THERMOMETER

THERMOMETER, an instrument for measuring the increase and decrease of the heat and cold of the air, by means of the elastic and expansive power of fluids.

The thermometer is an instrument of modern invention, but authors are not agreed on the person who was the first inventor. Sanctorius affirms, that he himself had this honour; and his assertion is corroborated by the testimonies of Borelli and Malpighi. Boerhaave, however, ascribes it to Cornelius Drebbel; Mueschenbroek to the Venetian Paul Sarpi; and Viviani to Galileo. It is not well known what was the form of the first instruments of this kind, only it is certain that they were far from being as perfect as they are at present. They were very clumsy; and air being the fluid employed, they acted also as a kind of barometers, and thus were entirely unfit for comparing the degrees of heat at the distance of any considerable time. The first improvement in their construction was made by Ferdinand II. great duke of Tuscany, or the members of the academy del Cimento under his protection. Spirit of wine inclosed in glass tubes hermetically sealed, was now employed; by which means evaporation was prevented, and likewise the pressure of the external air kept off. Instruments of this kind

were introduced into England by Mr Boyle, and very soon came to be universally used among the virtuosi in different parts of the world. However, they still laboured under great disadvantages; for as they were not adjusted to any common scale, tho' they would each of them show the comparative degrees of heat and cold in the bodies to which they were applied; yet they could never be compared with one another, nor could the observations made in different parts of the world be collected in such a manner as to answer any good purpose.

Mr Boyle was the first who proposed one certain and determinate degree of cold as a standard to be universally chosen from whence to begin the scale of thermometers, and which being at all times invariable, would keep the observations made by different thermometers sufficiently exact. The point he proposed was that in which the oil of aniseeds congeals; and he also mentioned that in which distilled water begins to freeze, as another which might be employed. At last, however, Mr Boyle laid aside both these standards; as thinking that the oil of aniseeds, water, and other liquors, might freeze in different degrees of heat. In this opinion he was followed by Dr Halley, who proposed for a standard the temperature of deep caves below ground. These indeed do always preserve the same temperature, but the use of them would be found not a little inconvenient for the purpose of adjusting thermometers; nor, after all, could there be any certainty even in this method; for we are by no means sure that the temperature of the ground is the same in all places at the same depth from the surface. Dr Halley therefore proposed another point, namely, that in which spirit of wine boils; and he also mentioned the heat of boiling water as a point very fixed and determined, and in which he has been followed by all that came after him.

From this one point of the heat of boiling water, it has been proposed to construct thermometers in such a manner that they shall all correspond with each other. This method is by marking the degree of expansion or contraction of the fluid in thermometers, as the heat applied is either greater or less than that of boiling water. Supposing, for example, the whole volume of this fluid to consist of 10,000 parts, we must mark on the tube, where that volume is expanded by heat or contracted by cold, 1, 2, 3, 4, &c. of these parts; all which may be done by different persons and in different places, so that they shall answer precisely to one another.—Yet this method, so plausible in appearance, is found not to be very practicable; it being difficult to determine exactly all the divisions from the alteration of the bulk of the fluid. The best method, therefore, is to have two fixed points of heat at a considerable distance the one from the other; such as that of boiling and freezing water; after which we are to divide the scale or tube between them into any convenient number of equal parts or degrees.

The method just now proposed is that which has for a long time been universally followed; and the only difficulties which remained in the construction of thermometers were the choice of a proper fluid, the adjusting of the two fixed points, and the division of the space between them in such a manner as to make proper allowance for any inequalities that might happen

Thermometer. to be in the diameter of the tube through which the fluid moves.

With regard to the fluid, quicksilver has universally obtained the preference, as being sooner heated and cooled than any other with which we are acquainted, and requiring so great a degree of cold to congeal it, that till very lately it was thought impossible to do so; though the contrary is now shown by undeniable experiments*. The heat also which quicksilver requires to make it boil, is very considerable, so that the scale on a quicksilver thermometer may be enlarged greatly beyond that made with any other fluid. In extreme degrees of cold, indeed, where quicksilver freezes, the thermometer should be filled with highly rectified spirit of wine, or the fluid petroleum, called naphtha; both of which are found unalterable in the greatest degrees of cold, either natural or artificial, hitherto observed in any part of the world.

