meter.

meter.

for the number 80, founded, as he thought, on philofophical Thermoreafons, made him decline following this advice. But he found by experience that the prediction of Condamine was too well founded.

Thermo- fnow just beginning to thaw, and mark the place where the mercury ftands with a 32; then immerge it in boiling water, and again mark the place where the mercury ftands in the tube, which mark with the num. 212, exceeding the former by 180; dividing therefore the intermediate fpace into 180 equal parts, will give the fcale of the thermometer, and which may afterwards be continued upwards and down wards at pleasure.

26

De l'ifle's

ter.

Plate DVII.

Other thermometers of a fimilar conftruction have been accommodated to common ufe, having but a portion of the above fcale. They have been made of a fmall fize and portable form, and adapted with appendages to particular purposes; and the tube with its annexed fcale has often been enclosed in another thicker glass tube, also hermetically fealed, to preferve the thermometer from injury. And all thefe are called Fahrenheit's thermometers.

In 1733, M. De l'Ile of Petersburgh conftructed a merthermome curial thermometer on the principles of Reaumur's fpirit thermometer. In his thermometer, the whole bulk of quickfilver, when immerged in boiling water, is conceived to be divided into 100,000 parts; and from this one fixed point the various degrees of heat, either above or below it, are marked in thefe parts on the tube or scale, by the various expanfion or contraction of the quick filver, in all imaginable varieties of heat.-Dr Martine apprehends it would have been better if De l'Ifle had made the integer 100,000 parts, or fixed point, at freezing water, and from thence computed the dilatations or condenfations of the quickfilver in those parts; as all the common observations of the weather, &c. would have been expreffed by numbers increafing as the heat increased, inftead of decreafing, or counting the contrary way. However, in practice it will not be very eafy to determine exactly all the divifions from the altera tion of the bulk of the contained fluid. And befides, as glafs itself is dilated by heat, though in a lefs proportion than quickfilver, it is only the excefs of the dilatation of the contained fluid above that of the glass that is obferved; and therefore if different kinds of glafs be differently affected by a given degree of heat, this will make a feeming difference in the dilatations of the quickfilver in the thermometers conftructed in the Newtonian method, either by Reaumur's rules or De l'Ifle's. Accordingly it has been found, that the quickfilver in De l'Ifle's thermometers has flood at different degrees of the fcale when immerged in thawing fnow: having stood in fome at 154o, while in others it has been at 156 or even 158°.

27

Reaumur's, r rather

De Luc's hermome

er.

The thermometer presently used in France is called Reaumur's; but it is very different from the one originally invent. ed by Reaumur in 1730, and defcribed in the Memoirs of the Academy of Sciences. The one invented by Reaumur was filled with spirit of wine; and tho' its fcale was divided by the author into 80 parts, of which o was the freezing point and 80 the boiling water point, yet in fact 80 was only the boiling point of the spirit of wine that he employed, which, as Dr Martine computes, correfponded with 180 of Fahrenheit. But the thermometer now in ufe in France is filled with mercury; and the boiling water point, which is at 80, corresponds with the 212th degree of Fahrenheit. The fcale indeed commences at the freezing point, as the old one did. The new thermometer ought more properly to be called De Luc's thermometer, for it was first made by De Luc; and is in fact as different from Reaumur's as it is from Sir Ifaac Newton's. When De Luc had fixed the fcale, and finished an account of it, he showed the manufcript to M. De la Condamine. Condamine advised him to change the number 80; remarking, that fuch was the inattention of phylicians, that they would probably confound it with Reaumur's. De Luc's modefty, as well as a predilection VOL. XVIII. Part II.

28

The thermometer of Celfius, which is used in Sweden, Celfius's has a scale of 100 degrees from the freezing to the boiling thermowater point.

meter.

ther.

29

These are the principal thermometers now ufed in Europe; How to and the temperatures indicated by any of them may be redu- compare ced into the correfponding degrees on any of the others by thefe toge means of the following fimple canons; in which R fignifies the degrees on the fcale of Reaumur, F thofe of Fahrenheit, and S thofe of the Swedish thermometer. 1. To convert the degrees of Reaumur into those of FahRX9 renheit; +32= F.

4

2. To convert the degrees of Fahrenheit into those of F 32 X 4 Reaumur ; = R. 9

3. To convert the Swedish degrees into thofe of FahrenSX9 +32= F. 5 4. To convert Fahrenheit's into Swedish;

heit;

F-32X5-S

Lavoifier's Elements of Chemistry.

