the years 1796, 1797, and 1798, are the most exact that have been made on this subject, and we shall therefore consider them more at large. They were made with the following apparatus. Having procured a cylindrical vessel made of tin plate, three feet deep and ten inches in diameter, they inserted into it two pipes directed downwards, so that water might pass through them into two bottles. One pipe was fixed near the bottom of the vessel, and the other about an inch from the top. The vessel was filled up for a few inches with gravel and sand, and all the rest with good fresh soil. It was then put into a hole in the ground, and the space around filled up with earth except on one side, for the convenience of putting bottles to the two pipes; then some water was poured on the earth to saturate it, and all that would drain off was suffered to escape. Hence the earth may be considered as saturated with moisture. The soil was kept for some weeks above the level of the upper pipe, but after that it was constantly allowed to be a little below it, thus preventing any water from running off through that pipe. The top of the soil for the first year was bare; but for the two last years it was covered with grass like other turf. The apparatus being thus prepared, a correct register was kept of the quantity of rain water which ran off from the surface of the earth by the upper pipe, as long as that was below the earth, and also of the quantity of water which passed through the three feet of earth, and ran off by the lower pipe; and a rain gauge of an equal diameter with the cylinder was kept near it, for the purpose of measuring the quantity of rain which fell in any corresponding time. Then, by subtracting the quantity of water which passed through the pipes from that in the rain gauge, the remainder was considered as equal to the quantity evaporated from the surface of the earth in the cylinder. The mean annual result of these experiments is shown in the following table.
| Water through the two pipes. | Mean. | Mean Rain. | Mean Evap. | |-----------------------------|-------|------------|------------| | 1796. Inch. | | | | | 1797. Inch. | | | | | 1798. Inch. | | | | | January | 1.897 | | | | February | 1.778 | | | | March | 4.31 | | | | April | 2.225 | | | | May | 2.027 | | | | June | 1.71 | | | | July | 1.53 | | | | August | | | | | September | | | | | October | | | | | November | | | | | December | | | | | Rain | 6.877 | | | | Evap. | 23.725| | |
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**Note:** The table values are given in inches. It appears from these experiments, that at Manchester the mean annual evaporation of water is above 25 inches; and if we add to this with Mr Dalton's inches for the dew which falls, the whole quantity evaporated in a year will be 30 inches. On the whole, we may perhaps estimate the mean annual evaporation from the whole surface of the globe at 35 inches from every square inch of surface, making the whole water annually evaporated over the whole globe equal to 94,450 cubic miles.
Were this prodigious mass of water all to subside in the atmosphere at once, it would increase its mass by about \( \frac{1}{2} \), and raise the barometer nearly 3 inches. But this never happens, no day passes without rain in some part of the earth; so that part of the evaporated water is continually precipitated again. Indeed it would be impossible for the whole of the evaporated water to subside in the atmosphere at once, at least in the state of vapour.
The higher regions of the atmosphere contain less vapour than the strata near the surface of the earth. This was observed both by M. de Saussure and M. de Luc.
At some height above the tops of mountains the atmosphere is probably still drier, for it was observed by Saussure, that on the tops of mountains the moisture of the air was rather less during the night than the day. And there can be little doubt that every stratum of air descends a little lower during the night than it was during the day, owing to the cooling and condensing of the stratum nearest the earth. Vapours, however, must ascend very high, for we see clouds forming far above the tops of the highest mountains.
Rain never begins to fall while the air is transparent; the invisible vapours first pass their maximum, and are changed into vesicular vapours; clouds are formed, and these clouds gradually dissolve in rain. Clouds, however, are not formed in all parts of the horizon at once; the formation begins in one particular spot, while the rest of the air remains clear as before; this cloud rapidly increases till it overpreads the whole horizon, and then the rain begins.
It is remarkable, that though the greatest quantity of vapour exists in the lower strata of the atmosphere, clouds never begin to form there, but always at some considerable height. It is remarkable too, that the part of the atmosphere at which they form has not arrived at the point of extreme moisture, nor near that point, even a moment before their formation. They are not formed then because a greater quantity of vapour had got into the atmosphere than could remain there without passing its maximum. It is still more remarkable, that when clouds are formed, the temperature of the spot in which they are formed is not always lowered, though this may sometimes be the case. On the contrary, the heat of the clouds themselves is sometimes greater than that of the surrounding air*. Nor is the formation of clouds owing to the capacity of air for combining with moisture being lessened by cold; so far from that, we often see clouds which had remained in the atmosphere during the heat of the day, disappear in the night, after the heat of the air was diminished.
The formation of clouds and rain cannot be accounted for by a single principle with which we are acquainted. It is neither owing to the saturation of the atmosphere, nor the diminution of the heat; nor the mixture of airs of different temperatures, as Dr Hutton supposes; for clouds are often formed without any wind at all either above or below them; and even if this mixture constantly took place, the precipitation, instead of accounting for rain, would be almost imperceptible.
It is a very remarkable fact, that evaporation often goes on for a month together in hot weather without any rain. This sometimes happens in this country; it happens every year in the torrid zone. Thus at Calcutta, during January 1785, it never rained at all; the mean of the thermometer for the whole month was 66°; there was no high wind, and indeed during great part of the month little wind at all.
The quantity of water evaporated during such a drought must be very great; yet the moisture of the air, instead of being increased, is constantly diminishing, and at last disappears almost entirely. For the dew, which is at first copious, diminishes every night; and if Dr Watson's experiment formerly mentioned be attended to, it will not be objected that the quantity of evaporation is also very much diminished. Of the very dry state to which the atmosphere is reduced during long droughts, the violent thunderstorms with which they often conclude is a very decisive proof. Now what becomes of all this moisture? It is not accumulated in the atmosphere above the country from which it was evaporated, otherwise the whole atmosphere would in a much less period than a month be perfectly saturated with moisture. If it be carried up daily through the different strata of the atmosphere, and wafted to other regions by superior currents of air, how is it possible to account for the different electrical state of the clouds situated between different strata, which often produces the most violent thunderstorms? They could not have remained in the lower strata of the atmosphere, and been daily carried off by winds to other countries; for there are often no winds at all during several days to perform this office; nor in that case would the dews diminish, nor could their presence fail to be indicated by the hygrometer.
It is impossible for us to account for this remarkable fact upon any principle with which we are acquainted. The water can neither remain in the atmosphere, nor pass through it in the state of vapour. It must therefore assume some other form; but what that form is, or how it assumes it, we know not. There are, therefore, two steps of the process which takes place between evaporation and rain, with which we are entirely unacquainted; first, the state of the vapour after it enters into the atmosphere, and second, the cause by which it is made to lay aside the new form which it assumed, return to its state of vapour, and descend in form of rain. Several theories have been contrived to account for this phenomenon, but they are all untenable on the present known laws of chemical action.
The mean annual quantity of rain is greatest at the equator, and decreases gradually as we approach the poles. Thus at Granada, Antilles, 12° N. Lat. it is 126 inches.
| Location | Latitude | Rainfall | |-------------------|----------|----------| | Cape François | 19° | 46" | | St Domingo | 19° | 46" | | Calcutta | 22° | 23" | | Rome | 41° | 54" | | England | 33° | 0" | | Peterburgh | 59° | 16" |
* De Luc la Meteorol. vol. ii. 100. On the contrary, the number of rainy days is smallest at the equator, and increases in proportion to the distance from it. From N. Lat. 12° to 43° the mean number of rainy days is 78; from 43° to 46° the mean number is 102; from 46° to 50° it is 134; from 50° to 60°, 161 days.
The number of rainy days is often greater in winter than in summer; but the quantity of rain is greater in summer than in winter. At Peterburgh, the number of rainy or snowy days during winter is 84, and the quantity which falls is only about 5 inches; during summer the number of rainy days is nearly the same, but the quantity which falls is about 11 inches.
More rain falls in mountainous countries than in plains. Among the Andes it is said to rain almost perpetually, while in Egypt it scarcely ever rains at all. If a rain-gauge be placed on the ground, and another at some height perpendicularly above it, more rain will be collected into the lower than into the higher; a proof that the quantity of rain increases as it descends, owing perhaps to the drops attracting vapour during their passage through the lower strata of the atmosphere where the greatest quantity resides. This, however, is not always the case, as Mr Copland of Dumfries discovered in the course of his experiments. He observed also, that when the quantity of rain collected in the lower gage was greatest, the rain commonly continued for some time; and that the greatest quantity was collected in the higher gage only either at the end of great rains, or during rains which did not last long. These observations are important, and may, if followed out, give us new knowledge of the causes of rain. They seem to show, that during rain the atmosphere is somehow or other brought into a state which induces it to part with its moisture; and that the rain continues as long as this state continues. Were a sufficient number of observations made on this subject in different places, and were the atmosphere carefully analysed during dry weather, during rain, and immediately after rain, we might soon perhaps discover the true theory of rain.
Rain falls in all seasons of the year, at all times of the day, and during the night as well as the day; though, according to M. Toaldo, a greater quantity falls during the day than the night. The cause of rain, then, whatever it may be, must be something which operates at all times and seasons. Rain falls also during the continuance of every wind, but oftener when the wind blows from the south. Falls of rain often happen likewise during perfect calms.
It appears from a paper published by M. Cotte in the Journal de Physique for October 1791, containing the mean quantity of rain falling at 147 places, situated between N. Lat. 11° and 65°, deduced from tables kept at these places, that the mean annual quantity of rain falling in all these places is 34.7 inches. Let us suppose then (which cannot be very far from the truth), that the mean annual quantity of rain for the whole is 34 inches. The superficies of the globe consists of $170,081,012$ square miles, or $686,401,498,471,475,200$ square inches. The quantity of rain therefore falling annually will amount to $23,337,650,812,030,156,800$ cubic inches, or somewhat more than $91,751$ cubic miles of water. This is $16,191$ cubic miles of water less than the quantity of water evaporated. It seems probable therefore, if the imperfection of our data warrant any conclusion, that some of the vapour is actually decomposed in the atmosphere, and converted into oxygen and hydrogen gas.
The dry land amounts to $52,745,253$ square miles; the quantity of rain falling on it annually will amount to $30,960$ cubic miles. The quantity of water running annually into the sea is $13,140$ cubic miles; a quantity of water equal to which must be supplied by evaporation from the sea, otherwise the land would soon be completely drained of its moisture.
The quantity of rain falling annually in Great Britain may be seen from the following table.
| Years of observation | Places | Rain in Inches | |----------------------|-------------------------|----------------| | 3 | Dover | 37.52 | | 5 | Ware, Hertfordshire | 23.6 | | 8 | London | 17.5 | | 8 | Kimbolton | 25.9 | | 45 | Lyndon | 22.21 | | 5 | Chatsworth, Derbyshire | 27.865 | | 8 | Manchester | 43.1 | | 18 | Liverpool | 34.41 | | 7 | Lancaster | 40.3 | | 5 | Kendal | 61.225 | | 14 | Dumfries | 36.127 | | 10 | Berwick | 31.26 | | 5 | Langholm | 36.73 | | 5 | Dalkeith | 25.124 | | 20 | Glasgow | 31 | | 8 | Hawkhill | 28.966 | | | Mean | 32.532 |
Mr Dalton has estimated the quantity of rain that falls in England at 21 inches; but as no account is taken of what falls in Wales and Scotland, this estimate probably falls much short of the real annual quantity. In this country it generally rains less in March than in November, in the proportion at a medium of 7 to 12. It generally rains less in April than October, in the proportion of 1 to 2 nearly at a medium. It generally rains less in May than September; the chances that it does so are at least at 4 to 3; but when it rains plentifully in May, it generally rains but little in September; and when it rains one inch, or less in May, it rains plentifully in September.