* See Cold, Congelation, and Quick-silver.

The adjustment of the two fixed points of heat, viz. that of boiling and that of freezing water, has been found a matter of considerable difficulty, as they vary considerably according to the height of the mercury in the barometer at the time. Hence the greatest philosophers have not thought it below them to bestow their labour in attempting to bring this matter to its utmost exactness; and for this purpose a committee of the Royal Society was lately appointed.

This inaccuracy in the heat of boiling water at different times, was observed by Mr Fahrenheit, a celebrated artist at Amsterdam, from whom the kind of thermometers now mostly used took their name. He supposed the variation to be much greater than it really is; but Mr de Luc, by a great number of experiments made at different heights above the level of the sea, found a rule by which the difference in the boiling point, answering to different heights in the barometer, is determined with great exactness. According to this rule, the alteration of the boiling point by the variation of the barometer from 29$\backslash frac\{1\}\{2\}$ to 30$\backslash frac\{1\}\{2\}$ inches is 1$\backslash circ$.59 of Fahrenheit. The committee of the Society (of whom M. de Luc was one), after a number of experiments which our limits will not allow us to insert, lay down the following practical rules for adjusting the boiling and freezing points.

“ Rules to be observed in adjusting the boiling point. — The most accurate way of adjusting the boiling point is, not to dip the thermometer into the water, but to expose it only to the steam, in a vessel closed up in the manner represented in fig. 4. where ABba is the vessel containing the boiling water, Dd the cover, E a chimney made in the cover intended to carry off the steam, and Mm the thermometer passed thro’ a hole in the cover. Those who would make use of this method, must take care to attend to the following particulars.

“ 1^st, The boiling point must be adjusted when the barometer is at 29.8 inches; unless the operator is willing to correct the observed point in the manner directed below.

“ 2^dy, The ball of the thermometer must be placed at such a depth within the pot, that the boiling point

Thermometer. shall rise very little above the cover; for otherwise part of the quicksilver in the tube will not be heated, and therefore the thermometer will not rise to its proper height. The surface of the water in the pot also should be at least one or two inches below the bottom of the ball; as otherwise the water, when boiling fast, might be apt to touch the ball: but it does not signify how much lower than that the surface of the water may be.

“ 3^dy, Care must be taken to stop up the hole in the cover through which the tube is inserted, and to make the cover fit pretty close, so that no air shall enter into the pot that way, and that not much steam may escape. A piece of thin flat tin plate must also be laid on the mouth of the chimney, so as to leave no more passage than what is sufficient to carry off the steam. The size of this plate should be not much more than sufficient to cover the chimney, that its weight may not be too great; and the mouth of the chimney should be made flat, that the plate may cover it more completely. It must be observed, that when the tin-plate is laid on the mouth of the chimney, it will commonly be lifted up by the force of the steam, and will rattle till it has slipped aside sufficiently to let the steam escape without lifting it up. In this case it is not necessary to put the plate back again, unless by accident it has slipped aside more than usual. If the artist pleases, he may tie each corner of this plate by a string to prongs fixed to the chimney, and standing on a level with the plate, as thereby it will necessarily be kept always in its place (A); but we would by no means recommend having it made with a hinge, as that might be apt to make it stick, in which case the included vapour might be so much compressed as to cause an error. We would also by no means advise lining the tin-plate with leather, or any other soft substance for the sake of making it shut closer, as that also might be apt to make it stick. The chimney also ought not to be made less than half a square inch in area: for though a smaller chimney would be sufficient to carry off the steam, unless the vessel is much larger than what we used; yet the adhesion which is apt to take place between it and the tin-plate when wet, might perhaps bear too great a proportion to the power which the included steam has to lift it off, if it was made much less. It is convenient that the chimney be not less than two or three inches long, as thereby the observer will be less incommoded by the steam; but it would be improper to make it much longer, for the longer the chimney is, the greater disposition has the air to enter into the pot between it and the cover.

“ It is most convenient not to make the cover fit on tight, but to take on and off easily; and to wrap some spun cotton round that part of the cover which enters into the pot, in order to make it shut closer; or, what seems to answer rather better, a ring of woollen cloth may be placed under the cover, so as to lie between the top of the pot and it. These methods of making the cover shut close can be used more conveniently when the cover is made to enter within the pot, as in the figure, than when it goes on on the outside.