9

5. To convert Swedish degrees into those of Reaumur ; = R.

SX4

RX5 4

5 6. To convert Reaumur's degrees into Swedish; = S. To fuch readers as are unacquainted with the algebraic expreffion of arithmetical formule, it will be fufficient to exprefs one or two of these in words to explain their use.1. Multiply the degree of Reaumur by 9, divide the product by 4, and to the quotient add 32, the fum expreffes the degree on the fcale of Fahrenheit.-2. From the degree of Fahrenheit fubtract 32, multiply the remainder by 4, and divide the produ✨ by 9, the quotient is the degree according to the fcale of Reaumur, &c.

As many other thermometers have been ufed befides these, and confequently obfervations taken by them, it is of importance to have them placed in fuch a point of view that they may be eatily compared with any of these four now in general ufe. We therefore give them in Plate DVII. in the fame order as they were arranged by Dr Martine in his valuable Effay on the Conftruction and Graduation of Thermometers, and at the fame time adding thofe of Celfus and De Luc. We call it by the name of De Luc for the fake of diftinguishing it from Reaumur's ipirit of wine thermometer, which may be feen in the fame Plate.

It is unneceffary to describe any of these more minutely, as they are no longer used. Those who wish to read a more particular account of them may confult Dr Martine's Efdays.

30

thermome

ters.

As in meteorological obfervations it is neceffary to attend Account to the greatest rife and fall of the thermometer, attempts of felf-regihave been made to construct a thermometer which might erg regifter the greatest degree of heat, or greatest degree of cold, which took place during the abfence of the obferver. In 31 1757 Lord Charles Cavendish prefented to the Royal So. Lord ciety of London a thermometer in two different forms; the Charles one contrived to mark the greatest degree of heat, and the thermome other the greatest degree of cold.

The firft confifts of a glafs tube AB, with a cylindrical bulb B at the lower end, and capillary at the top, over which there is fixed a glafs ball C. The bulb and part of the tube are filled with mercury, the top of which shows the 3 R

meter.

the index in the right hand tube rifes and marks the great Thermo eft heat.

Thermo- degrees of heat as ufual. The upper part of the tube above the mercury is filled with fpirit of wine; the ball C is alfo filled with the fame liquor almoft to the top of the capillary tube. When the mercury rifes the fpirit of wine is alfo raised, and falls into the ball C, which is fo made that the liquor cannot return into the tube when the mercury finks; confequently the height of the fpirit of wine in the ball, added to that in the tuke, will give the greateft degree of heat to which the thermometer has pointed fince lait obfer vation. When a new observation is to be made, the inftrument must be inclined till the liquor in the ball cover the end of the capillary tube.

3%

Mr Six's thermome

ter.

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

The thermometer for fhowing the greateft degree of cold is reprefented in fig. 4. by the crooked tube ABCD. This inftrument is filled with fpirit of wine, with the addition of as much mercury as is fufficient to fill both legs of the fyphon, 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 fpirit of wine. The degrees of heat are shown by the rife 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 finks by cold, that in the fhorter will rife and run over into the ball C, from which it cannot return when the mercury fubfides in the fhorter and rifes in the longer leg. The upper part of the fhorter leg will therefore be filled with a column of fpirits of a length proportional to the increase of heat; the bottom or lower furface of which, by means of a proper fcale, will show how much the mercury has been lower than it is; which being fubtracted from the prefent height will give the lowest point to which the mercury has fallen. That the thermometer may be fitted for a new obfervation, the mercury must be made to run back from the ball into the fhorter leg, by inclining the tube and heating the ball,