The degree of moisture that is present in the atmosphere at any given time, is measured by the hygrometer. Under the article Hygrometer we have amply described several of the most important instruments of that kind; but there is one hygrometer, viz. that of Mr Leslie, which remains to be described in this place. Figures of the instrument are given in Plate CCLXXVI. fig. 13, 14.
The principal part of the instrument is composed of Leslie's hygrostats, two glass tubes terminated by hollow balls, one transparent and the other opaque. The tubes are selected, as regular as possible, from 4 to 8 inches long, and about $\frac{1}{2}$ of an inch thick, or as slender as those employed for for thermometers, but with a much wider bore. This, in one tube, must be from $\frac{1}{8}$ to the $\frac{3}{8}$ of an inch in diameter, and an exact calibre, at least not differing by $\frac{1}{50}$ between both its extremities. To the end of it a small piece of black enamel is attached, and blown into an opaque ball, from $\frac{4}{7}$ to $\frac{1}{2}$ of an inch diameter. The corresponding tube may have its bore of the same, or rather a greater width, but its uniformity is not at all essential. Near the extremity it is swelled out into a thin cylinder, almost $\frac{1}{8}$ of an inch wide, and from $\frac{1}{8}$ to $\frac{6}{8}$ long; the inner cavity only being enlarged, without altering the exterior regularity of the tube. The short bit of glass where this cylinder terminates, is now blown into a thin pellucid ball, as nearly of the size of the former as the eye can judge. The exact equality of the balls would be unattainable, and fortunately the theory of the instrument does not require it. When a dark and a bright object are viewed together, the latter, from an optical deception, appears always larger than the reality; and for this reason, says Mr Leflie, I prefer making the clear ball a slight degree smaller than the black one. In the mean time a coloured liquor is prepared by dissolving carmine in concentrated sulphuric acid, in a phial with a ground stopper, taking care to avoid heat, as by this the colouring-matter would be charred, and the beauty of the liquor destroyed.
The tubes are now cut to nearly equal lengths, and the end of each swelled cut a little, to facilitate their junction. Close to the black ball, the tube is bent by the flame of a candle into a shoulder, such, that the root of the ball shall come into a line with the inner edge of the tube. This ball, being then warmed, the end of the tube is dipped into the acid liquor, and as much of it allowed to rise and flow into the cavity, as may be guessed sufficient to fill both tubes, excepting the cylinder. The two tubes are then, by the help of a blow-pipe, solidly joined together in one straight piece, without having any knot or protuberance. About half an inch from the joining, and nearer the cylinder, it is gently bent round by the flame of a candle, till the clear ball is brought to touch the tube $\frac{1}{4}$ inch directly below the black one. The instrument is now to be graduated; and the scale chosen by Mr Leflie is that which corresponds to the centigrade thermometer. Mr Leflie thus describes the mode of graduating the instrument.—The instrument is held in an oblique position, that the coloured liquor may collect at the bottom of the black ball, into which a few minute portions of air may, from time to time, be forced over, by heating the opposite ball with the hand. In this way, the interposed liquid will gradually be made to descend into the tube, and assume its proper place; and it should remain for a week or two in an inclined position, to let every particle drain out of the black ball. If any trace of fluid collects in rings within the bore, they are easily dispelled with a little dexterity and manipulation, which, though it would be difficult to describe, is most readily learnt and practised. The small cavity at the joining facilitates the rectification, by affording the means of sending a globule of air in either direction. In fixing the zero of the scale, Mr Leflie set the instrument in a remote corner of the room, or partly closed the window-shutters. When completely adjusted, the top of the coloured liquor, if held upright, should stand nearly opposite to the middle of the cylindrical reservoir.
In this state of preparation, the instrument is ready for being graduated. The clear ball and the contiguous part of the parallel tube are therefore covered with two or three folds of thin bibulous paper, moistened with pure water, to make it act as a hygrometer; and there is attached to the same tube a temporary scale, by means of a soft cement composed of bees-wax and rosin. A flat round piece of wood being provided with four or five pillars that screw into it, the instrument is fixed to one of them in an erect position, and on each side is disposed a fine corresponding thermometer, inverted, and at the same height, the one having its bulb covered with wet bibulous paper. Then half a yard of flannel is dried as much as possible without fingering, before a good fire, and rolling it up like a sleeve, it is lapped loosely round the lower part of the pillars, and the whole is inclosed under a large bell-glass. The flannel powerfully absorbs moisture from the confined air, and creates an artificial dryness of 80 or 100 degrees. In the space of a quarter or half an hour, the full effect is produced, and the quantities being noted at two or three separate times, the mean results are adopted. The deficient, measured by the temporary scale, being then augmented in the proportion of ten to the difference of the two thermometers, will give the length that corresponds to 100°. After the standard instrument is constructed, others are thence graduated with the utmost care; the first being planted in the centre, and the rest, with their temporary scales, stuck to the encircling pillars. For greater accuracy, the observation should be made in a room without a fire, or a screen ought to be interposed between the fire and the apparatus.
The slips of ivory intended for the scales are divided into equal parts, and should contain from 100° to 150°. The edges are filed down and chamfered, to fit easily between the parallel tubes; and they are secured in their place by a strong solution of sizingglass. The lower ball and its annexed cylinder, are covered with thin silk of the same colour as the upper ball, and a few threads are likewise lapped about that part of the tube which it touches. The instrument is lastly cemented into a piece of wood, either end of which admits a cylindrical case that serves equally to protect or to hold it. On other occasions, the hygrometer is inserted into the socket of a round bottom-piece where it stands vertical.
The above description refers particularly to fig. 14. Fig. 13 differs from this, only in having the balls of an equal height, and bended in opposite directions, which Mr Leflie considers as more convenient for some purposes to which the instrument is applied, to be mentioned hereafter, but which renders the instrument less portable.
The action of this hygrometer depends on the following principle; That the cold produced by evaporation in the instrument will accurately denote the degree of dryness of the air, or its distance from the point of saturation. To discover the dryness or humidity of the air, therefore, we have only to find the change of temperature induced in a body of water inflated, or exposed on all sides to evaporation. The steps which led Mr Leflie from these simple simple principles to the construction of the present ingenious instrument, are detailed by him in a paper published in Nicholson's Journal for January 1800, to which we must refer our readers for the particulars, contenting ourselves with the following summary view.
If two thermometers be filled with any expandible fluid, and having the bulb of the one wet and the other dry, they will, by their difference, denote the state of the air in respect to humidity. Mr Leflie's object was to combine two such instruments, so that they should indicate merely their difference of temperature; and this object he has completely attained by the present instrument. In ordinary cases, the intermediate liquor would continue stationary; for the air in both balls having the same temperature, and consequently the same elasticity, the opposite pressures would precisely counteract each other; but if, from the action of the external air on the moistened surface, one ball became colder, it is manifest the liquor would be pushed towards it by the superior elasticity of the air included in the other ball, so as to mark, by the space of its approach, the depression of temperature induced by evaporation.
This instrument does not merely point out the dryness of the air; it enables us to determine the absolute quantity of moisture which it is capable of imbibing; for the conversion of water into steam is found to consume 524° of the centigrade division; and evaporation, analogous in its effects, may be presumed to occasion the same waste of heat. If, therefore, air had the same capacity as water, for each degree of the hygrometer it would deposit as much heat as it would abstract by dissolving the $\frac{1}{3}$ part of its weight of humidity. But the capacity of air is to that of water as 11 to 5, and consequently it would require in that proportion a greater evaporation to produce the same effect. We may hence conclude, that, for each hygrometric degree, the air would require $\frac{1}{3} \times \frac{7}{3} = \frac{7}{9}$ part by weight of water to effect saturation.
Strictly speaking, the degrees marked by this hygrometer do not measure the dryness of the air at its actual temperature, but only its state of dryness when cooled down to the standard of the wet ball. The law, however, being known of the dissolving power of air as affected by heat, it is easy, from the dissipation of the air with respect to humidity at one temperature to derive that at any other. It will suffice to mention the result of a number of careful experiments:—Supposing air at the freezing point to be capable of holding 50 parts of moisture; at 10° centigrade, it will hold 100; at 20°, 200; at 30°, 400; thus doubling at each increase of 10°. Hence a table may be constructed by which these conversions will be easily made.
To omit nothing that tends to elucidate the theory of the instrument, we must observe that the air in its contact with the humid surface is not absolutely cooled to the same temperature; the air and water really meet each other at an intermediate point determined by their compounded density and capacity. Consequently the indications of the hygrometer ought to be augmented by the $\frac{1}{3} \times \frac{7}{3} = \frac{7}{9}$ part, or $\frac{7}{9}$. But this quantity is too small in any case to be regarded.
In considering the subject of winds, we shall first briefly detail their natural history, so far as it has not been already anticipated, and shall then endeavour to trace the laws by which they are regulated, or explain the manner in which their varieties are produced. As the direction of the winds is of the greatest consequence, especially in a commercial view, we shall first point out the direction of the most prevalent winds in various quarters of the world.
Between the tropics the winds are the most regular. Trade-winds lie near the equator, there is a regular wind during the whole year called the trade-wind. On the north side of the equator it blows from the north east, varying frequently a point or two towards the north or east; and on the south side of it, from the south-east, changing sometimes in the same manner towards the south or east. The space included between the second and fifth degrees of north latitude is the internal limit of these two winds. There the winds can neither be said to blow from the north nor the south; calms and violent storms are frequent. This space varies a little in latitude as the sun approaches either of the tropics. In the Atlantic ocean the trade winds extend farther north on the American than on the African coast; and as we advance westward, they become gradually more easterly, and decrease in strength. Their force diminishes likewise as we approach their utmost boundaries. It has been remarked also, that as the sun approaches the tropic of cancer, the south-east winds become gradually more southerly, and the north-east winds more easterly; exactly the contrary takes place when the sun is approaching the tropic of capricorn.
The trade-wind blows constantly in the Indian ocean from 10° south latitude to near 30°; but to the northward of this the winds change every six months, and blow directly opposite to their former course. These regular winds are called monsoons, from the Malay word moolin, which signifies a season. When they shift their direction, variable winds and violent storms succeed, which last for a month, and frequently longer; and during that time it is dangerous for vessels to continue at sea.
The monsoons in the Indian ocean may be reduced to two; one on the north and another on the south side of the equator; which extend from Africa to the longitude of New Holland and the east coast of China, and which suffer partial changes in particular places from the situation and influence of the neighbouring countries.