“ There are various easy ways by which the hole in the cover, through which the tube of the thermometer is passed, may be stopped up, and by which the thermometer may be suspended at the proper height. The hole in the cover may be stopped up by a cork, which must first have a hole bored through it big enough to receive the tube, and be then cut into two, parallel to the length of the hole. Another method more convenient in use, but not so easily made, is represented in fig. 6. which exhibits a perspective view of the apparatus; A a, is the cover; H, the hole through which the thermometer is passed; B b, a flat piece of brass fixed upon the cover; and D d E e, a sliding piece of brass, made so as either to cover the hole H, or to leave it uncovered, as in the figure, and to be tightened in either position by the screw S sliding in the slit M m; a semi-circular notch being made in the edge B b, and also in the edge D d, to inclose the tube of the thermometer: pieces of woollen cloth should also be fastened to the edges B b and D d, and also to the bottom of the sliding-piece D d E e, unless that piece and the cover are made sufficiently flat, to prevent the escape of the steam. In order to keep the thermometer suspended at the proper height, a clip may be used like that represented in fig. 6. which, by the screw S, must be made to embrace the tube tightly, and may rest on the cover. That part of the clip which is intended to bear against the tube had best be lined with woollen cloth, which will make it stick tighter to the tube, and with less danger of breaking it. Another method, which is rather more convenient, when the top of the tube of the thermometer is bent into a right angle, in the manner frequently practised at present for the sake of more conveniently fixing it to the scale, is represented in the same figure; G g F f, is a plate of brass, standing perpendicularly on the cover; and L l N n, a piece of brass, bent at bottom into the form of a loop, with a notch in it, so as to receive the tube of the thermometer, and to suffer the bent part to rest on the bottom of the loop; this piece must slide in a slit K k, cut in the plate L l N n, and be tightened at any height by the screw T.

“ 4thly, It is best making the water boil pretty briskly, as otherwise the thermometer is apt to be a great while before it acquires its full heat, especially if the vessel is very deep. The observer, too, should wait at least one or two minutes after the thermometer appears to be stationary, before he concludes that it has acquired its full height.

“ 5thly, Though, as was said before, this appears to be the most accurate way of adjusting the boiling-point; yet, if the operator was to suffer the air to have any access to the inside of the vessel, he would be liable to a very great error: for this reason, we strongly recommend it to all those who use this method, not to deviate at all from the rules laid down, without assuring themselves, by repeated trials with a pretty sensible thermometer, that such alteration may be used with safety. But the covering the chimney with the tin-plate ought by no means to be omitted; for though, if the cover of the pot fits close, it seldom signifies whether the plate is laid on or not, yet, if by accident the cover was not to fit close, the omitting the tin-plate would make a very great error. Making the

chimney very narrow would not answer the end properly; for if it was made so small as to make the vessel sufficiently close when the water boiled gently, it would not leave sufficient passage for the escape of the steam when the water boiled fast.

“ Another way of adjusting the boiling point is, to try it in a vessel of the same kind as the former, only with the water rising a little way, namely from one to three or four inches above the ball, taking care that the boiling point shall rise very little above the cover, as in the former method. In this method there is no need to cover the chimney with the tin-plate; and there is less need to make the cover fit close, only it must be observed, that the closer the cover fits, the less the operator will be incommoded by the steam. The height of the barometer at which the boiling point should be adjusted, when this method is used, is 29½ inches, or three-tenths of an inch less than when the former method is used.

“ It will be convenient to have two or three pots of different depths; for if a short thermometer is to be adjusted in the same pot which is used for a long one, it will require a great depth of water, which, besides taking up more time before it boils, makes the observation rather less accurate, as the heat seems to be less regular when the depth of water in the pot is very great, than when it is less.

“ Perhaps some persons, for the sake of heating the water more expeditiously, may be inclined to use an apparatus of such kind that the fire shall be applied to a considerable part of the sides of the pot as well as to the bottom; we would, however, caution them against any thing of that kind, as the observations are considerably less regular than when little more than the bottom of the pot is heated. If the pot is heated over a chating-dish or common fire, we apprehend that there can seldom be any danger of too much of the sides being heated; but if the operator should be apprehensive that there is, it is easily prevented by fastening an iron ring an inch or two broad round the pot near the bottom. This precaution is equally necessary when the thermometer is adjusted in steam, especially when there is not much water in the pot.