In 1782 Mr Six propofed another felf-regiftering ther mometer. It is properly a fpirit of wine thermometer, Fig. 5. though mercury is alfo employed for fupporting an index. ab is a thin tube of glafs 16 inches long, and ths of an inch caliber: cde and f g h are smaller tubes about th of an inch caliber. These three tubes are filled with highly rec tified fpirit of wine, except the fpace between d and which is filled with mercury. As the fpirit of wine contracts or expands in the middle tube, the mercury falls or rifes in the outside tubes. An index, fuch as that reprefented in fig. 6. is placed on the furface, within each of thefe tubes, fo light as to float upon it. k is a small glafs tube 4ths of an inch long, hermetically fealed at each end, and inclofing a piece of fteel wire nearly of its own length. At each end /, m, of this fmall tube, a fhort tube of black glafs is fixed, of fuch a diameter as to pals freely up and down within either of the outside tubes of the thermometer ce or fh. From the upper end of the index is drawn a fpring of glafs to the fineness of a hair, and about 4ths of an inch long; which being placed a little oblique, preffes. lightly against the inner furface of the tube, and prevents the index from defcending when the mercury defcends. Thefe indexes being inferted one into each of the outfide tubes, it is eafy to understand how they point out the great eft heat or cold that has happened in the obferver's abfence. When the fpirit of wine in the middle tube expands, it preffes down the mercury in the tube bf, and confequently raifes it in the tube ec; confequently the index on the left hand tube is left behind and marks the greateft cold, and

meter.

In 1790 a paper was given into the Royal Society of 33 Edinburgh, defcribing two thermometers, newly invented, Rutherby Dr John Rutherford of Middle Bailish; the one for re-ford's ther giftering the highest and the other for registering the low-mometer. eft degree of heat to which the thermometer has rifen or fallen during the abfence of the obferver. An account of them may be found in the third volume of the Transactions of the Society.

34

thermome

A new feif-registering thermometer has more lately been Mr Keith's invented by Mr Keith of Ravelftone, which we confider as ter. the most ingenious, fimple, and perfect, of any which has hitherto appeared. Its fimplicity is fo great, that it requires only a very fhort defcription to make it intelligible.

This is

AB is a thin glafs tube about 14 inches long and ths of Fig. 7. an inch caliber, close or hermetically fealed at top. To the lower end, which is open, there is joined the crooked glass tube BE, feven inches long, andths of an inch caliber, and open at top. The tube AB is filled with the ftrongest fpirit of wine, and the tube BE with mercury. properly a spirit of wine thermometer, and the mercury is ufed merely to fupport a piece of ivory or glass, to which is affixed a wire for raising one index or depreffing another, according as the mercury rifes or falls. E is a small conical piece of ivory or glafs, of fuch a weight as to float on the furface 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 a, moves eafily along the small harp fichord wire GK. LL are two indexes made of thin black oiled filk, which flide upwards or downwards with a force not more than two grains. The one placed above the knee points out the greatest rife, and the one placed below it points out the greateft fall, of the thermometer.

When the inftrument 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 rifes, the float and float wire, which can be moved with the fmalleft force, will be pushed upwards till the mercury become ftationary. As the knee of the float-wire moves upwards it will carry along with it the upper index L. When the mercury again fubfides, it leaves the index at the highest point to which it was railed, for it will not defcend by its own weight: As the mercury. falls the float-wire does the fame; it therefore brings along with it the lower index L, and continues to deprefs it till it again become stationary or afcend in the tube; in which cafe it leaves the lower index behind it as it had formerly left the upper. The fcale to which the indexes point is placed parallel to the flender harpfichord wire. It may be seen more diftin&tly in fig. 8. That the scale and indexes may not be injured by the wind and rain, a cylindrical glafs cover, clofe at top, and made fo as to exactly fit the part FG, is placed over it.

The ingenious inventor has another improvement in con templation, which, if upon trial it be found to anfwer, will make this thermometer as perfect as can be defired, provided there do not arise fome errors from the variable preffure of the atmosphere. He proposes to adopt clock-work to this thermometer, in fuch a way as to register with the ut moft precifion 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 fhall forbear to defcribe, hoping that the author himself, after his experiment has met with the fuccefs which we ardently with, will favour the world with his own account of it.

Thermo

meter.

35

M. De

Luc's fup

The fame ingenious gentleman has invented a felf-registering barometer, upon the fame principles with his felf-regiftering thermometer. We have had the pleasure of fecing both; and are convinced that they will fully gratify the withes of all who are engaged in meteorological ftudies. He is alfo in expectation of being foon able to produce an airthermometer 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 preffure of the atmosphere. M. De Luc-has described the beft method of construct pofed im- ing a thermometer, fit for determining the temperature of the air, in the menfuration of heights by the barometer. He has also shown how to divide the scale of a thermometer, fo as to adapt it for aftronomical purposes in the observation of refractions. Mr Caval- Mr Cavallo, in 1781, propofed the conftruction of a lo has pro- thermometrical barometer, which, by means of boiling wapofed a thermometer, might indicate the various gravity of the atmosphere, trical baro- or the height of the barometer. But as he does not fay meter. that the inftrument has been tried with the defired fuccefs, we forbear to describe it. Those who wish to know his ideas refpecting it may confult the Philofophical Tranfactions, vol. Ixxi. p. 524.

prove

ments.

