an instrument for measuring the degree of heat or cold in any body.

The thermometer was invented about the beginning of the 17th century; but, like many other useful inventions, it has been found impossible to ascertain to whom the honour of it belongs. Boerhaave ascribes it to Cornelius Drebbel of Alcmar, his own countryman. Fulgenzio attributes it to his master Paul Sarpi, the great oracle of the Venetian republic; and Viviani gives the honour of it to Galileo. But all these are posthumous claims. Sanctorio claims this honour to himself; and his assertion is corroborated by Berelli and Malpighi of the Florentine academy, whose partiality is not to be suspected in favour of a member of the Patavine school.

Perhaps the best way to reconcile these different claims would be, to suppose that the thermometer was really invented by different persons about the same time. We know that there are certain periods in the progress of the arts when the stream of human genius runs in the same direction, and moves towards the same object. That part of the current which reaches the object first may possess the title; but the other parts follow so rapidly and arrive so soon after, that it is impossible for a spectator to decide which is first in point of time.

The first form of this instrument for measuring the degrees of heat and cold, was the air-thermometer. It is a well known fact that air expands with heat so as to occupy more space than it does when cold, and that it is condensed by cold so as to occupy less space than when warmed, and that this expansion and condensation is greater or less according to the degree of heat or cold applied. The principle then on which the air-thermometer was constructed is very simple. The air was confined in a tube by means of some coloured liquor; the liquor rose or fell according as the air became expanded or condensed. What the first form of the tube was, cannot now perhaps be well known; but the following description of the air-thermometer will fully explain its nature.

The air-thermometer consists of a glass tube BE, fig. 1, connected at one end with a large glass ball A, and at the other end immersed in an open vessel, or terminating in a ball DE, with a narrow orifice at D; which vessel, or ball, contains some coloured liquor that will not easily freeze. Aquafortis tinged of a fine blue colour with a solution of vitriol or copper, or spirit of wine tinged with cochineal, will answer this purpose. But the ball A must be first moderately warmed so that a part of the air contained in it may be expelled through the orifice D; and then the liquor pressed by the weight of the atmosphere will enter the ball DE, and rise, for example, to the middle of the tube at C, at a mean temperature of the weather; and in this state the liquor by its weight, and the air included in the ball A, &c., by its elasticity, will counterbalance the weight of the atmosphere. As the surrounding air becomes warmer, the air in the ball and upper part of the tube, expanding by heat, will drive the liquor into the lower ball, and consequently its surface will descend; on the contrary, as the ambient air becomes colder, that in the ball is condensed, and the liquor pressed by the weight of the atmosphere will ascend; so that the liquor in the tube will ascend or descend more or less according to the state of the air contiguous to the instrument. To the tube is affixed a scale of the same length, divided upwards and downwards from the middle C into 100 equal parts, by means of which the ascent and descent of the liquor in the tube, and consequently the variations in the cold or heat of the atmosphere, may be observed.

This instrument was extremely defective; for the air in the tube was not only affected by the heat and cold of the atmosphere, but also by its weight.

The air being found improper for measuring with accuracy the variations of heat and cold according to the form of the thermometer which was first adopted, another fluid was proposed about the middle of the 17th century by the Florentine academy. This fluid was spirit of wine, or alcohol, as it is now generally named. The alcohol being coloured, was inclosed in a very fine cylindrical glass tube previously exhausted of its air, having a hollow ball at one end A, fig. 2, and hermetically sealed at the other end D. The ball and tube are filled with rectified spirit of wine to a convenient height, as to C, when the weather is of a mean temperature, which may be done by inverting the tube into a vessel of stagnant coloured spirit, under a receiver of the air-pump, or in any other way. When the thermometer is properly filled, the end D is heated red hot by a lamp, and then hermetically sealed, leaving the included air of about one-third of its natural density, to prevent the air which is in the spirit from dividing it in its expansion. To the tube is applied a scale, divided from the middle, into 100 equal parts, upwards and downwards.

As spirit of wine is capable of a very considerable degree of rarefaction and condensation by heat and cold, when the heat of the atmosphere increases the spirit dilutes, and consequently rises in the tube; and when the heat decreases, the spirit descends, and the degree or quantity of the motion is shown by a scale.

The spirit of wine thermometer was not subject to some of the inconveniences which attended the air thermometer. In particular, it was not affected by variations in the weight of the atmosphere; accordingly it soon came into general use among philosophers. It was, at an early period, introduced into Britain by Mr Boyle. To this instrument, as then used, there are, however, many objections. The liquor was of different degrees of strength, and therefore different tubes filled with it, when exposed to the same degree of heat, would not correspond. There was also another defect: The scale which was adjusted to the thermometer did not commence at any fixed point. The highest term was adjusted to the great sunshine heats of Florence, which are too variable and undetermined; and frequently the workman formed the scale according to his own fancy. While the thermometer laboured under such disadvantages it could not be of general use.