Between 3° and 10° of south latitude the south-east trade-wind continues from April to October; but during the rest of the year the wind blows from the north-west. Between Sumatra and New Holland this monsoon blows from the south during our summer months, approaching gradually to the south-east as we advance towards the coast of New Holland; it changes about the end of September, and continues in the opposite direction till April. Between Africa and Madagascar its direction is influenced by the coast; for it blows Winds blow from the north-east from October to April, and during the rest of the year from the south-west.
Over all the Indian ocean to the northward of the third degree of south latitude, the north-east trade-wind blows from October to April, and a south-west wind from April to October. From Borneo, along the coast of Malacca, and as far as China, this monsoon in summer blows nearly from the south, and in winter from the north by east. Near the coast of Africa, between Mozambique and Cape Guardafui, the winds are irregular during the whole year, owing to the different monsoons which surround that particular place. Monsoons are likewise regular in the Red sea; between April and October they blow from the north-west, and during the other months from the south-east, keeping constantly parallel to the coast of Arabia.
Monsoons are not altogether confined to the Indian ocean; on the coast of Brazil, between Cape St Augustine and the island of St Catharine, the wind blows between September and April from the east or north-east, and between April and September from the south-west. The bay of Panama is the only place on the west side of a great continent where the wind shifts regularly at different seasons; there it is easterly between September and March; but between March and September it blows chiefly from the south and south-west.
Such in general is the direction of the winds in the torrid zone all over the Atlantic, Pacific, and Indian oceans; but they are subject to particular exceptions, which we shall now endeavour to enumerate. On the coast of Africa, from Cape Bayador to Cape Verde, the winds are generally north-west; from thence to the island of St Thomas near the equator they blow almost perpendicular to the shore, bending gradually as we advance southwards, first to the west and then to the south-west. On the coast of New Spain likewise, from California to the bay of Panama, the winds blow almost constantly from the west or south-west, except during May, June, and July, when land-winds prevail, called by the Spaniards Popogayos. On the coast of Chili and Peru, from 20° to 30° south latitude, to the equator, and on the parallel coast of Africa, the wind blows during the whole year from the south, varying according to the direction of the land towards which it inclines, and extending much farther out to sea on the American than the African coast. The trade-winds are also interrupted sometimes by westerly winds in the bay of Campeachy and the bay of Honduras.
As to the countries between the tropics, we are too little acquainted with them to be able to give a satisfactory history of their winds.
In all maritime countries between the tropics, of any extent, the wind blows during a certain number of hours every day from the sea, and during a certain number towards the sea from the land; these winds are called the sea and land breezes. The sea breeze generally sets in about 10 in the forenoon, and blows till six in the evening; at seven the land breeze begins and continues till eight in the morning, when it dies away. During summer the sea breeze is very perceptible on all the coasts of the Mediterranean sea, and even sometimes as far north as Norway.
Vol. XIII. Part II.
In the island of St Lewis on the coast of Africa, in 16° north latitude, and 16° west longitude, the wind during the rainy season, which lasts from the middle of July to the middle of October, is generally between the south and the east; during the rest of the year it is for the most part east or north-east in the morning; but as the sun rises, the wind approaches gradually towards the north, till about noon it gets to the west of north, and is called a sea breeze. Sometimes it shifts to the east as the sun declines, and continues there during the whole night. In February, March, April, May and June, it blows almost constantly between the north and west. In the island of Bulama, which likewise lies on the west coast of Africa, in 11° north latitude, the wind during nine months of the year blows from the south-west; but in November and December, a very cold wind blows from the north-east.
In the kingdom of Bornou, which lies between 16° and 20° north latitude, the warm season is introduced about the middle of April by sultry winds from the south-east, which bring along with them a deluge of rain. In Fezzan, in 25° north latitude, and 35° east longitude, the wind from May to August blows from the east south-east, or south-west, and is intensely hot.
In Abyssinia the winds generally blow from the north-west, north, and north-east. During the Abyssinian months of June, July, August, September and October, the north and north-east winds blow almost constantly, especially in the morning and evening; and during the rest of the year they are much more frequent than any other winds.
At Calcutta, in the province of Bengal, the wind blows during January and February from the south-west and south; in March, April, and May from the south; in June, July, August and September, from the south and south-east; in October, November, and December, from the north-west. At Madras the most frequent winds are the north and north-east. At Tivoli in St Domingo, and the illes des Vaches, the wind blows oftenest from the south and south-east. From these facts it appears, that in most tropical countries with which we are acquainted, the wind generally blows from the nearest ocean, except during the coldest months, when it blows towards it.
In the temperate zones the direction of the wind is by no means so regular as between the tropics. Even in the same degree of latitude, we find them often blowing in different directions at the same time, while their changes are often so sudden and capricious, that to account for them has been hitherto found impossible. When winds are violent and continue long, they generally extend over a large tract of country; and this is more certainly the case when they blow from the north-east, than from any other points. By the multiplication and comparison of meteorological tables, some regular connection between the changes of the atmosphere in different places may in time be observed, which will at last lead to a satisfactory theory of the winds. It is from such tables chiefly that the following facts have been collected.
In Virginia, the prevailing winds are between the south-west, west, north, and north-west; the most frequent is the south-west, which blows more constantly in June, July, and August, than at any other season. The north-west winds blow most constantly in November. Winds, December, January, and February. At Ipswich in New England, the prevailing winds are also between the south-west, west, north, and north-east; the most frequent is the north-west. But at Cambridge, in the same province, the most frequent wind is the south-east. The predominant winds at New York are the north and west. In Nova Scotia north-west winds blow for three-fourths of the year. The same wind blows most frequently at Montreal in Canada, but at Quebec the wind generally follows the direction of the river St Lawrence, blowing either from the north-east or south-west. At Hudson's bay westerly winds blow for three-fourths of the year; the north-west wind occasions the greatest cold; but the north and north-east are the vehicles of snow.
It appears from these facts, that westerly winds are most frequent over the whole eastern coast of North America; that in the southern provinces south-west winds predominate, and that the north-west become gradually more frequent as we approach the frigid zone.
In Egypt, during part of May, and during June, July, August, and September, the wind blows almost constantly from the north, varying sometimes in June to the west, and in July to the west and the east; during part of September, and in October and November, the winds are variable, but blow more regularly from the east than any other quarter; in December, January, and February, they blow from the north, north-west, and west; towards the end of February they change to the south, in which quarter they continue till near the end of March; during the last days of March and in April they blow from the south-east, south, and south-west, and at last from the east; and in this direction they continue during a part of May.
In the Mediterranean the wind blows nearly three-fourths of the year from the north; about the equinoxes there is always an easterly wind in that sea, which is generally more constant in spring than in autumn. These observations do not apply to the gulf of Gibraltar, where there are seldom any winds except the east and the west. At Bastia, in the island of Corsica, the prevailing wind is the south-west.
In Syria the north wind blows from the autumnal equinox to November; during December, January, and February, the winds blow from the west and south-west; in March they blow from the south, in May from the east, and in June from the north. From this month to the autumnal equinox the wind changes gradually as the sun approaches the equator; first to the east, then to the south, and lastly to the west. At Bagdad the most frequent winds are the south-west and north-west; at Pekin, the north and the south; at Kamchatka, on the north-east coast of Asia, the prevailing winds blow from the west.
In Italy the prevailing winds differ considerably according to the situation of the places where the observations have been made. At Rome and Padua they are northerly, at Milan easterly. All that we have been able to learn respecting Spain and Portugal is, that on the west coast of these countries the west is by far the most common wind, particularly in summer; and that at Madrid the wind is north-east for the greatest part of the summer, blowing almost constantly from the Pyrenean mountains. At Berne in Switzerland, the prevailing winds are the north and west; at St Gotthard, the north-east; at Lausanne the north-west and south-west.
M. Cotte has given us the result of observations made at 86 different places of France, from which it appears, that along the whole south coast of that empire the wind blows most frequently from the north, north-west, and north-east; on the west coast, from the west, the winds south-west, and north-west; and on the north coast in France, from the south-west. That in the interior parts of France the south-west wind blows most frequently in 18 places; the west wind in 14; the north in 13; the south in 6; the north-east in 4; the south-east in 2; the east and north-west each of them in one. On the west coast of the Netherlands, as far north as Rotterdam, the prevailing winds are probably the south-west; at least this is the case at Dunkirk and Rotterdam. It is probable also, that along the rest of this coast, from the Hague to Hamburg, the prevailing winds are the north-west, at least these winds are most frequent at the Hague and at Franeker. The prevailing wind at Delft is the south-east, and at Breda the north and the east.
In Germany the east wind is most frequent at Gottingen, Munich, Weissenburg, Dusseldorf, Sagan, Erfurt, and at Buda in Hungary; the south-east at Prague and Wurtzburg; the north-east at Ratibon, and the west at Mainheim and Berlin.
From an average of 10 years of the register kept by order of the Royal Society, it appears, that at London the winds blow in the following order:
| Winds | Days | |-----------|------| | South-west | 112 | | North-east | 58 | | North-west | 50 | | West | 53 |
It appears from the same register, that the south-west wind blows at an average more frequently than any other wind during every month of the year, and that it blows longest in July and August; that the north-east wind blows most constantly during January, March, April, May, and June, and most seldom during February, July, September, and December; and that the north-west wind blows oftener from November to March, and more seldom during September and October, than any other months. The south-west winds are also most frequent at Bristol, and next to them are the north-east.
The following table of the winds at Lancaster has been drawn up from a register kept for seven years at that place:
| Winds | Days | |-----------|------| | South-west | 92 | | North-east | 67 | | South | 51 | | West | 41 |
The following table is an abstract of nine years observations made at Dumfries by Mr Copland.
| Winds | Days | |-----------|------| | South | 82½ | | West | 69 | | East | 68 | | South-west | 50½ |
The The following table is an abstract of seven years observations, made by Dr Meek at Cambullang, near Glasgow.
| Winds | Days | Winds | Days | |-------------|------|-----------|------| | South-west | 174 | North-east| 104 | | North-west | 40 | South-east| 47 |
It appears from the register from which this table was extracted, that the north-east wind blows much more frequently in April, May, and June, and the south-west in July, August, and September, than at any other period. We learn from the Statistical Account of Scotland, that the south-west is by far the most frequent wind all over that kingdom, especially on the west coast. At Saltcoats in Ayrshire, for instance, it blows three fourths of the year; and along the whole coast of Murray on the north-east side of Scotland, it blows for two-thirds of the year. East winds are common over all Great Britain during April and May; but their influence is felt most severely on the eastern coast.
The following table exhibits a view of the number of days during which the westerly and easterly winds blow in a year, at different parts of the island. Under the term westerly are included the north-west, west, south-west, and south; the term easterly is taken in the same latitude.
| Years of observation | Places | Wind | |---------------------|-----------------|------| | | | Westerly | Easterly | | 10 | London | 233 | 132 | | 7 | Lancaster | 216 | 149 | | 51 | Liverpool | 190 | 175 | | 9 | Dumfries | 227.5 | 137.5 | | 10 | Bransholm | 232 | 133 | | 7 | Cambullang | 214 | 151 | | 8 | Hawkhill near Edin.| 229.5 | 135.5 | | | Medium | 220.3 | 144.7 |
In Ireland the south-west and west are the grand trade-winds, blowing most in summer, autumn, and winter, and least in spring. The north-east blows most in spring, and nearly double what it does in autumn and winter. The south-west and north-west are nearly equal, and are most frequent after the south-west and west.
