THERMOMETER. 



Ic 



der ftanding pirallcl to the logs of the fiphon, and pointing 

 downwards. This cylinder contains the greatell part ot the 

 fluid. *nd is added only to make the thermometer more 

 fenlible than it xvould be, if the ball A was made ot a fut- 

 ficient nze to contain the proper quantity of fluid. 1 his 

 inllrument is filled with fpirit of wine, with the addition of 

 as much mercury « is fufficient to till both legs of the 

 Ijphon, and about a fourth or fifth part of the ball A. 1 he 

 common degrees of heat arc (hewn by the top of the mer- 

 cmt in tt"- longell leg, or by the top of the ipmt, in caie 

 any of it is left above the mercury. When the mercury in 

 the longcft leg finks by cold, that in the fhortcr leg will 

 rife, ail5 will run over into the ball A ; from whence it can- 

 not return back when the th.^rmometor rifes again, as the 

 furface of the mercury in the ball is below the orifice of the 

 tube n. Therefore the upper part of the Ihorter leg will be 

 filled with a column of fpirits of a length proportional to 

 tlie increafe of heat ; the bottom of which, by means of a 

 jroper fcalc, will (hew how much the tlicrmometer has been 

 .ower than it is ; which being fubtraftcd from the prefent 

 height, wnll j2,ive the lowed point that it has been at. In 

 order to prevent the mercury from falling into the ball A in 

 large drops, which would affeft the accuracy of the inftru- 

 ment, the top of the fliorter leg, clofe to the ball, is con- 

 trafted, by being held in the flame of a lamp, and the paf- 

 fage farther ftraightened by a folid thread of glafs placed 

 within the tube, and extending from the bottom of the 

 (hortcr leg to the part near the ball A, where it is moft con- 

 trafted. By this means, as foon as any fmall portion of mer- 

 cury is got beyond the thread of glafs, it breaks off, and 

 falls into the ball in very fmall drops. 



In order to fill the fliortor leg v.'ith mercury, for a new 

 expAriment, it mull be inclined till the mercury in the ball 

 covers the orifice of the tube n. The cylinder being then 

 heated, the mercury will be forced into the fhorter leg, and 

 will run down the thread of glafs in drops, which will foon 

 unite. Thus fuch a quantity of mercury mud be got into 

 the fhorter leg, as, upon the cooling of the inllrument, will 

 be fufficient to drive all the fpirit of wine into the ball, with 

 a lefs degree of cold than what the thermometer is likely to 

 be expofed to. The ball A mud always have fome mercury 

 in it, but never enough to fill it up to the orifice of the 

 tube n. It will be bed to leave a little of the fpirit above 

 the mercury in the longed leg ; in which cafe the top of the 

 fpirit will fhew the common degrees of heat. The Icale of 

 degrees on the (liorter leg will, in different feafons, be liable 

 to an error fimilar to that which was explained in the fird 

 mentioned thertnometcr ; but it will be lefs confiderable, as 

 the fpace between the two fcales is filled with mercury, 

 whofe expanfion is about fix times lefs than that of the fpirit 

 of wine. In the thermometer now defcfibed, the bore of the 

 tube is about c.054 inches ; and one inch of it contains 

 eight grains of mel-cury, and anfwers to feven degrees of 

 Fahrenheit's fcale. The drops of mercury which fall into 

 the ball A, anfwer to about one-eighth of a degree. 



Inftruments of this kind, with fome alteration in their 

 condruftion, would ferve for finding the temperature of the 

 fea at great depths, and alfo for finding that of the air at 

 confiderable heights. Lord Charles Cavendidi has ftiewn 

 how to adapt them for fuch purpofes. See Phil. Tranf. 

 vol. 1. art. 38. p. 300, &c. 



Since the publication of Mr. Canton's difcovery of the 

 compredibility (fee Compression) of fpirits of wine and 

 other fluids, there are two correftions necedary to be made 

 in the refult given by lord Charles Cavcndidi'S thermometer. 