To obtain some fixed unalterable point by which a determined scale might be discovered, to which all thermometers might be accurately adjusted, was the subject which next drew the attention of philosophers. Mr Boyle, who seems at an early period to have studied this subject with much anxiety, proposed the freezing of the essential oil of aniseeds as a convenient point for graduating thermometers; but this opinion he soon laid aside. Dr Halley next proposed that thermometers should... should be graduated in a deep pit under ground, where the temperature both in winter and summer is pretty uniform; and that the point to which the spirit of wine should rise in such a subterraneous place should be the point from which the scale should commence. But this proposal was evidently attended with such inconveniences that it was soon abandoned. He made experiments on the boiling point of water, of mercury, and of spirit of wine; and he seems rather to give a preference to the spirit of wine*. He objected to the freezing of water as a fixed point, because he thought that it admitted considerable latitude.

It seems to have been reserved to the all-conquering genius of Sir Isaac Newton to determine this important point, on which the accuracy and value of the thermometer depends. He chose, as fixed, those points at which water freezes and boils; the very points which the experiments of succeeding philosophers have determined to be the most fixed and convenient. Sensible of the disadvantages of spirit of wine, he tried another liquor which was homogeneous enough, capable of a considerable rarefaction, about 15 times greater than spirit of wine. This was linseed oil. It has not been observed to freeze even in very great colds, and it bears a heat about four times that of water before it boils. With these advantages it was made use of by Sir Isaac Newton, who discovered by it the comparative degree of heat for boiling water, melting wax, boiling spirit of wine, and melting tin; beyond which it does not appear that this thermometer was applied. The method he used for adjusting the scale of this oil thermometer was as follows: Supposing the bulb, when immersed in thawing snow, to contain 10,000 parts, he found the oil expand by the heat of the human body so as to take up 7/26 more space, or 10,256 such parts; and by the heat of water boiling strongly 10,725; and by the heat of melting tin 11,516. So that reckoning the freezing point as a common limit between heat and cold, he began his scale there, marking it 0, and the heat of the human body he made 12°; and consequently, the degrees of heat being proportional to the degrees of rarefaction, or 256 : 725 :: 12 : 34; this number 34 will express the heat of boiling water; and by the same rule, 72 that of melting tin+. This thermometer was constructed in 1701.

To the application of oil as a measure of heat and cold, there are insuperable objections. It is so viscid, that it adheres too strongly to the sides of the tube. On this account it ascends and descends too slowly in case of a sudden heat or cold. In a sudden cold, so great a portion remains adhering to the sides of the tube after the rest has subsided, that the surface appears lower than the corresponding temperature of the air requires. An oil thermometer is therefore not a proper measure of heat and cold.

All the thermometers hitherto proposed were liable to many inconveniences, and could not be considered as exact standards for pointing out the various degrees of temperature. This led Reaumur to attempt a new one, an account of which was published in the year 1730 in the Memoirs of the Academy of Sciences. This thermometer was made with spirit of wine. He took a large ball and tube, the dimensions and capacities of which were known; he then graduated the tube, so that the space from one division to another might contain 1000th part of the liquor; the liquor containing 1000 parts when it stood at the freezing point. He adjusted the thermometer to the freezing point by an artificial congelation of water; then putting the ball of his thermometer and part of the tube into boiling water, he observed whether it rose 80 divisions; if it exceeded these, he changed his liquor, and by adding water lowered it, till upon trial it should just rise 80 divisions; or if the liquor, being too low, fell short of 80 divisions, he raised it by adding rectified spirit to it. The liquor thus prepared suited his purpose, and served for making a thermometer of any size, whose scale would agree with his standard.

This thermometer was far from being perfect. As its defects, the bulbs were three or four inches in diameter, the surrounding ice would be melted before its temperature could be propagated to the whole spirits in the bulb; and consequently the freezing point would be marked higher than it should be.* Dr Martine accordingly found, that instead of coinciding with the 32d degree of Fahrenheit, it corresponded with the 34th, or a point a little above it. Reaumur committed a mistake also respecting the boiling point; for he thought that the spirit of wine, whether weak or strong, when immersed in boiling water, received the same degree of heat with the boiling water. But it is well known that highly rectified spirit of wine cannot be heated much beyond the 175th degree of Fahrenheit, while boiling water raises the quicksilver 37 degrees higher. There is another thermometer that goes by the name of Reaumur's, which shall be afterwards described.