At Copenhagen the prevailing winds are the east and south-east; at Stockholm, the west and north. In Russia, from an average of a register of 16 years, the winds blow from November to April in the following order:
| W. N.W. E. S.W. S. N.E. N. S.E. | |---------------------------------| | Days 45 26 23 22 20 19 14 12 |
And during the other six months,
| W. N.W. E. S.W. S. N.E. N. S.E. | |---------------------------------| | Days 27 27 19 24 22 15 32 18 |
The west wind blows during the whole year 72 days; the north-west 53, the south-west and north 40 days each. During summer it is calm for 41 days, and during winter for 21. In Norway the most frequent winds are the south, the south-west and south east. The wind at Bergen is seldom directly west, but generally south-west or south-east; a north-west, and especially a north-east wind, are but little known there.
From the whole of these facts, it appears that the most frequent winds on the south coasts of Europe are the north, the north-east and north-west, and on the western coast the south-west; that in the interior parts which lie most contiguous to the Atlantic ocean, south-west winds are also most frequent; but that easterly winds prevail in Germany. Westerly winds are also most frequent on the north-east coast of Asia.
It is probable that the winds are more constant in the south temperate zone, which is in a great measure covered with water, than in the north temperate zones, where their direction must be frequently interrupted and altered by mountains and other causes.
M. de la Bailie, who was sent thither by the French king to make astronomical observations, informs us, at the Cape of Good Hope the main winds are the south-east and north-west; that other winds seldom last longer than a few hours; and that the east and north-east winds blow very seldom. The south-east wind blows in most months of the year, but chiefly from October to April; the north-west prevails during the other five months, bringing along with it rain, and tempests, and hurricanes. Between the Cape of Good Hope and New Holland the winds are commonly westerly, and blow in the following order: north-west, south-west, west, north.
In the great South sea, from latitude 30° to 40° in the Pacific ocean, the south-east trade-wind blows most frequently, especially when the sun approaches the tropic of Capricorn; the wind next to it in frequency is the north-west, and next to that is the south-west.
Thus it appears that the trade-winds sometimes extend farther into the south temperate zone than their usual limits, particularly during summer; that beyond their influence the winds are commonly westerly, and that they blow in the following order: north-west, south-west, west.
We have now considered pretty much at large the theory of direction of the winds in different parts of the earth's surface. Another very curious part of the history of the winds relates to their violence, and the effects with which they are attended, or to the history of hurricanes, whirlwinds, tornadoes, &c. Of some of these we have already treated under the articles Hurricane and Harmattan; and the confined limits of this article oblige us to refer our readers for more particulars to Capper's Observations on the Winds and Monsoons.
As to the velocity of the wind, its variations are almost infinite, from the gentlest breeze, to the hurricane, which tears up trees and blows down houses. Our most violent winds take place when neither the heat nor the cold is greatest; violent winds generally extend over a large tract of country, and they are accompanied with sudden and great falls in the mercury of the barometer. The wind is sometimes very violent at a distance from the earth, while it is quite calm at its surface. On one occasion Lunardi went at the rate of 70 miles an hour in his balloon, though it was quite calm at Edinburgh when he ascended, and continued so during his whole voyage. A pretty good idea of the velocity of the wind, under different circumstances, may be formed from the following table, which was drawn up by Mr Smeaton.
| Miles per Hour | Feet per Second | Perpendicular force on one square foot, in Avoirdupois pounds and Paris | |---------------|-----------------|------------------------------------------------------------------------| | 1 | 1.47 | .005 Hardly perceptible. | | 2 | 2.93 | .020 Just perceptible. | | 3 | 4.4 | .044 Gently pleasant. | | 4 | 5.87 | .079 Gently pleasant. | | 5 | 7.33 | .123 Pleasant, brisk. | | 10 | 14.67 | .492 Pleasant, brisk. | | 15 | 22. | 1.107 | | 20 | 29.34 | 1.968 Very brisk. | | 25 | 36.67 | 3.075 High wind. | | 30 | 44.01 | 4.429 Very high wind. | | 35 | 51.34 | 6.027 Storm or tempest. | | 40 | 58.68 | 7.873 Great storm. | | 45 | 66.01 | 9.963 Hurricane. | | 50 | 73.35 | 12.300 Hurricane that tears up trees and carries buildings before it. | | 60 | 88.02 | 17.715 | | 80 | 117.36 | 31.490 | | 100 | 146.7 | 49.200 |
For the means of ascertaining the velocity of the winds, see ANEMOMETER and ANEMOSCOPE.
We shall now endeavour to explain the phenomena that we have been describing, or to form a plausible theory of the winds.
The atmosphere is a fluid surrounding the earth, and extending to an unknown height. Now all fluids tend invariably to a level: if a quantity of water be taken out of any part of a vessel, the surrounding water will immediately flow in to supply its place, and the surface will become level as before; or if an additional quantity of water be poured into any part of the vessel, it will not remain there, but diffuse itself equally over the whole. Such exactly would be the case with the atmosphere. Whatever therefore destroys the equilibrium of this fluid, either by increasing or diminishing its bulk in any particular place, must at the same time occasion a wind.
Air, besides its qualities in common with other fluids, is also capable of being dilated and compressed. Suppose a vessel filled with air: if half the quantity which it contains be drawn out by means of an air-pump, the remainder will still fill the vessel completely; or if twice or three times the original quantity be forced in by a condenser, the vessel will still be capable of holding it.
Rarefied air is lighter, and condensed air heavier than common air. When fluids of unequal specific gravities are mixed together, the heavier always descend and the lighter ascend. Were quicksilver, water, and oil, thrown into the same vessel together, the quicksilver would uniformly occupy the bottom; the water the middle, and the oil the top. Were water to be thrown into a vessel of oil, it would immediately descend, because it is heavier than oil. Exactly the same thing takes place in the atmosphere. Were a quantity of air, for instance, to be suddenly condensed at a distance from the surface of the earth, being now heavier than before, it would descend till it came to air of its own density; or, were a portion of the atmosphere at the surface of the earth to be suddenly rarefied, being now lighter than the surrounding air, it would immediately ascend.
If a bladder half filled with air be exposed to the heat of a fire, the air within will soon expand, and distend the bladder; if it be now removed to a cold place, it will soon become flaccid as before. This shows that heat rarefies, and that cold condenses air. The surface of the torrid zone is much more heated by the rays of the sun than the frozen or temperate zones, because the rays fall upon it much more perpendicularly. This heat is communicated to the air near the surface of the torrid zone, which being thereby rarefied, ascends, and its place is supplied by colder air, which rushes in from the north and south.
The diurnal motion of the earth is greatest at the equator, and diminishes gradually as we approach the poles, where it ceases altogether. Every spot of the earth's surface at the equator moves at the rate of 15 geographical miles in a minute; at 40° of latitude it moves at about 11 miles and a half in a minute, and at the 30° at nearly 13 miles. The atmosphere, by moving continually round along with the earth, has acquired the same degree of motion, so that those parts of it which are above the equator move faster than those which are at a distance. Were a portion of the atmosphere to be transported in an instant from latitude 30° to the equator, it would not immediately acquire the velocity of the equator; the eminences of the earth, therefore, would strike against it, and it would assume the appearance of an easterly wind. This is the case in a smaller degree with the air that flows towards the equator, to supply the place of the rarefied air which is continually ascending; and this, when combined with its real motion from north to south, must cause it to assume the appearance of a north-easterly wind on this side the equator, and of a south-westerly beyond it.
The motion westward occasioned by this difference in celerity alone, would be very small; but it is increased by another circumstance. Since the rarefaction of the air in the torrid zone is owing to the heat derived from the contiguous earth, and since this heat is owing to the perpendicular rays of the sun, those parts must be hottest where the sun is actually vertical; and consequently the air above them must be most rarefied; the contiguous parts of the atmosphere will therefore be drawn most forcibly to that particular spot. Now, since the diurnal motion of the earth is from east to west, this hottest spot will be continually shifting westwards, and this will occasion a current of the atmosphere in that direction. That this cause really operates, appears from a circumstance already mentioned: When the sun approaches either of the tropics, the trade-wind on the same side of the equator assumes a more easterly direction, evidently from the cause here mentioned, while the opposite trade-wind being deprived of this additional impulse, blows in a direction more perpendicular to the equator.
The westerly direction of the trade-wind is still farther increased by another cause. Since the attraction of the sun and moon produces so remarkable an effect... Winds effect upon the ocean, we cannot but suppose that an effect equally great, at least, is produced upon the atmosphere. Indeed as the atmosphere is nearer the moon than the sea is, the effects produced by attraction upon it ought to be greater. When we add to this the elasticity of the air, or that disposition which it has to dilate itself when freed from any of its pressure, we cannot but conclude, that the tides in the atmosphere are considerable. Now since the apparent diurnal motion of the moon is from east to west, the tides must follow it in the same manner, and consequently produce a constant motion in the atmosphere from east to west. This reasoning is confirmed by the observations of several philosophers, particularly of M. Caffon, that in the torrid zone the barometer is always two-thirds of a line higher twice every 24 hours than during the rest of the day; and that the time of this rise always corresponds with the tides of the sea; a proof that it proceeds from the same cause.
All these different causes probably combine in the production of the trade-winds; and from their being sometimes united, and sometimes distinct or opposite, arise all those little irregularities which take place in the direction and force of the trade-winds.
Since the great cause of these winds is the rarefaction of the atmosphere by the heat of the sun, its ascension and the consequent rushing in of colder air from the north and south, the internal boundary of the trade-winds must be that parallel of the torrid zone which is hottest, because there the ascension of the rarefied air must take place. Now since the sun does not remain stationary, but is constantly shifting from one tropic to the other, we ought naturally to expect that this boundary would vary together with its exciting cause; that therefore, when the sun is perpendicular to the tropic of Cancer, the north-east trade-wind would extend no farther south than north latitude $23^\circ 30'$; that the south-east wind would extend as far north; and that, when the sun was in the tropic of Capricorn, the very contrary would take place. We have seen, however, that though this boundary be subject to considerable changes from this very cause, it may in general be considered as fixed between the second and fifth degrees of north latitude.
Though the sun be perpendicular to each of the tropics during part of the year, he is for one half of it at a considerable distance, so that the heat which they acquire, while he is present, is more than lost during his absence. But the sun is perpendicular to the equator twice in a year, and never farther distant from it than $23\frac{1}{2}^\circ$; being therefore twice every year as much heated, and never so much cooled as the tropics, its mean heat must be greater, and the atmosphere in consequence generally most rarefied at that place. Why then, it will be asked, is not the equator the boundary of the two trade-winds? To speak more accurately than we have hitherto done, the internal limit of these winds must be that parallel where the mean heat of the earth is greatest. This would be the equator, were it not for a reason that shall now be explained.