“ The greatest inconvenience of this method of adjusting the boiling point is the trouble of keeping a proper depth of water in the pot; as to do this it is necessary first to find the height of the boiling point coarsely by trying it in an open vessel, and then to put such a quantity of water into the pot that it shall rise from one to three or four inches above the ball, when the thermometer is placed at such a depth within the pot that the boiling point shall rise very little above the cover. The operator must be very careful that the quantity of water in the pot be not so small as not entirely to cover the ball.

“ A third way of adjusting the boiling point is to wrap several folds of linen rags or flannel round the tube of the thermometer, and to try it in an open vessel, taking care to pour boiling water on the rags, in order to keep the quicksilver in the tube as nearly of the heat of boiling water as possible. The best way is to pour boiling water on the rags three or four times, waiting a few seconds between each time, and to wait some seconds after the last time of pouring on water before the boiling point is marked, in order that

the water may recover its full strength of boiling, which is in good measure checked by pouring on the boiling water.

"In this method the boiling point should be adjusted when the barometer is at 29.8 inches, that is, the same as when the first method is used; the water should boil fast, and the thermometer should be held upright, with its ball two or three inches under water, and in that part of the vessel where the current of water ascends (B).

"Whichever of these methods of adjusting the boiling point is used, it is not necessary to wait till the barometer is at the proper height, provided the operator will take care to correct the observed height according to the following table.

"To make use of this table, seek the height which the barometer is found to stand at in the left-hand column, if the boiling point is adjusted either in the first or third method, and in the second column if it is adjusted in the second method; the corresponding number in the third column shows how much the point of 212° must be placed above or below the observed point, expressed in thousandth parts of the interval between the boiling and freezing point: for example, suppose the boiling point is adjusted in steam when the barometer is at 29 inches, and that the interval between the boiling and freezing points is 11 inches; the nearest number to 29 in the left-hand column is 29.03, and the corresponding number in the table is 7 higher, and therefore the mark of 212° must be placed higher than the observed point by $\backslash frac\{7\}\{1000\}$ of the interval between boiling and freezing, that is, by $\backslash frac\{7\; \backslash times\; 11\}\{1000\}$, or .077 of an inch.

"This method of correcting the boiling point is not strictly just, unless the tube is of an equal bore in all its parts; but the tube is very seldom so much unequal as to cause any sensible error, where the whole correction is so small. The trouble of making the correction will be abridged by making a diagonal scale, such as is represented in fig. 4.

"It is not very material what kind of water is used for adjusting the boiling point, so that it is not at all fust; only, if any kind of hard water is used, it is better that it should be kept boiling for at least ten

minutes before it is used. But we would advise all those desirous of adjusting thermometers in the most accurate manner for nice experiments, to employ rain or distilled water, and to perform the operation in the first-mentioned manner, that is, in steam.

"On the freezing point.—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. In the generality of thermometers, indeed, the distance of the freezing point from the ball is so small, that the greatest error which can arise from neglecting this precaution is not very considerable, unless the weather is warmer than usual; but as the freezing point is frequently placed at a considerable distance from the ball, the operator should always be careful either to pile the pounded ice 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 he must correct the observed point, upon that account, according to the following table;

"The first column of this table is the heat of the air, and the second is the correction expressed in 1000th parts of the distance between the freezing point and the surface of the ice: for example, 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 $7\; \backslash times\; .00261$, or .018 of an inch lower than the observed point. This correction also would be made more easy by the help of a diagonal scale, similar to that proposed for the boiling point.

"On the precautions necessary to be observed in making observations with thermometers.—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."