At length a different fluid was proposed, by which Mercurial thermometers could be made free from most of the defects hitherto mentioned. This fluid was mercury, and seems first to have occurred to Dr Halley in the last century; but it was not adopted by him on account of its having a smaller degree of expansibility than the other fluids used at that time*. Boerhaave says that the mercurial thermometer was first constructed by Tram, vol. Olaus Roemer; but the honour of this invention is generally given to Fahrenheit of Amsterdam, who presented an account of it to the Royal Society of London in 1724.

That we may judge the more accurately of the propriety of employing mercury, we will compare its qualities with those of the fluids already mentioned, air, alcohol, and oil.

Air is the most expansible fluid, but it does not receive nor part with its heat so quickly as mercury. Alcohol does not expand much by heat. In its ordinary state it does not bear a much greater heat than 175 of Fahrenheit; but when highly rectified it can bear a greater degree of cold than any other liquor hitherto employed as a measure of temperature. At Hudson's Bay, Mr Macnab, by a mixture of vitriolic acid and snow, made it to descend to 69 below 0 of Fahrenheit. This is an inconvenience, however, attending the use of this liquor; it is not possible to get it always of the same degree of strength. As to oil, its expansion is about 15 times greater than that of alcohol; it sustains a heat of 600°; and its freezing point is so low that it has not been determined; but its viscosity renders it useless.

Mercury is superior to alcohol and oil, and is much more manageable than air. As far as the experiments of mercury... ments already made can determine, it is of all the fluids hitherto employed in the construction of thermometers, that which measures most exactly equal differences of heat by equal differences of its bulk; its dilatations are in fact very nearly proportional to the augmentations of heat applied to it (a). 2. Of all liquids it is the most easily freed from air. 3. It is fitted to measure high degrees of heat and cold. It sustains a heat of 600° of Fahrenheit's scale, and does not congeal till it fall 39 or 40 degrees below c. 4. It is the most sensible of any fluid to heat and cold, even air not excepted. Count Rumford found that mercury was heated from the freezing to the boiling point in 58 seconds, while water took two minutes 13 seconds, and common air 10 minutes and 17 seconds. 5. Mercury is a homogeneous fluid, and every portion of it is equally dilated or contracted by equal variations of heat. Any one thermometer made of pure mercury is, ceteris paribus, possessed of the same properties with every other thermometer made of pure mercury. Its power of expansion is indeed about six times less than that of spirit of wine, but it is great enough to answer most of the purposes for which a thermometer is wanted.

The fixed points which are now universally chosen for adjusting thermometers to a scale, and to one another, are the boiling and freezing points of water. The boiling water point, it is well known, is not an invariable point, but varies some degrees according to the weight and temperature of the atmosphere. In an exhausted receiver, water will boil with a heat of 98° or 100°; whereas in Papin's digester it will require a heat of 412°. Hence it appears that water will boil at a lower point, according to its height in the atmosphere, or to the weight of the column of air which presses upon it. In order to ensure uniformity therefore in the construction of thermometers, it is now agreed that the bulb of the tube be plunged in the water when it boils violently, the barometer standing at 30 English inches (which is its mean height round London, and the temperature of the atmosphere 55°). A thermometer made in this way, with its boiling point at 212°, is called by Dr Horsley Bird's Fahrenheit, because Mr Bird was the first person who attended to the state of the barometer in constructing thermometers.

As artists may be often obliged to adjust thermometers under very different pressures of the atmosphere, philosophers have been at pains to discover a general rule which might be applied on all occasions. M. de Luc, in his Recherches sur les Mod. de l'Atmosphere, from a series of experiments, has given an equation for the allowance on account of this difference, in Paris measure, which has been verified by Sir George Shuckburgh; also Dr Horsley, Dr Maskelyne, and Sir George Shuckburgh, have adapted the equation and rules to English measures, and have reduced the allowances into tables for the use of the artist. Dr Horsley's rule deduced from De Luc's, is this:

\[ \frac{99}{899000} \log_e x - 92.804 = h, \]

where \( h \) denotes the height of a thermometer plunged in boiling water, above the point of melting ice, in degrees of Bird's Fahrenheit, and \( x \) the height of the barometer in 10ths of an inch. From this rule he has computed the following table, for finding the heights, to which a good Bird's Fahrenheit will rise when plunged in boiling water, in all states of the barometer, from 27 to 31 English inches; which will serve, among other uses, to direct instrument-makers in making a true allowance for the effect of the variation of the barometer, if they should be obliged to finish a thermometer at a time when the barometer is above or below 30 inches; though it is best to fix the boiling point when the barometer is at that height.