It has been shewn by astronomers, that the orbit of the earth is an ellipse, and that the sun is placed in one of the foci. Were this orbit to be divided into two parts by a straight line perpendicular to the transverse axis, and passing through the centre of the sun, one of these parts would be less than the other; and the earth during its passage through the smaller part of its orbit, would constantly be nearer the sun than while it moved through the other portion. The celerity of the earth's motion in any part of its orbit is always proportioned to its distance from the sun; the nearer it is to the sun it moves the faster; the farther distant, the slower. The earth passes over the smaller portion of its orbit during our winter, which must therefore be shorter than our summer, both on account of this part of the orbit being smaller than the other, and on account of the increased celerity of the earth's motion. The difference, according to Cassini, is 7 days, 23 hours, 53 minutes. While it is winter in the northern, it is summer in the southern hemisphere; wherefore the summer in the southern hemisphere must be just as much shorter than the winter, as our winter is shorter than our summer. The difference, therefore, between the length of the summer in the two hemispheres is almost 16 days. The summer in the northern hemisphere consists of 190$\frac{1}{2}$ days, while in the southern it consists only of 174$\frac{1}{2}$. They are to one another nearly in the proportion of 14 to 12.8; and the heat of the two hemispheres may probably have nearly the same proportion to one another. The internal limit of the trade-winds ought to be that parallel where the mean heat of the globe is greatest; this would be the equator, if both hemispheres were equally hot; but since the northern hemisphere is the hottest, that parallel ought to be situated somewhere in it; and since the difference between the heat of the two hemispheres is not great, the parallel ought not to be so far distant from the equator.
The trade-wind would blow regularly round the whole globe if the torrid zone were all covered with water. If the Indian ocean were not bounded by land on the north, it would blow there in the same manner as it does in the Atlantic and Pacific oceans. The rays of light pass through a transparent body without communicating any, or at least but a small degree of heat. If a piece of wood be inclosed in a glass vessel, and the focus of a burning-glass directed upon it, the wood will burn to ashes, while the glass through which all the rays passed is not even heated. When an opaque body is exposed to the sun's rays, it is heated in proportion to its opacity. If the bulb of a thermometer be exposed to the sun, the mercury will not rise so high as it would do if this bulb were painted black. Land is much more opaque than water; it becomes therefore much warmer when both are equally exposed to the influence of the sun. For this reason, when the sun approaches the tropic of Cancer, India, China, and the adjacent countries, become much hotter than the ocean which washes their southern coasts. The air over them becomes rarefied, and ascends, while colder air rushes in from the Indian ocean to supply its place. As this current of air moves from the equator northward, it must, for a reason already explained, assume the appearance of a south-west wind; and this tendency eastward is increased by the situation of the countries to which it flows. This is the cause of the south-west monsoon, which blows during summer in the northern parts of the Indian ocean. Between Borneo and the coast of China, its direction is almost due north, because... cause the country to which the current is directed lies rather to the west of north; a circumstance which counteracts its greater velocity.
In winter, when the sun is on the south side of the equator, these countries become cool, and the north-east trade-wind resumes its course, which, had it not been for the interference of these countries, would have continued the whole year.
As the sun approaches the tropic of Capricorn, it becomes almost perpendicular to New Holland; that continent is heated in its turn, the air over it is rarified, and colder air rushes in from the north and west to supply its place. This is the cause of the north-west monsoon, which blows from October to April, from 3° to 15° south latitude. Near Sumatra its direction is regulated by the coast; this is the case also between Africa and Madagascar.
The same cause which occasions the monsoons, gives rise to the winds which blow on the west coasts of Africa and America. The air above the land is hotter and rarer, and consequently lighter than the air above the sea; the sea air, therefore, flows in, and forces the lighter land atmosphere to ascend.
The same thing will account for the phenomena of the sea and land breezes. During the day, the cool air of the sea, loaded with vapors, flows in upon the land, and takes the place of the rarefied land air. As the sun declines, the rarefaction of the land air is diminished; thus an equilibrium is restored. As the sea is not so much heated during the day as the land, neither is it so much cooled during the night, because it is constantly exposing a new surface to the atmosphere. As the night approaches, therefore, the cooler and denser air of the hills (for where there are no hills there are no sea and land breezes) falls down upon the plains, and preluding upon the now comparatively lighter air of the sea, causes the land breeze.
The rarefied air which ascends between 2° and 5° north latitude, has been shewn to be the principal cause of the trade-winds. As this air ascends, it must become gradually colder, and consequently heavier; it would therefore descend again if it were not buoyed up by the constant ascent of new rarefied air. It must therefore spread itself to the north and south, and gradually mix in its passage with the lower air; and the greater part of it probably does not reach far beyond 30°, which is the external limit of the trade-wind. Thus there is a constant circulation of the atmosphere in the torrid zone; it ascends near the equator, diffuses itself toward the north and south, descends gradually as it approaches 30°, and, returning again towards the equator, performs the same circuit. It has been the opinion of the greater part of those who have considered this subject, that the whole of the rarefied air which ascends near the equator, advances towards the poles and descends there. But if this were the case, a constant wind would blow from both poles towards the equator, and the trade winds would extend over the whole earth; for otherwise the ascent of air in the torrid zone would very soon cease. A little reflection must convince us that it cannot be true. Rarefied air differs in nothing from the common air, except in containing a greater quantity of heat. As it ascends, it gradually loses this superfluous heat. What then should hinder it from descending, and mixing with the atmosphere below?
That there is a constant current of superior air, however, towards the poles, cannot be doubted; but it consists principally of hydrogen gas. We shall immediately attempt to assign the reason why its accumulation at the pole is not always attended with a north wind.
If the attraction of the moon and the diurnal motion of the sun have any effect upon the atmosphere, and that they have some effect can hardly be disputed, there must be a real motion of the air westwards within the limits of the trade-winds. When this body of air reaches America, its further passage westwards is stopped by the mountains which extend from one extremity of that continent to the other. From the momentum of this air, when it strikes against the sides of these mountains, and from its elasticity, it must acquire from them a considerable velocity, in a direction contrary to the first, and would therefore return eastwards again if this were not prevented by the trade-winds. It must therefore rush forwards in that direction where it meets with the least resistance; that is, towards the north and south. As air is nearly a perfectly elastic body, when it strikes against the sides of the American mountains, its velocity will not be perceptibly diminished, though its direction be changed. Continuing to move, therefore, with the velocity of the equator, when it arrives at the temperate zones it will assume the appearance of a north-east or south-east wind. To this is to be ascribed the frequency of south-west winds over the Atlantic ocean and western parts of Europe. Whether these winds are equally frequent in the northern Pacific ocean, we have not been able to ascertain; but it is probable that the mountains in Asia produce the same effect as those in America.
It is not impossible that another circumstance may also contribute to the production of these winds. The oxygen, which is rather heavier than common air, may mix with the atmosphere; but the hydrogen (a cubic foot of which weighs only 41.41 grains, while a cubic foot of oxygen weighs 59.332 grains) may ascend to the higher regions of the atmosphere.
By what means the decomposition is accomplished (if it takes place at all) we cannot tell. There are probably a thousand causes in nature of which we are entirely ignorant. Whether heat and light, when long applied to vapors, may not be able to decompose them, by uniting with the hydrogen, which seems to have a greater attraction for heat than oxygen has, or whether the electrical fluid may not be capable of producing this effect, are questions which future observations and experiments must determine. Dr Franklin filled a glass tube with water, and passed an electrical shock through it; the tube was broken in pieces, and the whole water disappeared. He repeated the experiment with ink instead of water, and placed the tube upon white paper; the same effects followed, and the ink, though it disappeared completely, left no stain on the paper. Whether the water in these cases was decomposed or not, it is impossible to say; but the supposition that it was, is not improbable. An experiment might easily be contrived to determine the point.
This decomposition would account for the frequency of south-west winds, particularly in summer; for this new air is furnished to supply the place of that which is forced northwards by the causes already explained. Perhaps it may be a confirmation of this conjecture, that the south-west winds generally extend over a greater tract of country than most other winds which blow in the temperate zones. What has been said of south-west winds holds equally with regard to north-west winds in the south temperate zone.
After south-west winds have blown for some time, a great quantity of air will be accumulated at the pole, at least if they extend over all the northern hemisphere; and it appears, from comparing the tables kept by some of our late navigators in the northern Pacific ocean with similar tables kept in this island, that this is sometimes the case so far as relates to the Atlantic and Pacific oceans. When this accumulation becomes great, it must, from the nature of fluids, and from the elasticity of the air, press with a considerable and increasing force on the advancing air; so that in time it becomes stronger than the south-west wind. This will occasion at first a calm, and afterwards a north wind, which will become gradually easterly as it advances southwards, from its not affuming immediately the velocity of the earth. The mass of the atmosphere will be increased in all those places over which this north-east wind blows; this is confirmed by the almost constant rise of the barometer during a north-east wind.
Whatever tends to increase the bulk of the atmosphere near the pole, must tend also to increase the frequency of north-east winds; and if there be any reason when this increase takes place more particularly, that reason will be most liable to these winds. During winter the northern parts of Europe are covered with snow, which is melted in the beginning of summer, when the heat of the sun becomes more powerful. Great quantities of vapour are during that time raised, which will augment both the bulk and weight of the atmosphere, especially if the conjecture about the conversion of vapour into air has any foundation. Hence north-east winds are most prevalent during May and June.
But it will be said, if this hypothesis were true, the south-west and north-east winds ought to blow alternately, and continue each of them for a stated time; whereas the south-west wind blows sometimes longer and sometimes shorter, neither is it always followed by a north-east wind.
If the conjecture about the decomposition of vapour in the torrid zone be true, the hydrogen which formed a part of it will ascend from its lightness, and form a stratum above the atmospheric air, and gradually extend itself, as additional hydrogen rises, towards the north and south, till at last it reaches the poles. The lightness of hydrogen is owing to the great quantity of heat which it contains; as it approaches the poles it must lose a great part of this heat, and may in consequence become heavy enough to mix with the atmosphere below. Oxygen makes a part of the atmosphere; and its proportion near the poles may sometimes be greater than ordinary, on account of the additional quantity brought thither from the torrid zone. Mr Cavendish mixed oxygen and hydrogen together in a glass jar; and upon making an electrical spark pass through them, they immediately combined and formed water.
That there is electric matter at the poles, cannot be doubted. The abbe Chappe informs us, that he saw thunder and lightning much more frequently at Tobolsk and other parts of Siberia, than in any other part of the world. In the north of Europe, the air, during very cold weather, is exceedingly electric; sparks can be drawn from a person's hands and face, by combing his hair, or even powdering him with a puff. Aepinus was an eye-witness to this fact, and to still more astonishing proofs of the electricity of the atmosphere during great colds.
May not the appearance of the aurora borealis be owing to the union of oxygen and hydrogen by the intervention of the electric fluid? That it is an electrical phenomenon, at least, can hardly be doubted. Artificial electricity is much strengthened during an aurora, as Mr Volta and Mr Canton have observed; and the magnetic needle moves with the same irregularity during an aurora that has been observed in other electrical phenomena. This fact we learn from Bergman and De la Lande. Many philosophers have attempted to demonstrate, that aurora borealis are beyond the earth's atmosphere; but the very different results of their calculations evidently prove that they were not possessed of sufficient data.