After having adjusted the fixed points of thermometers in this manner, the division of the scale is easy; for by making a certain quantity of mercury, suppose as much as fills the tube the length of half an inch, pass through the whole length of it when open at both ends, we can perceive what length of space it occupies in different parts of the tube, and divide accordingly. However, even after thermometers are constructed in this manner with the utmost accuracy, a very considerable inconvenience attends the using them, namely, that the observer's eye must be on the instrument the very instant that the mercury stands at the highest or lowest degree; for since the time when that may happen is utterly uncertain, if it be not immediately noticed, it can never afterwards be known. The sultry heat of the summer's day, and the freezing cold of the winter's night, render it very

"(n) In a vessel of boiling water one may almost always perceive the current of water to ascend on one side of the vessel, and to descend on the other.

unpleasant to be abroad at such times in the open air, although it is absolutely necessary that the thermometer should be so. It would therefore be a very great improvement could a thermometer be constructed in such a manner as to show the greatest height to which it had ascended, or the lowest point to which it descended, in the observer's absence. One of this kind has been constructed by Mr James Six, of which the following account is given by him in the 72d vol. of the Phil. Trans.

“Fig. 1. ab is a tube of thin glass, about 16 inches long, and five sixteenths of an inch in diameter; cde fgh, a smaller tube with the inner diameter, about one fortieth, joined to the larger at the upper end b, and bent down, first on the left side, and then, after descending two inches below ab, upwards again on the right, in the several directions cde fgh, parallel to and one inch distant from it. On the end of the same tube at b, the inner diameter is enlarged to half an inch from b to i, which is two inches in length. This glass is filled with highly rectified spirits of wine to within half an inch of the end i, excepting that part of the small tube from d to g, which is filled with mercury. From a view of the instrument in this state, it will readily be conceived, that when the spirit in the large tube, which is the bulb of the thermometer, is expanded by heat, the mercury in the small tube on the left side will be pressed down, and consequently cause that on the right side to rise; on the contrary, when the spirit is condensed by cold, the reverse will happen, the mercury on the left side will rise as that on the right side descends. The scale, therefore, which is Fahrenheit's, beginning with 0 at the top of the left side, has the degrees numbered downwards, while that at the right side, beginning with 0 at the bottom, ascends. The divisions are ascertained by placing this thermometer with a good standard mercurial one in water gradually heating or cooling, and marking the divisions of the new scale at every 5° (c). Thus far our thermometer resembles in some respects those of Mr Bernoulli and Lord Charles Cavendish: but the method of showing how high the mercury had risen in the observer's absence, the essential property of an instrument of this kind, is wholly different from theirs, and effected in the following manner. Within the small tube of the thermometer, above the surface of the mercury on either side, immersed in the spirit of wine, is placed a small index, so fitted as to pass up and down as occasion may require: that surface of the mercury which rises carries up the index with it, which index does not return with the mercury when it descends; but by remaining fixed, shows distinctly, and very accurately, how high the mercury had risen, and consequently what degree of heat or cold had happened. Fig. 2. represents one of these indexes drawn larger than the real ones, to render it more distinct. a is a small glass tube, three quarters of an inch long, hermetically sealed at each end, inclosing a piece of steel wire nearly of the

same length; at each end cd is fixed a short piece of a tube of black glass, of such a diameter as to pass freely up and down within the small tube of the thermometer. The lower end floating on the surface of the mercury, is carried up with it when it rises, while the piece at the upper end being of the same diameter, keeps the body of the index parallel to the sides of the thermometrical tube. From the upper end of the body of the index at e is drawn a spring of glass to the fineness of a hair, about five-sevenths of an inch in length, which being set a little oblique, presses lightly against the inner surface of the tube, and prevents the index from following the mercury when it descends, or being moved by the spirit passing up or down, or by any sudden motion given to the instrument by the hand or otherwise; but at the same time the pressure is so adjusted as to permit this index to be readily carried up by the surface of the rising mercury, and downwards whenever the instrument is to be rectified for observation. To prevent the spirit from evaporating, the tube at the end i is closely sealed (d). Fig. 3. represents the thermometer on its frame; the plates on which the scale is graven on either side are made to slide out, and the frame is open to the back behind the large tube, which does not touch it, except at each end. The cap a, and the base b, are made to fix on with screws, and only cover the turning of the small tube. By a screw at the bottom of the frame, it may be made fast to the wall against which it is to hang without doors, to prevent its being shaken by violent winds. Towards evening I usually visit my thermometer, and see at one view, by the index on the left side, the cold of the preceding night; and by that on the right, the heat of the day. These I minute down, and then apply a small magnet to that part of the tube against which the indexes rest, and move each of them down to the surface of the mercury: thus, without heating, cooling, separating, or at all disturbing the mercury, or moving the instrument, may this thermometer, without a touch, be immediately rectified for another observation. When I wish to put the thermometer out of my hand without hanging it up, I have a stand to place it on; for if the mercury presses against the index while the instrument lies in an horizontal position, it is in danger of passing by it, which is avoided by keeping the thermometer in a position nearly vertical. To prevent the mercury shifting its place in the spirits within the tube (which I apprehend it might do on account of the superiority of its specific gravity, especially when kept for a considerable time, very high on one side, and low on the other), I made that part of the small tube from e to f with the inner diameter exceeding small; and found upon trial, that after the summer's heat had kept the mercury for a long time high on one side, the winter's cold brought it again as accurately to the freezing point on the other as at first (e). This thermometer may be made a mercurial