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(a) We have affirmed that the expansions of the bulk of quicksilver by heat are nearly (for they are not strictly so) in a regular arithmetical progression, according to the quantity of heat it is exposed to; and such seems to be the case according to the Table published by M. de Luc, at page 309 of his first volume on the Modifications of the Atmosphere. The following extract of this table shows these variations: and the first and second differences are added, in order to render these irregularities more sensible. They are such as can hardly be conceived from the nature of any substance, without the influence of extraneous and accidental causes, which may have escaped the attention of the observer; neither have they been found exactly true by Dr Crawford. M. de Luc supposes the whole heat from melting ice to that of boiling water to be divided into 80 parts; by the fractional subdivisions of which he expresses the absolute quantities of heat, answering to each 5 or 10 degrees of Reaumur's thermometer (=22.5 of Fahrenheit's scale); so that the whole sum of these fractions amounts exactly to the assumed number 80. They are as follow:

| Degrees | Reaumur's Thermometer | Fahrenheit's Thermometer | Quantities of heat | First differences | Second differences | |---------|-----------------------|-------------------------|-------------------|------------------|------------------| | 80 | | | | | | | 70 | | | | | | | 60 | | | | | | | 50 | | | | | | | 40 | | | | | | | 30 | | | | | | | 20 | | | | | | | 10 | | | | | | | 0 | | | | | |

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Equation Equation of the Boiling Point.

| Barometer | Equation | Difference | |-----------|----------|------------| | 31.0 | + 1.57 | 0.78 | | 30.5 | + 0.79 | 0.79 | | 30.0 | 0.00 | 0.80 | | 29.5 | - 0.80 | 0.82 | | 29.0 | - 1.62 | 0.83 | | 28.5 | - 2.45 | 0.85 | | 28.0 | - 3.31 | 0.86 | | 27.5 | - 4.16 | 0.88 | | 27.0 | - 5.04 | |

The numbers in the first column of this table express heights of the quicksilver in the barometer in English inches and decimal parts: the second column shows the equation to be applied, according to the sign prefixed, to 212° of Bird's Fahrenheit, to find the true boiling point for every such state of the barometer. The boiling point for all intermediate states of the barometer may be had with sufficient accuracy, by taking proportional parts, by means of the third column of differences of the equations. See Philosophical Transactions, vol. lxiv. art. 30.; also Dr Maskelyne's Paper, vol. lxiv. art. 20.

In the following table we have the result of fifteen different observations made by Sir George Shuckburgh compared with the result of M. de Luc's rules.

| Height of the Barometer reduced to the same temperature of 30° | Mean Boiling Point by observation | Boiling Point by De Luc's Rules | Height of Barometer | Boiling Point by Observation | Boiling Point by De Luc's Rules | |---------------------------------------------------------------|----------------------------------|-------------------------------|---------------------|-------------------------------|-------------------------------| | Inch. | Inch. | Inch. | Inch. | Inch. | Inch. | | 26.498 | 207.07 | 208.54 | 30.008 | 213.22 | 213.47 | | 27.241 | 208.64 | 208.84 | 30.207 | 213.58 | 213.79 | | 27.954 | 209.87 | 210.03 | 30.489 | 214.15 | 214.23 | | 28.377 | 210.50 | 210.81 | 30.763 | 214.37 | 214.66 | | 28.699 | 211.27 | 211.34 | 30.847 | 214.83 | 214.79 | | 28.898 | 211.50 | 211.67 | 30.937 | 214.90 | 214.96 | | 29.999 | 211.60 | 211.83 | | | | | 29.447 | 212.55 | 212.74 | | | | | 29.805 | 212.93 | 213.15 | | | |

Sir George Shuckburgh has also subjoined the following general table for the use of artists in constructing the thermometer, both according to his own observations and those of M. de Luc.

The Royal Society, fully apprised of the importance of adjusting the fixed points of thermometers, appointed a committee of seven gentlemen to consider of the best method for this purpose; and their report is published in the Phil. Trans. vol. lxvii. part ii. art. 37.

They observed, that though the boiling point be placed so much higher on some of the thermometers now made than on others, yet this does not produce any considerable error in the observations of the weather, at least in this climate; for an error of 1½° in the position of the boiling point, will make an error only of half a degree in the position of 92°, and of not more than a quarter of a degree in the point of 62°. It is only in nice experiments, or in trying the heat of hot liquors, that this error in the boiling point can be of much importance.

