VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS, 51 



than the second, for which reason the latter set should not be continued within 

 those limits. I would willingly have given rules for the construction of this 

 second set of divisions, but am obliged to omit it, as it cannot be done properly 

 without first determining, by experiment, how much the quicksilver in the tube 

 is heated by immersing the ball in hot liquors. 



In a spirit thermometer, the error proceeding from the fluid in the tube being 

 not of the same heat as that in the ball, is much greater ; as spirits of wine 

 expand much more by heat than quicksilver: for which reason spirit thermome- 

 ters are not so proper for trying the heat of liquors, as those of quicksilver. 

 Another circumstance which ought to be attended to, in adjusting the boiling 

 point of a thermometer is, that the ball should not be immersed deep in the 

 water; for if it is, the fluid which surrounds it will be compressed by consider- 

 ably more than the weight of the atmosphere, and will therefore acquire a 

 sensibly greater heat than it would otherwise do. Mr. C. here describes a vessel 

 to inclose the thermometer in, to adjust that point, with a chimney to carry off 

 the steam. If such a vessel as this is used, the thermometer will be found to stand 

 not sensibly higher when the water boils vehemently, than when it boils gently ; 

 and if the mouth of the chimney is covered by any light body, in such manner 

 as to leave no more passage for the steam than what is necessary to prevent the 

 body from being blown ofi^ by the pressure of the included vapour, the ther- 

 mometer will stand only half or three quarters of a degree higher, if the ball 

 is immersed a little way in the water, than if it is exposed only to the steam. 

 But if the covering of the chimney is removed, the thermometer will immedi- 

 ately sink several degrees, when the ball is exposed only to the steam, at least 

 if the cover does not fit close ; whereas when the ball is immersed in the water, the 

 removal of the covering has scarcely any effect on it. Whence it appears, that 

 the steam of water boiling in a vessel, from which the air is perfectly excluded, 

 is a little, but not much, cooler than the water itself, but is considerably so if the 

 air has the least admission to the vessel. Perhaps a still more convenient 

 method of adjusting the boiling point would be, not to immerse the ball in the 

 water at all, but to expose it only to the steam, as thus the trouble of keeping 

 the water in the vessel to the right depth would be avoided ; and besides, several 

 thermometers might be adjusted at the same time, which cannot be done with 

 proper accuracy when they are innnersed in the water, unless tiie distance of 

 the boiling point from the ball is nearly the same in all of them. At present 

 there is so little uniformity observed in the manner of adjusting thermometers, 

 that the boiling point, in instruments by our best artists, difl^er from each 

 other by not less than 2|» ; owing partly to a difference in the height of the 

 barometer at which they were adjusted, and partly to the quicksilver in the tube 

 being more heated in the method used by some persons, than in that used by 



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