Thermo-meter. rial one by inverting the glass, and filling with mercury that part which in the first is filled with spirits, and with spirits that part of the small tube from d to g which in the former is filled with mercury; the indexes in either case may be the same, and will be carried up in the same manner upon the surface of the mercury; but the end of the tube at i, instead of being sealed, must then be left open, and stand inverted in a bulb, or small cistern of mercury, into which the external air has free access. The diameter of the tube ab should be considerably increased if the degrees on the scale are required to be as wide as those in the spirit-thermometers. It is indeed better in this case to have a double rather than a larger single tube; but finding the weight of so great a quantity of mercury in a thin glass tube attended with many disadvantages, and the motion of the fluids in the spirit-ones perfectly agreeing with, and being as readily excited by change of heat and cold as in the mercurial thermometers, I preferred the former as much more commodious. A person cannot approach near to the thermometer first described when the air is very cold (especially with a light, which by night is necessary) without causing the spirits presently to expand, and consequently the mercury on the left side immediately to descend. This sensibility is here attended with every advantage, without the inconvenience to which common thermometers in this case are liable (r); for the index will accurately show the greatest height to which the mercury had risen, although, before the exact degree can well be distinguished, it will appear separated from the index, and descending apace. As the scale is 16 inches long, and divided into 100° only, which are more than sufficient for the temperature of the air, they are large enough to be subdivided at pleasure. The indexes, though of a tender and delicate nature, when once placed in the tube, are not liable to suffer any alteration by time or accident; and the thermometer may be exposed to rain at all times, without suffering the least injury in any respect.

“ In constructing the thermometer before-mentioned, I at first hit on a plan by which the same end was obtained by a different method; and though in some respects, and for some purposes, it may not be so proper as that already described, yet for some others it may be found useful; and therefore I shall briefly describe it. The glass of this instrument is in all respects the same as in the former, excepting that the diameters of the tubes are something larger. It is likewise filled with spirits of wine and mercury, in the same manner; but the indexes are different, being only a small tube of black glass, about five-sevenths of an inch in length, hermetically sealed at each end, containing a piece of steel wire. An index of this sort is placed in the thermometer on either side, which ha-

ving no spring to support them, sink down in the spirits, and rest upon the mercury. Whenever the mercury descends, the index will follow it; but when it rises, the index will not rise with it, and by remaining at the place to which the mercury had descended, will show the greatest degree of heat or cold which had happened. In this manner do these indexes answer the same purpose, though they move directly contrary to the others in the other thermometer; but this instrument is not so easily rectified as the former, for the most powerful magnet will not bring the index up again while the mercury above presses against them; and although it is possible to remove the mercury, and by that means set the index at liberty, yet inconveniences will be incurred from which the other is entirely free.