In adjusting the freezing as well as the boiling point, the quicksilver in the tube ought to be kept of the same heat as that in the ball. When the freezing point is placed at a considerable distance from the ball, the pounded ice should be piled to such a height above the ball, that the error which can arise from the quicksilver in the remaining part of the tube not being heated equally with that in the ball, shall be very small, or the observed point must be corrected on that account according to the following table:

| Heat of the Air | Correction | |-----------------|------------| | 42° | .00087 | | 52° | .00174 | | 62° | .00261 | | 72° | .00384 | | 82° | .00435 |

The correction in this table is expressed in thousandths of the distance between the freezing point and the surface of the ice: e.g. if the freezing point stands seven inches above the surface of the ice, and the heat of the room is 62, the point of 32° should be placed 7×00261, or .018 of an inch lower than the observed point. A diagonal scale will facilitate this correction. The committee observe, that in trying the heat of liquors, care should be taken that the quicksilver in the tube of the thermometer be heated to the same degree as that in the ball; or if this cannot be done conveniently, the observed heat should be corrected on that account; for the manner of doing which, and a table calculated for this purpose, we must refer to their excellent report in the Phil. Trans. vol. lxvii. part ii. art. 37.

With regard to the choice of tubes, they ought to be exactly cylindrical. But though the diameter should vary a little, it is easy to manage that matter in the manner proposed by the Abbé Nollet *, by making a small portion of the quicksilver, e. gr. as much as fills up an inch or half an inch, slide backward and forward in the tube; and thus to find the proportions of all its inequalities, and from thence to adjust the divisions to a scale of the most perfect equality. The capillary tubes are preferable to others, because they require smaller bulbs, and they are also more sensible, and less brittle. The most convenient size for common experiments has the internal diameter about the 45th or 50th of an inch, about nine inches long, and made of thin glass, that the rise and fall of the mercury may be better seen.

The next thing to be considered, is of what number of degrees or divisions the scale ought to consist, and from what point it ought to commence. As the number of the divisions of the scale is an arbitrary matter, the scales which have been employed differ much from one another in this circumstance. Fahrenheit has made 180 degrees between the freezing and boiling water point. Amounts made 73, and Sir Isaac Newton only 34. There is, however, one general maxim, which ought to be observed: That such an arithmetical number should be chosen as can easily be divided and subdivided, and that the number of divisions should be so great that there shall seldom be occasion for fractions. The number 82 chosen by Reaumur answers extremely well in this respect, because it can be divided by several figures without leaving a remainder; but it is too small a number; the consequence of which is, that the degrees are placed at too great a distance from one another, and fractions must therefore be often employed. We think, therefore, that 162 would have been a more convenient number. Fahrenheit's number 180 is large enough, but when divided its quotient soon becomes an odd number.

As to the point at which the scale ought to commence, various opinions have been entertained. If we knew the beginning or lowest degree of heat, all philosophers would agree, that the lowest point of the thermometer ought to be fixed there; but we know neither the lowest nor the highest degrees of heat; we observe only the intermediate parts. All that we can do, then, is to begin it at some invariable point, to which thermometers made in different places may easily be adjusted. If possible too, it ought to be a point at which a natural well-known body receives some remarkable change from the effects of heat or cold. Fahrenheit began his scale at the point at which snow and salt congeal. Kirwan proposes the freezing point of mercury. Sir Isaac Newton, Hales, and Reaumur adopted the freezing point of water. The objection to Fahrenheit's lowest point is, that it commences at an artificial cold never known in nature, and to which we cannot refer our feelings, for it is what few can ever experience. There would be several great advantages gained, we allow, by adopting the freezing point of mercury. It is the lowest degree of cold to which mercury can be applied as a measure; and it would render unnecessary the use of the signs plus and minus, and the extension of the scale below 0. But we object to it, that it is not a point well known; for few, comparatively speaking, who use thermometers, can have an opportunity of seeing mercury congealed. As to the other advantage to be gained by adopting the freezing point of mercury, namely, the abolition of negative numbers, we do not think it would counterbalance the advantage, to be enjoyed by using a well-known point. Besides, it may be asked, Is there not a propriety in using negative numbers to express the degree of cold, which is a negative thing? Heat and cold we can only judge of by our feelings: the point then at which the scale should commence, ought to be a point which can form to us a standard of heat and cold; a point familiar to us from being one of the most remarkable that occurs in nature, and therefore a point to which we can with most clearness and precision refer to in our minds on all occasions. This is the freezing point of water chosen by Sir Isaac Newton, which of all the general changes produced in nature by cold is the most remarkable. It is, therefore, the most convenient point, for the thermometers to be used in the temperate and frigid zones; we may say over the globe, for even in the hottest countries of the torrid zone many of the mountains are perpetually covered with snow.

