THERMOMETER. 



an inch, anfwers to a,tl. part of ti.e breadth of the httle 

 piece of chv. When one gage .s accurately adjulted to 

 the proportional meafures above Hated, two pieces of brals 

 (hould be made, one fitting exaaiy ii.to one end and the 

 other into the other; which w.U fcrve as ftandards for the 

 nidv adjuftmeut of other gages to the dnneniions of the 

 ongi'nal, and thus we may be affured, that thermometers on 

 this principle, though made by different perfons, and m 

 different countries, wiU all be equally affeftcd by equal 

 degrees of heat, and all fpeak the fame language, ihe 

 fcale commences at a red heat fully vifible m day hght ; 

 and the grtratell heat which Mr. Wedgwood has hitherto 

 obtained in his experiments, is i6o=. Swedifh copper has 

 been found to melt at 27", fiWer at 28', and gold at 32°, 

 of this thermometer. Brafs is in fufion at 21°: the 

 welding heat of iron is from 90" to 95^ and the greateft 

 heat that could be produced in a common fmith's forge, 

 1 25''. Caft iron melted at 130° ; and the heat by which 

 iron is run down among the fuel for carting is 150°. A 

 Heffian crucible melted ir.to a flag-hke fubftance at about 

 1 50'. The fonding heat of glafs furnaces, or that by which 

 the perfeft yitrifications of the materials is produced, was 

 at one of them 1 14° for flint-glafs, and 124° for plate-glafs. 

 Delft-ware is fired by a heat of 40° or 41'^ ; Queen's- ware 

 by 86°; and ftone-ware by 102°, which degree of heat 

 changes it to a true porcelain texture. The thermometer 

 pieces begin to acquire a porcebin texture at about 1 10°. 

 A piece of an Etrufcan vafe melted completely at 33° ; 

 pieces of other vafe and Roman ware about 36^ ; Worcefter 

 china vitrified at 94° ; Mr. Sprimont's Chelfea china at 

 105"; the Derby at 112°; the Bow at 121°; but Briftol 

 china (hewed no appearance of vitrification at 135°. The 

 common fort of Chinefe porcelain does not perfeftly vitrify 

 by any fire which Mr. Wedgwood could produce : but 

 began to foften about 120°, and at 156° became fo foft as 

 to fink down and apply itfelf cloie upon an irregular furface 

 underneath. The true ftone Nankeen does not foften in 

 the leaft, by this ftrong heat ; nor even acquire a porcelain 

 texture. The Drefden porcelain is more refraftory than 

 the common Chinefe, but not equally fo with the ftone 

 Nankeen. The cream-coloured or Queen's-ware bears the 

 fame heat as the Drefden. Mr. Pott fays, that to melt a 

 mixture of chalk and clay in certain proportions, which 

 appear from his tables to be equal parts, is " among the 

 mafter-pieces of art." This mixture melts into a perfeft 

 glafs at 123' of this thermometer. For other curious par- 

 ticulars, fee Phil. Tranf. vol. Ixxii. part ii. p. 305, &c. 



This thermometer, fays Dr. Young, (Left, on Nat. 

 Philof. vol. i. 648.) may be extremely ufeful for identifying 

 the degree of heat which is required for a particular pur- 

 pofe ; but for the comparifon of temperatures by an ex- 

 tcnfion of the numerical fcale, we have not foificient evidence 

 of its accuracy to allow us to depend on its indications ; and 

 it is fcarcely credible, that the operation of furnaces of any 

 kind, can produce a heat of fo many thoufand degrees of a 

 natural fcale, as Mr. Wedgwood's experiments have led him 

 to fuppofe ; nor is the fuppofilion confiftent with the ob- 

 fervations of other philofophcrs. 