If this conjecture be true, part of the atmosphere near the poles must at times be converted into water. This would account for the long continuance of south-west winds at particular times; when they do so, a decomposition of the atmosphere is going on at the pole. It would render this conjecture more probable, if the barometer fell always when a south-west wind continues long.
If this hypothesis be true, a south-west wind ought always to blow after aurora borealis; and we are in winds very formed by Mr Winn, that this is actually the case, common after aurora borealis. This he found never to fail in 23 instances. He observed also, that when the aurora was bright, the gale came on within 24 hours, but did not last long; but if it was faint and dull, the gale was longer in beginning, and less violent, but it continued longer. This looks like a confirmation of our conjecture. Bright auroras are probably nearer than those which are dull. Now, if the aurora borealis be attended with a decomposition of a quantity of air, that part of the atmosphere which is nearest must first rush in to supply the distant parts. Just as if a hole were bored in the end of a long vessel filled with water, the water nearest the hole would flow out immediately, and it would be some time before the water at the other end of the vessel began to move. The nearer we are to the place of precipitation, the sooner will we feel the south-west wind. It ought therefore to begin sooner after a bright aurora, because it is nearer than a dull and faint one. Precipitations of the atmosphere at a distance from the pole cannot be so great as those which take place near it; because the cold will not be sufficient to condense so great a quantity of hydrogen; south-west winds, therefore, ought not to last so long after bright as after dull auroras. Winds are more violent after bright auroras, because they are nearer the place of precipitation; just as the water near the hole of the vessel runs swifter than that which is at a considerable distance.
If these conjectures have any foundation in nature, there are two sources of south-west winds; the first has causes of its origin in the trade-winds, the second in precipitations of the atmosphere near the pole. When they originate from the first cause, they will blow in countries farther farther south for some time before they are felt in those which are farther north; but the contrary will take place when they are owing to the second cause. In this last case, too, the barometer will sink considerably; and it actually does so constantly after aurora, as we are informed by Mr Madison, who paid particular attention to this subject. By keeping accurate meteorological tables in different latitudes, it might easily be discovered whether these consequences be true, and of course whether the above conjectures be well or ill grounded.
It appears that winds generally commence at that point towards which they blow; and hence they must arise from a rarefaction and consequent displacement of the air in some particular place, by the action of heat, or which they come other cause. Perhaps, according to the idea of Mr Williams, this cause may be an increased precipitation of the superior strata of air, rendered unusually dense from its being furnished with moisture in the place where the wind begins to blow, or from an increased evaporation from a humid surface in the opposite direction.
Hurricanes are constantly preceded by a great depression of the thermometer; and in these cases the wind often seems to blow from every direction towards the quarter where this fall of the barometer is observed.
Violent winds from the north-east have repeatedly been observed to begin at the quarter towards which they blow. In 1749 Dr Franklin was prevented from observing an eclipse of the moon at Philadelphia by a north-east storm, which came about seven o'clock in the evening. He was surprised to find afterwards that it had not come on at Boston till near 11 o'clock; and, upon comparing all the accounts which he received from the several colonies of the beginning of this and other storms of the same kind, he found it to be always an hour later the farther north-east, for every 100 miles. "From hence (says he) I formed an idea of the course of the storm, which I will explain by a familiar instance. Suppose a long canal of water stopped at the end by a gate. The water is at rest till the gate is opened; then it begins to move out through the gate, and the water next the gate is first in motion, and moves on towards the gate, and so on successively, till the water at the head of the canal is in motion, which it is last of all. In this case the water moves indeed towards the gate; but the successive times of beginning the motion are in the contrary way, viz., from the gate back to the head of the canal. Thus to produce a north-east storm, I suppose some great rarefaction of the air in or near the gulf of Mexico; the air rising thence has its place supplied by the next more northern, cooler, and therefore denser and heavier air; a successive current is formed, to which our coast and inland mountains give a north-east direction."
Several instances of a similar kind have occurred. In 1822, Dr Mitchell observed a storm which began at Charlestown on the 21st of February, at two o'clock P.M., but was not observed at Washington, several hundred miles to the north-east, till five o'clock; at New York till 10, nor at Albany till daybreak of the following morning. Hence it appears that it must have moved at the rate of 1100 miles in 11 hours, or 100 miles an hour.
A remarkable storm of this kind, in which the wind was easterly, and attended with a heavy fall of snow, was observed in Scotland on the 8th of February 1799; but the motion of the wind was much slower. It began to snow at Falkirk on the 7th of February at six in the evening, but at Edinburgh not till one o'clock A.M. on the 8th; and the snow was not observed at Dunbar till seven hours after. The storm continued 11 hours, during which time it did not travel more than 100 miles.
Currents of air from the poles naturally assume a north-east direction as they advance southwards, because their diurnal motion becomes less than that of the earth. Various circumstances, however, may change this direction, and cause them to become north, or even north-west winds. The south-west winds themselves may often prove sufficient for this; and violent rains, or great heat, by lessening or rarefying the atmosphere in any country, will produce the same effect in countries to the westwards, when north winds happen to be blowing.
In North America, the north-west winds become gradually more frequent as we advance northwards. The east coast of this continent, where the observations were made from which this conclusion was drawn, is alone cultivated; the rest of the country is covered with wood. Now cultivated countries are generally considered as warmer than those which are uncultivated, though Mr Williams is of a different opinion; and on this circumstance founded his hypothesis of the climate of Britain being much deteriorated during the last 50 years. The air, therefore, in the interior parts of the country should be constantly colder than the east coast. This difference will scarcely be perceptible in the southern parts, because there the influence of the sun is very powerful; but it will become gradually greater as we advance northwards, because the influence of the sun diminishes, and the continent becomes broader. Hence north-west winds ought to become more frequent upon the east coast as we advance northwards; and they will probably cease to blow so often as soon as the whole continent of North America becomes cultivated.
There is one curious circumstance which deserves attention. One current of air is often observed to blow at currents of the surface of the earth, while a current in the contrary ten appear direction is flowing in a superior part of the atmosphere, sphere at Dr Thomson on one occasion observed three currents the same of this kind blowing all at the same time in contrary directions. It has been affirmed that changes of weather commonly commence in the upper strata, and that they are gradually extended by the current of air that commences above, proceeding towards the lower parts of the atmosphere.
Besides these more general winds, there are others Partial which extend only over a very small part of the earth, winds. These originate from many different causes. The atmosphere is principally composed of three different kinds of air, oxygen, azote, and carbonic acid, to which may be added water. Great quantities of each of these ingredients are constantly changing their aerial form, and combining with various substances; or they are separating from other bodies, assuming the form of air, and mixing with the atmosphere. Partial deficiencies, therefore, and partial accumulations, must be continu- ally taking place in different parts of the atmosphere, which will occasion winds varying in direction, violence, and continuance, according to the suddenness and the quantity of air destroyed or produced. Besides these, there are many other ingredients constantly mixing with the atmosphere, and many partial causes of condensation and rarefaction in particular places. To these, and probably to other causes hitherto unknown, are to be ascribed all those winds which blow in any place besides the general ones already explained; and which, as they depend on causes hitherto at least reckoned contingent, will probably for ever prevent uniformity and regularity in the winds. All these causes, however, may, and probably will, be discovered: the circumstances in which they will take place, and the effects they will produce, may be known; and whenever this is the case, the winds of any place may in some measure be reduced to calculation.
**CHAP. V. Of Meteors.**
The principal luminous phenomena denominated meteors, have been fully considered under Atmospheric Electricity. Those meteors that burst in the air, and are followed by the falling of stones or other mineral substances, have been fully described and accounted for under Meteorolite. We have here only to notice briefly the meteors called falling stars, and ignis fatuus.
The falling or shooting star is a very common phenomenon, and takes place more especially at those seasons and in those situations where the aurora borealis is most frequently observed. Indeed they are considered by most philosophers as modifications of the same phenomenon, and depending on the same cause. We have good reason to conclude that the aurora borealis is an electrical meteor; and if the falling star is so nearly allied to the aurora as is supposed, it must also be produced by electricity. Mr G. Morgan seems to have no doubt of the electrical nature of this meteor, and remarks that if what appears as an undulating flash in the aurora, could be concentrated, or confined within smaller dimensions, it would probably assume the appearance of a falling star. He founds this opinion chiefly on the following experiment.
Into a tube 48 inches long, and \( \frac{1}{4} \) inch diameter, Mr Morgan conveyed as much air, as under the common pressure of the atmosphere, would fill two inches in length of the same tube. (The tube we presume was previously exhausted of air.) One extremity of the tube he connected with the ground by means of good conductors, and fastened to the other a metallic ball. Through the tube thus filled with rarefied air, he sent electric sparks of different magnitudes, by bringing the ball within the striking distance of different fixed conductors. When the sparks were small, a flash like that of the aurora borealis, seemed to fill the whole tube; but when the spark was what might be made to strike through 10 inches in the open air, it appeared to strike through the whole length of the tube, with all the brilliancy and straightness of a falling star. If, however, he extracted part of the air out of the tube, by the air-pump, he could never make the electric fluid assume any form excepting that of a flash; but by exchanging the tube for another with a thermometrical ball, and treating it in the same manner as the preceding, the flash never appeared, but the fluid in its passage assumed all the brilliancy of a falling star.
It is easy to trace the similarity of circumstances that take place in this experiment, and in the natural phenomenon of the falling star. Both take place in rarefied air; both are remarkable for the brightness of their light, and for the straightness of their direction. That falling stars are frequently, if not always, the concentration of an aurora borealis, may be inferred from their being the constant attendants of a very electrical state of the atmosphere; and from their frequent appearance near that portion of the heavens which is illumined by the northern lights at the time of their appearance.
Mr Morgan was riding towards Norwich late at night, when to the north east of the town he beheld a fine conical stream of the aurora borealis. The whole body every now and then flashed, as if an additional quantity of electric fluid were thrown into it; and nearly at the same instant he perceived what is vulgarly called a falling star, darting from its summit. This appearance he observed twice successively.
The ignis fatuus, or will-with-the-wisp, that appears so often in boggy, marshy and damp situations, thus decoying the unwary traveller, and terrifying the superstitious vulgar, seems to be rather of a phosphoric than an electric nature, similar to the light which is emitted by stale fish, rotten wood, and other putrefactive substances. Sir Isaac Newton defined it to be a vapour shining without heat.
A remarkable ignis fatuus was observed by Mr Derham, in some boggy ground, between two rocky hills. He was so fortunate as to be able to approach it within two or three yards. It moved with a brisk and delusive motion about a dead thistle, till a slight agitation of the air, occasioned, as he supposed, by his near approach to it, occasioned it to jump to another place; and as he approached, it kept flying before him. He was near enough to satisfy himself, that it could not be the shining of glow-worms or other insects—it was one uniform body of light.
M. Beccaria mentions two of these luminous appearances, which were frequently observed in the neighbourhood of Bologna, and which emitted a light equal to that of an ordinary faggot. Their motions were unequal, sometimes rising, and sometimes sinking towards the earth; sometimes totally disappearing, though in general they continued hovering about six feet from the ground. They differed in size and figure; and indeed, the form of each was fluctuating, sometimes floating like waves, and dropping sparks of fire. He was assured there was not a dark night in the whole year in which they did not appear; nor was their appearance at all affected by the weather, whether cold or hot, snow or rain. They have been known to change their colour from red to yellow; and generally grew fainter as any person approached, vanishing entirely when the observer came very near to them, and appearing again at some distance.