The thermometers which are at present in most general use, are Fahrenheit's, Del'Isle's, Reaumur's, and Celsius's. Fahrenheit's is used in Britain, Del'Isle's in Russia, Reaumur's and the thermometre centigrade in France, and Celsius's, the same as the last named, in Sweden. They are all mercurial thermometers. For their description and the method of comparing them together, see Chemistry, No 198—201. See also Plate DXXXIV.

As in meteorological observations it is necessary to attend to the greatest rise and fall of the thermometer, attempts have been made to construct a thermometer which might register the greatest degree of heat, or greatest degree of cold, which took place during the absence of the observer. In 1757 Lord Charles Cavendish presented to the Royal Society of London a thermometer in two different forms; the one contrived to mark the greatest degree of heat, and the other the greatest degree of cold.

The first consists of a glass tube AB, fig. 3, with a cylindrical bulb B at the lower end, and capillary at the top, over which there is fixed a glass ball C. The bulb and part of the tube are filled with mercury, the top of which shows the degrees of heat as usual. The upper part of the tube above the mercury is filled with spirit of wine; the ball C is also filled with the same liquor almost to the top of the capillary tube. When the mercury rises, the spirit of wine is also raised, and falls into the ball C, which is so made that the liquor cannot return into the tube when the mercury sinks; consequently the height of the spirit of wine in the ball, added to that in the tube, will give the greatest degree of heat to which the thermometer has pointed since last observation. When a new observation is to be made, the instrument must be inclined... Inclined till the liquor in the ball cover the end of the capillary tube.

In this thermometer it is evident that the mercury must be affected by the weight and elasticity of the spirit of wine, and therefore it will not correspond to any of the common mercurial thermometers.

The thermometer for showing the greatest degree of cold is represented in fig. 4, by the crooked tube ABCD. This instrument is filled with spirit of wine, with the addition of as much mercury as is sufficient to fill both legs of the syphon, and about a fourth or fifth part of the hollow ball C. We are not told what the proportion of mercury was to that of spirit of wine. The degrees of heat are shown by the rise or fall of the mercury in the leg AB. The thermometer marks the greatest fall by means of the hollow ball C. When the mercury in the longer leg sinks by cold, that in the shorter will rise and run over into the ball C, from which it cannot return when the mercury subsides in the shorter and rises in the longer leg. The upper part of the shorter leg will therefore be filled with a column of spirits of a length proportional to the increase of heat; the bottom or lower surface of which, by means of a proper scale, will show how much the mercury has been lower than it is; which being subtracted from the present height will give the lowest point to which the mercury has fallen. That the thermometer may be fitted for a new observation, the mercury must be made to run back from the ball into the shorter leg, by inclining the tube and heating the ball.

In 1782 Mr Six proposed another self-registering thermometer. It is properly a spirit of wine thermometer, though mercury is also employed for supporting an index. a b, fig. 5, is a thin tube of glass 16 inches long, and \( \frac{1}{3} \)ths of an inch caliber; c d e and f g h are smaller tubes about \( \frac{1}{8} \)th of an inch caliber. These three tubes are filled with highly rectified spirit of wine, except the space between d and g, which is filled with mercury. As the spirit of wine contracts or expands in the middle tube, the mercury falls or rises in the outside tubes. An index, such as that represented in fig. 6, is placed on the surface, within each of these tubes, so light as to float upon it. k is a small glass tube \( \frac{1}{3} \)ths of an inch long, hermetically sealed at each end, and inclosing a piece of steel wire nearly of its own length. At each end l, m, of this small tube, a short tube of black glass is fixed, of such a diameter as to pass freely up and down within either of the outside tubes of the thermometer c e or f h. From the upper end of the index is drawn a spring of glass to the fineness of a hair, and about \( \frac{1}{3} \)ths of an inch long; which being placed a little oblique, presses lightly against the inner surface of the tube, and prevents the index from descending when the mercury descends. These indexes being inserted one into each of the outside tubes, it is easy to understand how they point out the greatest heat or cold that has happened in the observer's absence. When the spirit of wine in the middle tube expands, it presses down the mercury in the tube A f, and consequently raises it in the tube c e; consequently the index on the left hand tube is left behind and marks the greatest cold, and the index in the right hand tube rises and marks the greatest heat.

