THERMOMETER. 



-d, 



dn 



. Confequeiitly the ball is equal to as many 

 dm + dn 



cylinders having a diameters of the tube, for their refpeftive 

 height, and i diameter for the bafe, as are contained in this 

 laft quotient ; and, therefore, its cylindric folidity ex- 

 prefTed in the cyhndric fohdities of the tube will be = a x 



r m — dm — d n , , r . • . ., • 



r ; . i3ut tlie diameter of this ball is equal to 



dm -^ dn ^ 



thebafeof thecylindor in whicliit maybe iiifcribed,and the fo- 

 hdity of thiscyhnder is equal to Jds the folidity of the circum- 

 fcribing fphere. Confequently the folidity of this cyhiider 



MI . , cm — dm — dn , , ,• r 



wiU be = + a X -r" 1 > "Uid the diameter or 



dm -^^ an 



its bafe equal to the diameter of the ball, will be = 

 -77 



■) / f I 



dm + dn 



sj la 



d y. m + n 



It is evident that, ceteris paribus, the larger the bulb is, in 

 proportion to the diameter of the cavity of the tube, or 

 tiie narrower the latter is in proportion to the former, the 

 greater will the motion of tlic furface of the fluid be in the 

 tube. But it mull be obferved, that when the bulb is very 

 large, the thermometer will not eafily arrive at tlie precile 

 temperature of any place, wherein it may be fituated. 

 Some perfons, in order to give the bulb a greater furface, 

 and of courfe to render it more capable of readily attaining a 

 given temperature, have made it not globular, but cylin- 

 drical (which ihape was adopted by Fahrenheit), or flat, or 

 bell-like, &c. ; but thofe fliapes are improper, becaufe they 

 are liable to be altered by the varying gravity of th^ atmo- 

 fphere, confequently thofe thermometers cannot be ac- 

 curate. The bulb fltould be clean and colourlefs ; fince 

 coloured furfaces are apt to be partially heated by a ftrong 

 light. If you take two equal thermometers, and paint the 

 bulb of one of them black, or of any dai'k colour, and ex- 

 pofe them both to the fun ; the mercury in that whofe bidb 

 is painted will rife feveral degrees higher than in the other : 

 even a ftrong day-light, independently of the diretft rays of 

 the fun, will aflfeft them differently. The ball of the ther- 

 mometer ftiould not be in contaft with the fubftance of the 

 fcale, left it (hould be influenced by the temperature of that 

 fubftance. 



When a proper tube and ball are procured, and their pro- 

 portion afcertained, the next objedl which requires peculiar 

 attention is that of fiUing the thermometer. For this pur- 

 pofe the tubes Ihould be clean and dry, and the mercury 

 very pure. (See Mercury and Barometer.) The mer- 

 cury may be introduced into the tube by means of a kind of 

 refervoir fixed at th^.top of it, and pr,oportioned in fize to 

 the bulk of the ball, oi Sy rolling upon the tube a (lip of 

 fine paper, about two or thi:* inches broad. In order to 

 clear the tube of its air and man 'ire, it ftiould be held over 

 a gentle fire, fo difpofed, as that i. may heat at once the 

 whole extent of the tube, till its heat^^comes too great for 

 the operator's hand to bear, who therel>.-e ufes a glove or 

 nippers for this purpofe ; care being taken "hat the ball is 

 not heated at the fame time. After the incloied air is thus 

 rarefied, and the particles that might obftruft the free motion 

 of the mercury are made to float in vapours within the bore 

 of the tube, the tube is to be held upright, and the ball fud- 

 denly heated, by which means the air contained in it will be 

 dilated, and carry off the impurities of the tube, fo that it 

 will be rendered clean and free from air. When the ball is 

 heated to a coniidcrable degree, the mercury may be poured 

 Vol. XXXV. 



into the refervoir fixed at the top of the tube, through a 

 fmall corner of the paper. When the refervoir is almoft 

 full, the ball ftiould be withdrawn from the fire, and the air 

 will then be condenfed, and the fpace left by it will be foon 

 occupied by the mercury. By alternately heating and cool- 

 ing the ball, it may thus be filled with mercur\' ; but when 

 it is nearly full, the mercury contained in it muft be made to 

 boil, by placing it over burning coals, in order to purge it 

 of its air. However, as a fmall quantity of air will be left 

 in the ball after this operation, it will be expedient to re- 

 move the mercury, which remains in the refervoir, imme- 

 diately after the thermometer is withdrawn from the fire ; 

 and thus the whole column, unfupplied with mercury from 

 the refervoir, will defcend into the ball by the condenfation 

 of that which is contained in it, and the tube being empty, 

 the fmall bubble of air will efcape. Let the tube be again 

 heated fucceffively through its whole length, commencing 

 from the bottom, and preferving the heat of the ball, that 

 the mercury may occupy it entirely, and no air be allowed 

 to enter. During this operation, when the mercury of the 

 thermometer begins to appear in the refervoir, let the mer- 

 cury contained in a paper funnel be poured into it in fuch a 

 quantity as will more than fill the thermometer, which is 

 then to be removed from the fire. The mercury of the 

 tube, and that diicharged from the funnel, will unite, and 

 pafs together into the thermometer, and thus it will be 

 wholly filled. In this ft ate it may be left for any time at 

 pleafure, without any apprehenfion of its imbibing either air 

 or moifture. Nothing now remains but to get rid of the 

 fuperfluous quickfilver, and to fealthe tube. For this pur- 

 pofe the thermometer is held in the hand and heated, till a 

 drop of mercury falls out of it, and is then left to recover 

 the temperature of the air ; by which means there will re- 

 main at the top of the tube a fmall empty fpace. Then 

 with a blow-pipe and the flame of a candle, let the end of 

 the tube be formed into a fine point, of fuch a length as 

 will admit of its being properly fealed. When this prepa- 

 ratory procefs is completed, let the thermometer be gradually 

 plunged into boiling water, fo that the fuperfluous mercury 

 may ifl'ue from it flowly ; and when it ceafes to be dif- 

 charged, withdraw the thermometer from the boiling water ; 

 wipe it dry, and as foon as poflible, put the ball of it over 

 a fmall fire, covered with afties, and previoufly prepared for 

 the purpofe. In this part of the operation, it is neceffary 

 to be quick, that the mercury may not have time to con- 

 denfe, and the air enter into the tube. In this ftate the 

 thermometer may be left to lieat, till it parts with more or 

 fewer drops of the mercury, according to the proportion 

 which the length of the tube bears to that of the fcale ap- 

 plied to it. The thermometer is then fealed, by melting 

 only the end of the point above mentioned, and at the fame 

 inftant withdrawing it from the fire. 



The method of filling the thermometer with a paper tqbe, 

 or funnel, is as follows. Let the ball be heated, fo that 

 the mercury may rife to the top of the tube ; whilft it ap- 

 proaches it, apply the tube of paper to the end of the tube, 

 fo that it may ferve for a refervoir. The thermometer be- 

 ing placed near the fire, fo that it may always preferve the 

 fame degree of dilatation, take fome well purified mercury 

 in a paper cornet, and communicate a little more heat to 

 the ball. When the mercury rifes, and forms a fmall drop 

 at the end, poQr the mercury of the cornet into the refer- 

 voir of paper, and withdraw the ball from the fire. Hav- 

 ing removed the paper refervoir, place the ball again over 

 the fire, and leal the point of the tube at the moment 

 when the mercury rifes to it, and withdraw the thermometer 

 from the fire. This operation will be acquired by ufe. 



3 T Thermo- 