 For in eftimating, e. g. the temperature of the fea at any 

 depth, the thermometer will appear to have b^n colder than 



6 



it rcallv was : and befidet, the expanfion of fpirits of wine 

 1)V any' given number of degrees of Fahrenheit's thermome- 

 ter is o-reater in the higher degrees than in the lower. For 

 the metliod of making thefe two corredlions by Mr. Caven- 

 didi, fee Phipps's Voyage to the North Pole, p. 14^. 



Indruments of this kind, for determining the degree of 

 heat or cold in the abfence of the obferver, have been in- 

 vented and defcribed by others. Van Swinden (Did. fur la 

 Comparifondu Therm, p. 253 — 255.) defcribes one, which, 

 he fays, was the fird of the kind made on a plan communi- 

 cated by M. Bernouilli to M. Leibnitz. M. Kraft, he 

 alfo tells us, made one nearly like it. Mr. Six, in 1782, 

 propofed another condruftion of a thermometer of the fame 

 kind, which has been well received. 



This is properly a fpirit thermometer, though mercury is 

 employed in it for the purpofe of fupporting a certain in- 

 dex : a b {fig- 14. ) is a tube of thin glafs, about fixteen 

 inches long, and rVths of an inch in diameter; cdifgh is 

 a fmaller tube, with the inner diameter about -^^th, joined 

 to a larger al the upper end b, and bent down firft on the 

 left fide, and then, after defcending two inches below a b, 

 upwards again on the right, in the feveral direftions Ci/ir, 

 fgh, parallel to, and one inch didant from it. At the end 

 of the fame tube at /;, the inner diameter is enlarged to half 

 an inch from h to ?, which is two inches in length. This 

 glafs is filled with highly reftified fpirit of wine to within 

 half an inch of the end /', excepting that part of the fmall 

 tube from d to g, which is filled with mercury. From a 

 view of the indrument it will be readily conceived, that when 

 the fpirit in the large tube is expanded by heat, the mercury 

 in the fmall tube on the left fide will be preffed down, and 

 caufe that on the right fide to rife : on the contrary, when 

 the fpirit is condenfed by cold, the reverfe will happen. 

 Fahrenheit's fcale, which begins with o at the top of the 

 left fide, has the degrees numbered downwards, while that 

 at the right fide, beginning with o at the bottom, afcends. 

 The divifions are afcertained by placing the thermometer 

 with a good ftandard mercurial one in water, gradually heat- 

 ing or cooling, and marking the divifions of the new fcale 

 at every five degrees. The divifions below the freezing point 

 are taken by means of a mixture of fea-falt and ice, as de- 

 fcribed by ISIollet, De Luc, and others. In order to (hew 

 how high the mercury has rifen in the obferver's abfence, 

 there is placed within the fm.all tube of the thermometer, 

 above the furface of the mercury on either fide, immerfed in 

 the fpirit of wine, a fmall index, fo fitted as to pafs up and 

 down as occafion may require. One of thefe indices is re- 

 prefented in^^. 15; a is a fmall glafs tube, three-quarters 

 of an inch long, hermetically fealed at each end, inclofing 

 a piece of deel wire nearly of the fame length ; at each end 

 c, d, is fixed a fliort piece of a tube of black glafs, of fuch 

 a diameter as to pais freely up and down within the fmall 

 tube of the thermometer. The lower end, floating on the 

 furface of the mercury', is carried up with it when it rifes, 

 while the piece at the upper end, being of the fame diame- 

 ter, keeps the body of the index parallel to the fides of the 

 thermometrical tube. From the upper end of the body of 

 the index at c is drawn a fpring of glafs to the finenefs of a 

 hair, about five-fourths of an inch in length, which being 

 fet a little oblique, preffes lightly againd the furface of the 

 tube, and prevents the index from foUowing the mercury 

 when it defcends, or being moved by the fpirit palling up 

 and down, or by any fudden motion given to the indru- 

 ment ; but at the fame time the predure is fo adjulled as to 

 permit this index to be readily carried up by the furface of 

 the rifing mercury, and downwards, whenever the inilru- 

 infent fs testified for obfervation. This index, by not re- 



turnincr 