In 1790 a paper was given in to the Royal Society of Edinburgh, describing two thermometers, newly invented, by Dr John Rutherford of Middle Bailish; the one for registering the highest and the other for registering the lowest degree of heat to which the thermometer has risen or fallen during the absence of the observer. An account of them may be found in the third volume of the Transactions of the Society.

A new self-registering thermometer has been invented by Mr Keith of Ravelstone, which we consider thermometers as the most ingenious, simple, and perfect, of any which has hitherto appeared. Its simplicity is so great, that it requires only a very short description to make it intelligible.

AB, fig. 7, is a thin glass tube about 14 inches long Fig. 7 and \( \frac{1}{3} \)ths of an inch caliber, close or hermetically sealed at top. To the lower end, which is open, there is joined the crooked glass tube BE, seven inches long, and \( \frac{1}{3} \)ths of an inch caliber, and open at top. The tube AB is filled with the strongest spirit of wine, and the tube BE with mercury. This is properly a spirit of wine thermometer, and the mercury is used merely to support a piece of ivory or glass, to which is affixed a wire for raising one index or depressing another, according as the mercury rises or falls. F is a small conical piece of ivory or glass, of such a weight as to float on the surface of the mercury. To the float is joined a wire called the float-wire, which reaches upwards to H, where it terminates in a knee bent at right angles. The float-wire, by means of an eye at o, moves easily along the small harpsichord wire GK. LL are two indexes made of thin black oiled silk, which slide upwards or downwards with a force not more than two grains. The one placed above the knee points out the greatest rise, and the one placed below it points out the greatest fall, of the thermometer.

When the instrument is to be prepared for an observation, both indexes are to be brought close to the knee H. It is evident, that when the mercury rises, the float and float-wire, which can be moved with the smallest force, will be pushed upwards till the mercury become stationary. As the knee of the float-wire moves upwards, it will carry along with it the upper index L. When the mercury again subsides, it leaves the index at the highest point at which it was raised, for it will not descend by its own weight: As the mercury falls the float-wire does the same; it therefore brings along with it the lower index L, and continues to depress it till it again becomes stationary or ascend in the tube; in which case it leaves the lower index behind it as it had formerly left the upper. The scale to which the indexes point is placed parallel to the slender harpsichord wire. It may be seen more distinctly in fig. 8. Fig. 8. That the scale and indexes may not be injured by the wind and rain, a cylindrical glass cover, close at top, and made so as exactly to fit the part G F, is placed over it.

The ingenious inventor has another improvement in contemplation, which, if upon trial it be found to answer, will make this thermometer as perfect as can be desired, provided there do not arise some errors from the variable pressure of the atmosphere. He proposes to adapt clock-work to this thermometer, in such a way as to register with the utmost precision the degrees of heat and cold for every month, day, and minute in the year. The principles on which this clockwork is to be formed we shall forbear to describe, hoping that the author himself, after his experiment has met with the success which we ardently wish, will favour the world with his own account of it.

The same ingenious gentleman has invented a self-registering barometer, upon the same principles with the self-registering thermometer. We have had the pleasure of seeing both; and are convinced that they will fully gratify the wishes of all who are engaged in meteorological studies. He is also in expectation of being soon able to produce an air-thermometer free from the defects of those which were formerly made, as he has found out a way of preventing it from being affected by the pressure of the atmosphere.

M. de Luc's supposed improvements.

Mr Cavallo has described the best method of constructing a thermometer, fit for determining the temperature of the air, in the mensuration of heights by the barometer. He has also shown how to divide the scale of a thermometer, so as to adapt it for astronomical purposes in the observation of refractions.

Mr Cavallo, in 1781, proposed the construction of a thermometrical barometer, which, by means of boiling water, might indicate the various gravity of the atmosphere, or the height of the barometer. But as he does not say that the instrument has been tried with the desired success, we forbear to describe it. Those who wish to know his ideas respecting it may consult the Philosophical Transactions, vol. lxxi. p. 524.

The thermometers hitherto described are very limited in their extent; they indeed point out to us the lowest degrees of heat which are commonly observed even in cold climates, but they by no means reach to those degrees of heat which are very familiar to us. The mercurial thermometer extends no farther than to 600° of Fahrenheit's scale, the heat of boiling mercury; but we are sure that the heat of solid bodies, when heated to ignition, or till they emit light, far exceeds the heat of boiling mercury.