Dr Shaw also describes a singular ignis fatuus, which he saw in the Holy Land. It was sometimes globular, or in the form of the flame of a candle; and immediately afterwards spread itself so much, as to involve the whole company in a pale insensible light, Weather, and then was observed to contract itself again, and suddenly disappear. In less than a minute, however, it would become visible as before, and run along from one place to another; or would expand itself over more than three acres of the adjacent mountains. The atmosphere at this time was thick and hazy.
All these luminous appearances are probably owing to the extrication of hydrogen gas so slightly impregnated with phosphorus as to continue emitting a faint light, without producing that brilliant flash which follows the sudden extrication into the air, of the common phosphorated hydrogen gas obtained in the usual chemical experiment of throwing phosphuret of lime into water.
**Chap. VI. Of the Application of Meteorology to Prognosticating the Weather.**
It has ever been a principal object among mankind, to foretell the changes of weather that are likely to follow particular appearances in the sky, among the heavenly bodies, &c.; and it has been often alleged, that in this respect the philosopher is far behind the husbandman and the shepherd. Were the former, however, to add to his scientific researches the observations to which the latter are indebted for their judgement of the weather, he would soon be far superior to them in this respect.
Dr Kirwan has lately endeavoured to discover probable rules for prognosticating the weather in different seasons, as far as regards this climate, from tables of observation alone; and from comparing a number of these observations made in England, from 1677 to 1789, he found,
1. That when there has been no storm before or after the vernal equinox, the ensuing summer is generally dry, at least five times in six.
2. That when a storm happens from an easterly point, either on the 19th, 20th, or 21st of May, the succeeding summer is generally dry four times in five.
3. That when a storm arises on the 26th, 27th, or 29th of May (and not before), in any point, the succeeding summer is generally dry four times in five.
4. If there be a storm at south-west or west-south-west on the 19th, 20th, 21st, or 22d of March, the succeeding summer is generally wet five times in six.
In this country winters and springs, if dry, are most commonly cold; if moist, warm; on the contrary, dry summers and autumns are usually hot, and moist summers cold. So that if we know the moistness or dryness of a season, we can judge pretty accurately of its temperature.
From a table of the weather kept by Dr Rutty, in Dublin, for 41 years, Dr Kirwan endeavoured to calculate the probabilities of particular seasons being followed by others. Though his rules relate chiefly to the climate of Ireland, yet as probably there is not much difference between that island and Britain, in the general appearance of the seasons, we shall mention his conclusions here.
In 41 years there were six wet springs, 22 dry, and 13 variable; 20 wet summers, 16 dry, and five variable; 11 wet autumns, 11 dry, and 19 variable. A season according to Dr Kirwan, is counted wet, when it contains two wet months. In general, the quantity of rain which falls in dry seasons is less than five inches; in wet seasons more. Variable seasons are those in which there falls between 30 and 36 pounds, a pound being equal to .157637 of an inch.
The order in which the different seasons succeeded each other, was as in the following table:
| Season | Times | Probability | |-------------------------|-------|-------------| | A dry spring | | | | A wet spring | | | | A variable spring | | | | A dry summer | | | | A wet summer | | | | A variable summer | | | | A dry spring and dry summer | | | | A dry spring and wet summer | | | | A wet spring and dry summer | | | | A wet spring and wet summer | | | | A wet spring and variable summer | | | | A dry spring and variable summer | | | | A variable spring and dry summer | | | | A variable spring and wet summer | | | | A variable spring and variable summer | | |
Hence Dr Kirwan deduced the probability of the Rules for kind of seasons which would follow others. This probability is expressed in the last column of the table, and is to be understood in this manner. The probability that... Weather. that a dry summer will follow a dry spring is $\frac{4}{5}$; that a wet summer will follow a dry spring, $\frac{3}{4}$; that a variable summer will follow a dry spring, $\frac{2}{3}$, and so on.
This method of Dr Kirwan, if there is such a connection between the different seasons that a particular kind of weather in one has a tendency to produce a particular kind of weather in the next, as it is reasonable to expect from theory, may in time, by multiplying observations, come to a great degree of accuracy, and may at last, perhaps, lead to that great desideratum, a rational theory of the weather. As we wish to throw as much light as possible on this important subject, we shall add to these a few maxims, the truth of which has either been confirmed by long observation, or which the knowledge we have already acquired of the causes of the weather has established on tolerably good grounds.
1. A moist autumn with a mild winter is generally followed by a cold and dry spring, which greatly retards vegetation. Such was the year 1741.
2. If the summer be remarkably rainy, it is probable that the ensuing winter will be severe; for the unusual evaporation will have carried off the heat of the earth. Wet summers are generally attended with an unusual quantity of seed on the white thorn and dog-rose bushes. Hence the unusual fruitfulness of these shrubs is a sign of a severe winter.
3. The appearance of cranes and birds of passage early in autumn announces a very severe winter; for it is a sign it has already begun in the northern countries.
4. When it rains plentifully in May, it will rain but little in September, and vice versa.
5. When the wind is south-west during summer or autumn, and the temperature of the air unusually cold for the season, both to the feeling and the thermometer, with a low barometer, much rain is to be expected.
6. Violent temperatures, as storms or great rains, produce a sort of crisis in the atmosphere, which produces a constant temperature, good or bad, for some months.
7. A rainy winter predicts a sterl year: a severe autumn announces a windy winter.
To the above we shall add the following maxims, drawn from observation, and with these shall conclude this article.—Sea and fresh water-fowls, such as cormorants, sea-gulls, snipe-heron, &c. flying from sea, or the fresh waters, to land, show bad weather at hand: land fowls flying to waters, and these shaking, washing, and noisy, especially in the evening, denote the same; geese, ducks, cats, &c. picking, shaking, washing, and noisy; rooks and crows in flocks, and suddenly disappearing; pyes and jays in flocks, and very noisy; the raven or hooded-crow crying in the morning, with an interruption in their notes, or crows being very clamorous at even; the heron, bittern, and swallow lying low; birds forsaking their meat and flying to their nests; poultry going to roost, or pigeons to their dove-house; tame fowls grubbing in the dust, and clapping their wings; small birds seeming to duck and wash in the sand; the late and early crowing of the cock, and clapping his wings; the early ringing of wood-larks; the early chirping of sparrows; the early note of the chaffinch near houses; the dull appearance of robin-redbreast near houses; peacocks and owls unusually clamorous.
Sea and fresh-water fowls gathering in flocks to the banks, and there sporting, especially in the morning; birds wild-geese flying high, and in flocks, and directing their course eastward; coots restless and clamorous; the hoopoe loud in his note; the kingfisher taking to land; rooks darting or shooting in the air, or sporting on the banks of fresh waters; and lastly, the appearance of the malefigie at sea, is a certain forerunner of violent winds, and (early in the morning) denotes horrible tempests at hand.
Halcyons, sea-ducks, &c. leaving the land and flocking to the sea; kites, herons, bitterns, and swallows flying high and loud in their notes; lapwings restless and clamorous; sparrows after sunrise restless and noisy; ravens, hawks, and kestrels (in the morning), loud in their notes; robin-redbreast mounted high, and loud in his song; larks soaring high, and loud in their songs; owls hunting with an easy and clear note; bats appearing early in the evening.
Asses braying more frequently than usual; hogs playing, scattering their food, or carrying straw in their mouths; oxen sniffing the air, looking to the south, while lying on their sides, or licking their hoofs; cattle galloping for air at noon; calves running violently and gamboling; deer, sheep, or goats, leaping, fighting, or pushing; cats washing their face and ears; dogs eagerly scraping up earth; foxes barking, or wolves howling; moles throwing up earth more than usual; rats and mice more restless than usual; a grumbling noise in the belly of hounds.
Worms crawling out of the earth in great abundance; spiders falling from their webs; flies dull and restless; ants hastening to their nests; bees hastening home, and keeping close in their hives; frogs and toads drawing nigh to houses; frogs croaking from ditches; toads crying on eminences; gnats fingering more than usual; but, if gnats play in the open air, or if hornets, wasps, and glow-worms appear plentifully in the evening, or if spiders webs are seen in the air, or on the grass, or trees, these do all denote fair and warm weather at hand.
Sun rising dim or watery; rising red with blackish beams mixed along with his rays; rising in a muzzy or muddy colour; rising red and turning blackish; setting under a thick cloud; setting with a red sky in the east.
N.B. Sudden rains never last long; but when the air grows thick by degrees, and the sun, moon, and stars shine dimmer and dimmer, then it is like to rain six hours usually.
Sun rising pale and setting red, with an iris; rising large in surface; rising with a red sky in the north; setting of a bloody colour; setting pale, with one or more dark circles, or accompanied with red streaks; seeming concave or hollow; seeming divided, great storms; parhelia, or mock suns, never appear, but are followed by tempests.
Sun rising clear, having set clear the night before; rising while the clouds about him are driving to the west; rising with an iris around him; and that iris wearing away equally on all sides, then expect fair and settled weather; rising clear and not hot; setting in red clouds, according to the old observation:
The The evening red and morning gray, Is the sure sign of a fair day.
Moon pale in colour, rain; horns blunt at first rising, rain; horns blunt, at or within two or three days after the change, denotes rain for that quarter; an iris with a south wind, rain next day; wind south third night after change, rain next day; the wind south, and the moon not seen before the fourth night, rain most of that month; full moon in April, new and full moon in August, for most part bring rain; mock moons are the forerunners of great rains, land floods, and inundations.
Moon seeming greatly enlarged; appearing of a red colour; horns sharp and blackish; if included with a clear and ruddy iris; if the iris be double or seem to be broken in parts, tempests.
N.B. On the new moon, the wind for the most part changes.
When the moon, at four days old, has her horns sharp, she foretells a tempest at sea, unless she has a circle about her, and that too entire, because, by that she shews that it is not like to be bad weather, till it is full moon.
Moon seeming to exhibit bright spots; a clear iris with full moon; horns sharp fourth day, fair till full; horns blunt at first rising, or within two or three days after change, denotes rain for that quarter; but fair weather the other three quarters; moon clear three days after change and before full, always denotes fair weather. After every change and full, rains for the most part, succeeded by fair settled weather; moon clear and bright, always fair weather.
Stars seeming large, dull, and pale of colour, rain; or when their twinkling is not perceptible, or if encompassed within it is. In summer, when the wind is at east, and stars seem greater than usual, then expect sudden rain; stars appearing great in number, yet clear and bright, seeming to shoot or dart, denote fair weather in summer, and in winter frosts.