36

37 The thermometers

defcribed above too

limited.

38

Sir Ifaac

Newton's

the scale of the thermometer.

Martine's Elays.

The thermometers hitherto described are very limited in their extent; they indeed point out to us the loweft 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 fure that the heat of folid bodies, when heated to ignition, or till they emit light, far exceeds the heat of boiling mercury.

In order to remedy this defect, Sir Ifaac Newton, whofe method of genius overcame those obstacles which ordinary minds could extending not approach, attempted by an ingenious experiment to extend the scale to any degree required. Having heated a mafs of iron red-hot, and expofed it to the cold air, he obferved the time which elapfed till it became cold, or of the fame temperature with the air; and when the heat fo far decreased that he could apply fome known measure (as a thermometer) to it, he obferved the degrees of heat loft in given times; and thence drew the general conclufion, that the quantities of heat loft 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 progreffion, the decrements of the heat of the iron would be continually proportional. Ha ving by this proportion found out the decrements of heat in a given time after it came to a known point, it was eafy, by carrying upwards the fame proportion to the beginning of its cooling, to determine the greatest heat which the body had acquired. This proportion of Sir Ifaac's was found by Dr Martine to be fomewhat inaccurate. The heat of a cooling body does not decrease exactly in proportion to that which the body retains. As the refult of many obfervations, he found that two kinds of proportion took place, an arithmetical as well as the geometrical proportion which Sir Ifaac Newton had adopted; namely, that the decrements of heat were partly proportional to the times (that is, that quantities of heat are loft 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 fufficiently accurate. By the geometrical proportion which Sir Ifaac Newton adopted he discovered the heat of

metals red-hot or in fufion.

meter.

39

thermome.

This method, fo fuccefsfully purfued by Sir Ifaac, was Thermofufficient to form a scale of high degrees of heat, but was hot convenient for practical purpofes. Accordingly the ingenious Mr Jofiah Wedgwood, who is well known for Mr his great improvement in the art of pottery, applied himself Wedgin order to discover a thermometer which might be eafily wood's managed. After many experiments recorded in the Phi- ter for lofophical Tranfactions, but which it is unneceffary to detail measuring in this place, he has invented a thermometer which marks high dewith much precifion the different degrees of ignition from grees of a dull red heat vifible in the dark to the heat of an air-heat furnace. This thermometer is extremely fimple. It confifts of two rulers fixed upon a fmooth flat plate, a little farther afunder 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 fuch a fize as just to enter at the wide end; they are then heated in the fire along with the body whofe heat we wish to determine. The fire, according to the degree of heat it contains, diminishes or contracts the earthy body, so that when applied to the wide end of the gage, it will flide on towards the narrow end, lefs or more according to the degree of heat to which it has been exposed.

40

That this inftrument may be perfectly underflood, we Described. have given a reprefentation of it in Plate DVI. fig. 9. ABCD is a fmooth flat plate; and EF and GH two rulers or flat pieces, a quarter of an inch thick, fixed flat upon the plate, with the fides that are towards one another made perfectly true, a little farther afunder at one end EG than at the other end FH: thus they include between them a long converging canal, which is divided on one fide into a number of fmall equal parts, and which may be confidered as performing the offices both of the tube and scale of the common thermometer. It is obvious, that if a body, fo ad- Philofopbijufted as to fit exactly at the wider end of this canal, be after-cal Tranf wards diminished in its bulk by fire, as the thermometer actions, vol. pieces are, it will then pafs further in the canal, and more Ixxiv. and more fo according as the diminution is greater; and conversely, that if a body, fo adjusted as to país on to the narrow end, be afterwards expanded by fire, as is the cafe with metals, and applied in that expanded state to the scale, it will not pafs fo far; and that the divifions on the fide will be the measures of the expanfions of the one, as of the contractions of the other, reckoning in both cafes from that point to which the body was adjusted at first.

I is the body whofe alteration of bulk is thus to be mea fured. This is to be gently pushed or flid along towards the end FH, till it is topped by the converging fides of the canal.

41

3 R

papers