In order to remedy this defect, Sir Isaac Newton, whose genius overcame those obstacles which ordinary minds could not approach, attempted by an ingenious method of extending the scale of experiment to extend the scale to any degree required. Having heated a mass of iron red hot, and exposed it to the cold air, he observed the time which elapsed till it became cold, or of the same temperature with the air; and when the heat so far decreased that he could apply some known measure (as a thermometer) to it, he observed the degrees of heat lost in given times; and thence drew the general conclusion, that the quantities of heat lost in given small spaces are always proportional to the heat remaining in the body, reckoning the heat to be the excess by which it is warmer than the ambient air. So that taking the number of minutes which it took to cool after it came to a determined point in an arithmetical progression, the decrements of the heat of the iron would be continually proportional. Having by this proportion found out the decrements of heat in a given time after it came to a known point, it was easy, by carrying upwards the same proportion to the beginning of its cooling, to determine the greatest heat which the body had acquired. This proportion of Sir Isaac's was found by Dr Martine to be somewhat inaccurate. The heat of a cooling body does not decrease exactly in proportion to that which the body retains. As the result of many observations, he found that two kinds of proportion took place, an arithmetical as well as the geometrical proportion which Sir Isaac Newton had adopted; namely, that the decrements of heat were partly proportional to the times (that is, that quantities of heat are lost in equal times), as well as partly in proportion to the remaining heat; and that if these two are added together the rule will be sufficiently accurate. By the geometrical proportion which Sir Isaac Newton adopted he discovered the heat of metals red hot or in fusion.

This method, so successfully pursued by Sir Isaac, was sufficient to form a scale of high degrees of heat, but was not convenient for practical purposes. Accordingly the late Mr Wedgwood invented a very simple thermometer which marks with much precision the different degrees of ignition, from a dull red heat visible in the dark to the heat of an air-furnace. It consists of two rulers fixed upon a smooth flat plate, a little farther asunder at the one end than at the other, leaving an open longitudinal space between them. Small pieces of alum and clay mixed together are made of such a size as just to enter at the wide end; and are heated along with the body whose heat we wish to determine. The earthly body contracts according to the degree of heat, so that when applied to the wide end of the gage, it will slide on towards the narrow end, less or more according to the degree of heat to which it has been exposed.

ABCD, fig. 9, is a smooth flat plate; and EF and GH two rulers or flat pieces, a quarter of an inch thick, fixed flat upon the plate, with the sides that are towards one another made perfectly true, a little farther asunder at one end EG than at the other end FH: thus they include between them a long converging canal, which is divided on one side into a number of small equal parts, and which may be considered as performing the offices both of the tube and scale of the common thermometer. It is obvious, that if a body, so adjusted as to fit exactly at the wider end of this canal, can be afterwards diminished in its bulk by fire, as the thermometer pieces are, it will then pass further in the canal, and more and more so according as the diminution is greater; and conversely, that if a body, so adjusted as to pass on to the narrow end, be afterwards expanded by fire, as is the case with metals, and applied in that expanded state to the scale, it will not pass so far; and that the divisions on the side will be the measures of the expansions of the one, as of the contractions of the other, reckoning in both cases from that point to which the body was adjusted at first.

I is the body whose alteration of bulk is thus to be measured. This is to be gently pushed or slid along towards the end FH, till it is stopped by the converging sides of the canal. See Chemistry, N° 1412.

A very ingenious application of Fahrenheit's thermometer has been made by Mrs Lovi, glass-blower in Edinburgh, for ascertaining the temperature of compost dunghills, for regulating the temperature of hot-beds, and observing the changes of temperature in corn and haystacks when they are put up damp. This may be called an agricultural thermometer, and has been found of great use for the above purposes.

THERMOPYLÆ, in Ancient Geography, a narrow pass or defile, between the Sinus Maliacus on the east, and a steep branch of Mount Oeta, covered with unpassable woods, on the west; leading from Thessaly to Locris and Boeotia. These moun- The mountains divide Greece in the middle, in the same manner as the Apennine does Italy; forming one continued ridge from Leucate on the west to the sea on the east, with thickets and rocks interspersed. The valley is of small breadth, and completely occupied by a marsh, through which it is impossible for horse or foot to penetrate. Through the middle of this marsh a paved road was made by the Greeks, which still remains, and affords the only path to Greece from Thessaly. The Thermae or hot springs, from which the place had its name, are near the narrowest part of the pass; and their temperature was found by Dr Clarke to be 111°.

At the south entrance of the defile is a small eminence, on which Dr Clarke found a tumulus, with the remains of a massive pedestal of red breccia much time-worn, and evidently the tomb of the 300 Spartans, to whose immortal memories this monument was consecrated. The rising ground on which the monument stands is the spot where the Spartans fell. The remains of the wall which shut up the pass are also yet visible. For an account of the battle of Thermopylae, see Sparta.