“ In some cases it may be found expedient, instead of the double thermometer first described, to make two single ones; one to show the greatest degree of heat only, and the other the cold, each having its proper index (see fig. 4. and 5.) The first has the small tube as this thermometer was principally to be used immersed in a bulb or small cistern of mercury, to which the external air has free access; the other has the small tube turned up on the right side, with some mercury let down to the bottom, and the upper end closely sealed, as in the double instrument. Making a standard mercurial thermometer, by which the scale of the spirit-one was to be divided, I endeavoured to obtain as wide degrees as possible, that the motion of the mercury might thereby be rendered more conspicuous, and the height of it ascertained with greater precision. It is true, the larger the degrees, the larger in some measure must be the bulb, and therefore the fluid contained in it not likely to be so soon affected by any change bent down on the left side, and the lower end immersed in heat or cold in the atmosphere as in a smaller. But fed in a large quantity of water, gradually heating or cooling, little or no disadvantage could arise from making the bulb somewhat larger than those commonly made use of in the air. Not being able, however, to procure glass tubes so long as I had occasion for, whose inner diameters were perfectly equal, I took the following method to adjust the divisions on the scale to the inequality of the tubes. Choosing a tube of a length suitable to my purpose, with a proper bulb at the end, I put into it a small quantity of mercury (c) sufficient to form a column about one inch in length. Drawing then on a board the three lines aa, bb, cc, fig. 6. I placed the glass tube on the line aa, and while the mercury remained at rest at the end of the tube, near the bulb, I made two pencil-marks on the line aa, one at d, and the other at e, perfectly coinciding with the two ends of the column of mercury: then causing the mercury to move slowly on farther from the bulb, till that end of the column which was first at d coincided

“ (r) The most sensible mercurial thermometers commonly have the column of mercury, as well as the degrees, very small; and a person assisted with a light can hardly view them near enough, when the weather is very cold, without causing the mercury to rise before the degrees where it stood can be well ascertained.

“ Freezing fogs also, which with us usually attend the greatest degrees of cold, by covering the glass with frost, render the mercury invisible, and cannot well be removed without causing the mercury to rise, or at least render the observation doubtful, which at such a time is very disagreeable; for in proportion to the extraordinary degree of cold, so is our curiosity likely to be excited.

“ (g) To put in a small quantity of mercury, and measure its length at different parts of the tube, as described by Abbe Nollet, vol. iv. p. 370. Lecons Physique, is a very excellent method to discover the error; but in what manner readily to adjust the scale so as to avoid any inaccuracy from such inequality (which in tubes of the length I had occasion for seemed to me unavoidable), was a matter concerning which I could meet with no information.

Thermometer, Thermopyle.

ded with the mark at $e$, and letting it rest again, I made another mark at $f$; after which, causing the mercury to move on as before, and continuing to mark its length at every part of the tube till it reached the end farthest from the bulb; by these means I obtained the several intermediate points on the line $aa$. Through these several points I drew dotted lines parallel to each other, and at right angles with the line $aa$ to the line $bb$. Taking now, with a pair of compasses, the widest intervals between any of the dotted parallels, which in this case is from $d$ to $e$, I inserted that distance successively between the several parallels, beginning at the lowest pair, as from $d$ to $e$, from $e$ to $f$, from $f$ to $g$, and so on to $h$, as exhibited in the figure; and the aggregate of these lines may be considered as one continued line, without any error of consequence in this matter. Having now the thermometer completely filled with mercury, the air expelled, the point of the scale at $102^\backslash circ$, and the freezing point properly taken ($ii$) and marked upon the tube, which was now hermetically sealed, I again applied the tube to the line $aa$, and marked on that line the point of $102^\backslash circ$ and the freezing point. Through those points I drew the lines $ii$, $kk$, and divided that part of the compound line $db$ included between $ii$ and $kk$ into 14 equal parts, beginning at $o$, the point where $ii$ cuts the line $db$, continuing afterward six divisions now on that line below $kk$, making in all 20 equal divisions. If now lines be drawn through each of the dividing points, from $o$ to 20 to the line $cc$, at right angles with the same, they will give on the line $cc$ the true thermometrical scale to every $5^\backslash circ$ from 2 to 102, properly adjusted to the inequality of the tube ($1$), which in this case is nearly of the same diameter at each end, but smaller towards the middle. Tubes may indeed be found of some considerable length with less inequality than what this scale exhibits; but the error is here enlarged to render the method of correcting it more conspicuous."

Though Fahrenheit's thermometer is that most commonly used, yet as others have been invented by different persons, we have thought proper, in Plate CCLXXXV. to give a comparative view of their different scales; by which if the reader happens to meet with any of those instruments, or to peruse the accounts of observations made by them, they may be easily reduced to one standard.