Thermometee, Differential, a curious fort of thermo- 

 pneter invented by proteffor Leflie, which exprefles not the 

 abfolute degree of heat, but the difference, when any exifts, 

 between the temperatures of the two fpots where its two 

 bulbs are placed. The method of conftrufting it is as 

 follows. {^K PlateXVl. Pneumatics, Jig. l-j.) Seleft two 

 thermometer tubes with bores rather wider than ufual, and 

 one a Uttle wider than the other. Let the balls be blown 



as equal as the eye can judge, and from .4 to .7 of an iiicli 

 in diameter, and let the open end of the tube alfo be widened 

 in a flight degree. The tubes muft be of unequal length, 

 the longeil being nearly twice the length of the other. Then 

 introduce into the longer tube a little fulphuric acid tinged 

 with carmine, fufficient to fill about an inch oi its cavity ; 

 join the two tubes together by the blow-pipe, and when 

 joined, bend them in the form of the letter U, with the 

 bulbs about three or four inches afundcr, making one 

 flexure juft below the junfture of the two tubes, where the 

 fmall cavity (which is reprefented in the plate) facilitates 

 the adjuftment of the inftrument, which by a little dexterity 

 is performed by forcing a few globules of air by the heat 

 of the hand from one bulb to the other. Attach a gra- 

 duated fcale to the (horter tube, making the zero about the 

 middle of it, and adjuft. to it the quantity of air in each bulb, 

 fo that when the bulbs are at the fame temperature, the 

 upper furface of the coloured liquor may juft correfpond 

 with the zero. Sulphuric acid is chofen as the liquor in- 

 terpofed between the bulb, on account of its bearing any 

 heat or cold that would be ufed witliout being evaporated 

 or congealed. 



In this inftrument the air inclofed in the bulbs is the fub- 

 ftance, which, by its expanfion or contraftion, caufes the 

 motion of the coloured liquor up or down the fcale, and as 

 gafes are much more expanfible than liquids, the inftrument 

 is fooner affefted by minute changes of heat. But as the 

 two bulbs are of equal fize, and both filled with air, and 

 feparated from each other by the intervening liquor, it is 

 obvious that when the temperature is the fame in each bulb, 

 be it high or low, the preffure on each fide of the liquor is 

 alfo equal, and it muft remain ftationary : fo that it can only 

 move when one bulb is warmer than the other. Hence the 

 particular and fole ufe of this inftrument as a differential 

 thermometer. The lower part of the inftrument (or the 

 fpace included between the two bends) is cemented to an 

 upright ftera, by which it is fupported. 



This inftrument has been employed by the inventor in a 

 variety of curious experiments on caloric, or the matter of 

 heat. The peculiar advantage which this inftrument pof- 

 feffes is, that, bcfides its extreme fenfibility, in ufing it the 

 common temperature of the furrounding air may, in gene- 

 ral, be difregarded ; this being always the zero of the fcale, 

 whatever be the aftual variation of heat in the furrounding 

 atmofphere ; and hence a much greater degree of fimplicity 

 is introduced into the delicate refearcheson this fubjeft. For 

 the refleftion of heat mirrors were employed, generally of 

 block-tin, highly polifhed, and hammered to fit a wooden 

 gage, the fegment of a parabolic curve, by which much of 

 the difperfion produced by a fimple concave form was 

 avoided ; fo that when expofed to the direft rays of the 

 fun, they collefted them into a pretty diftinft focus, of 

 about half an inch in diameter. The fubftance employed to 

 generate the radiant heat was a hollow cubic tin canifter, 

 placed direftly in front of the mirror of its focal point, 

 and when ufed, filled vrith boiling water, and fitted with a 

 common thermometer, pafGng through a hole in the cover, 

 and immerfed in the water. The cubical form of the ca- 

 nifter allowed of four fides, of perfeftly equal dimenfions, 

 each of which, when turned to the mirror, afforded a 

 heated furface for the tranfmifllon of radiant caloric, and' 

 they were occafioiially coated with various fubftances to 

 afcertain the effeft of colour, pohih, and the hke, in re- 

 tarding or promoting the radiation of the heat within. 

 With this apparatus, and his differential thermometer, 

 Mr. Leflie performed a variety of interefting experiments. 