In cloudy weather, when the wind falls, rain follows; clouds growing bigger, or seeming like rocks or towers settling on tops of mountains; coming from the south, or often changing their course; many in number at north-west in the even; being black in colour from the east, rain at night; but out of the west, rain next day; being like fleece of wool, from the east, rain for two or three days; lying like ridges about mid day in the south-west, thaws great storms both of wind and rain to be nigh. Clouds flying to and fro; appearing suddenly from the south or west; appearing red, or accompanied with redness in the air, especially in the morning; being of a leadish colour in the north-west; single clouds denote wind from whence they come; but if at sunset, clouds appear with golden edges, or diminish in bulk, or small clouds sink low, or draw against the wind, or appear small, white, and scattered in the north-west (such as are vulgarly called mackerel) when the sun is high, there are signs of fair weather.
N.B. It is often observed, that though the mackerel sky denotes fair weather for that day, yet for the most part, rain follows in a day or two after.
After a long drought, the rainbow denotes sudden and heavy rains; if green be the predominant colour, it denotes rain, but if red, wind with rain; if the clouds grow darker, rain; if the bow seems broken, violent storms; if appearing at noon, much rain; if in the west great rain, with thunder.
N.B. It is observed, that if the last week in February, and the first fortnight of March, be mostly rainy, and attended with frequent appearances of the bow, a wet spring and summer may be expected.
The rainbow appearing after rains, denotes fair weather at hand, if the colours grow lighter, fair; if the bow suddenly disappears, fair; if the bow appears in the morning, it is the sign of small rains, followed by fair weather; and if appearing at night, fair weather; if appearing in the east in the evening, fair; if the bow appear double, it denotes fair weather at present but rain in a few days; if in autumn, it continues fair for two days after the appearance of the aurora borealis, expect fair weather for at least eight days more.
If mists be attracted to the tops of hills then expect rain from rain in a day or two; if, in dry weather, they be observed to ascend more than usual, then expect sudden rain; mists in the new moon foretell rain in the old; mists also in the old moon denote rain to happen in the new; a milky white scare, in a clear sky in the south-east, is always a forerunner of rain.
If mists dissipate quickly, or descend after rain, it is a sure sign of fair weather; a general mist before sunrise near the full moon, denotes fair weather for about a fortnight running. If after sunset or before sunrise, a white mist arise from the waters and meads, it denotes warm and fair weather next day. A milky dew on the inside of glass windows thaws fair weather for that day.
Wood swelling, or stones seeming to sweat; lute or rain from violent strings breaking; printed canvas or panted maps animate relaxing; salt becoming moist; rivers linking, or floods bodies, suddenly abating; remarkable halo about the candle; great dryness of the earth; pools seeming troubled or muddy; yellow fum on the surface of stagnant waters; dandelion or pimpinella shutting up; trefoil dwelling in stalk, while the leaves bow down.
N.B. A dry spring is always attended with a rainy winter.
Wind shifting to the opposite point; sea calm, with a wind from murmuring noise; a murmuring noise from the woods; and rocks when the air is calm; leaves and feathers seeming much agitated; tides high when the thermometer is high; trembling or flexuous burning of flames; coal burning white with a murmuring noise; thunder in the morning with a clear sky; thunder from the north.
N.B. Whosoever the wind begins to shift, it will not rest till it come to the opposite point; and if the wind be in the north, it will be cold; if in the north-east colder; if in the south; it brings rain; but if in the south-west more rain.
The sudden cloathing of gaps in the earth; the remarkable rising of springs or rivers; if the rain begins an hour or two before sunrise it is like to be fair after sunrise; but if an hour or two after sunrise; it for the most part happens to continue all day and then to cease; when it begins to rain from the south with a high wind for two or three hours, and that the wind falls, and it still continues raining, it is then like to continue for 12 hours or more, and then to cease.
N.B. N. B. These long rains seldom hold above 24 hours, or happen above once a year.
A hasty shower after raging winds is a sure sign of the storm being near an end. If the water ripples and frequent bubbles arise, or if the halcyon or kingfisher attempts the sea while the storm lasts, or moles come out of their holes, or sparrows chirp merrily, these are all certain signs of the storm ceasing.
Both sea and fresh water fishes by their frequent rising and fluttering on the surface of the water, foretell the storm nigh over, but especially dolphins spouting up water in a storm foretell a calm.
N. B. Let the wind be in what quarter it will, upon the new moon, it presently changes.
Clouds white, inclining to yellow, and moving heavily though the wind be high, is a sure sign of hail; if the eastern sky before sunrise be pale, and refracted rays appear in thick clouds, then expect great storms of hail; white clouds in summer are a sign of hail, but in winter they denote snow, especially when we perceive the air to be a little warm; in spring or winter, when clouds appear of blueish white, and expand much, expect small hail or drizzling, which properly is no other than frozen mists.
Meteors shooting in the summer's evening, or chops and clefts in the earth, when the weather is sultry, always foretel thunder is nigh; in summer or harvest, when the wind has been south two or three days, and the thermometer high, and clouds rise with great white tops like towers, as if one were upon the top of another, and joined with black on the nether side, expect rain and thunder suddenly; if two such clouds arise, one on either hand, it is then time to look for shelter, as the thunder is very nigh.
N. B. It is observed that it thunders most with a south wind and least with an east.
Sea-pyes, starlings, fieldfares, with other migratory birds, appearing early, denote a cold season to ensue; the early appearance of small birds in flocks, and of robin-redbreasts near houses; sun in harvest setting in a mist or broader than usual; moon bright, with sharp horns, after change; wind shifting to the east or north after change; sky full of twinkling stars; small clouds hovering low in the north; snow falling small, while clouds appear on heaps like rocks.
N. B. Frosts in autumn are always succeeded with rain.
Snow falling in large flakes while the wind is at south; cracks appearing in the ice; sun looking watery; the moon's horns blunted; stars looking dull; wind turning to the south; wind extremely shifting. It is also observed, that, if October and November be frost and snow, January and February are like to be open and mild.
Fair weather for a week together, while the wind is all that time in the south, is, for the most part, followed by a great drought; if February be for most rainy, signs of spring and summer quarters are like to be too; but drought, if it happen to be altogether fair, then expect a drought to follow; if lightning follow after 24 hours of dry and fair weather, drought will follow, but if within 24 hours, expect great rains.
A moist and cold summer, and mild autumn, are signs of sure signs of a hard and severe winter; store of hips, hard wind, and haws denote the same; the hazel-tree flowering, is ever observed to foretel the same; acorns found without any insect is a sure prognostic of a hard winter.
A dry and cold winter with a southerly wind; a sign of very rainy spring, sickness in summer; if summer be plentiful, dry with the wind northerly, great sickness is likely to follow; great heats in spring time without winds; roots having a lucious taste, while the wind has been long southerly without rain; and lastly, great quantities of flinting atoms, insects or animals, as flies, frogs, snakes, locusts, &c.
Inclofe the leech worm in an eight ounce phial experiments with a bit of linen; let the water be changed once a week in summer, and once a fortnight in winter.
If the leech lies motionless at the bottom in a spiral form, fair weather; if crept to the top, rain; if restless, wind; if very restless, and without the water, thunder; if in winter at bottom, frost; but if in the winter it pitches its dwelling on the mouth of the phial, snow. See HELMINTHOLOGY (f).
In calm weather, when the air is inclined to rain, the mercury is low; but when tending to fair, it will weather from the rise; in very hot weather when falling, it foretells thunder; if rising in winter, frost; but if falling in frost, thaw; if rising in a continued frost, snow; if foul weather quickly on its falling, soon over; if fair weather quickly on its rising, soon over; also if rising high in foul weather, and so continuing for two or three days, before the foul weather is over, then expect a continuance of fair weather; but, if in fair weather the mercury fall low, and so continue for two or three days, then expect much rain, and probably high winds.
N. B. In an east wind, the mercury always rises and falls lowest before great winds.*
It was intended to insert in this article a summary view of the opinions of Toaldo, Cotte, and Lamarck, p. 149, respecting the influence of the moon in producing changes in our atmosphere; but peculiar circumstances render it necessary to postpone this view till we come to the article MOON.
*Nicholson's Journal, Feb. 1804.
INDEX. INDEX.
A.
Atmosphere, density of, least at the equator, and greatest at the poles, No 13 weight of, the same all over the globe, ib. forms two inclined planes, meeting at the equator, ib. in the northern hemisphere less inclined in our summer, and v. v., ib. August, the warmest month in the southern latitudes, 16
B.
Barometer, stands highest at the level of the sea, medium height there, 30 inches, varies very little in the torrid zone, tropical daily variation corresponds to the tides, table of the range of, range of, much less in N. America, seems to have a tendency to rise towards evening, range of, greater in winter, high in serene weather, and on the approach of easterly and northerly winds, low in calm weather, on the approach of rain, high winds, or with a southerly wind, axioms on, by Cotte, variation of, accounted for, why highest in winter in northern latitudes, whether affected by the sun and moon, C.
Capper on the winds, Clouds, always form at some height above the earth, theory of, uncertain, Congelation, perpetual term of, tables of, Cotte's writings on meteorology, p. 715 axioms on the barometer, on the thermometer, Currents of air, different, in the atmosphere at once, D.
Dalton's writings on meteorology, table of the quantity of vapour at various temperatures, p. 715 and Hoyle's experiments on evaporation, No 29.
Drought, signs of, No 108
E.
Evaporation, confined to the surface, proportional to the temperature of the air, rate of, how estimated, p. 715 goes on continually, mean annual, at Liverpool, over the globe, No 31 from land, experiments on by Dalton and Hoyle, may go on for a month together without rain, F.
Falling star probably of an electrical origin, analogous to the aurora borealis, ib.
H.
Hail, signs of, Howard's (Luke) writings on meteorology, remarks on the influence of the sun and moon on the barometer, Hygrometer, Leslie's described, January, the coldest month in all latitudes, Ignis fatuus, probably a phosphoric phenomenon, July, the warmest month in northern latitudes, K.
Kirwan's writings on meteorology, mode of calculating the mean annual and monthly temperature of the air, p. 710, note (x), and p. 711, note (x), mode of estimating the rate of diminution of the air's temperature, conclusions on the weather, L.
Lamarck's writings on meteorology, Leech, experiments with, as to its powers of prognosticating the weather, Leslie's hygrometer described, explained, Luc de, vindicated from the charge of plagiarism, p. 706, note (a)
M.
Metals, Meteorology, object of, connection of with chemistry, still in its infancy, 3
N.
Meteorology, means of improving, importance of, writers on, division of, Monsoons, direction of, Moon, effect of, on the barometer, Morgan's remarks on the falling star, R.
Rain never begins in a clear sky, theory of, uncertain, mean annual quantity of, greatest at the equator, in Great Britain, falls most in the day, proportional quantity in different months, often most frequent in winter, signs of from birds, from beasts, from insects, from the sun, from the moon, from the clouds, from a rainbow, from mists, from inanimate bodies, signs of its ceasing, S.
Saussure's writings on meteorology, Seasons, probable succession of, petilential, signs of T.
Temperature of the atmosphere tends towards a mean in all climates, mean annual greatest at the equator, No 15 table of, how calculated, p. 710, note (d), mean monthly table of p. 711 how calculated, ib. note (e), of the air diminishes as we ascend above the earth, No 17 diminishes in arithmetical progression, owing to the air's conducting power, of the north pacific ocean, of the southern hemisphere, of small seas, of North America, of islands, of open plains, of woody countries Thaw,