VOL. XVII.] PHILOSOPHICAL TRANSACTIONS. 607 



sible of a gentle warmth, and also not subject to evaporate without a good; 

 degree of fire, might most properly he applied to the construction of thermo- 

 meters, were its expansion more considerable. However, small as it is, it is 

 sufficient to disturb the precise nicely of the mercurial barometers, showing the 

 counterpoise of the pressure of the atmosphere by a cylinder of mercury ; (ota 

 if mercury be more expanded, and consequently lighter, in warm weather than 

 in cold, it will necessarily follow, that the same weight of atmosphere will 

 require a taller cylinder in summer, and a shorter in winter to counterpoise it*- 

 And if the extremity of weather do but occasion an 150th part difference, as 

 it is probable it does, its effect on a barometer will be a 10th of an inch, above 

 and below the mean, or a 5th in the whole. 



3. I filled the smaller bolt-head with spirits of wine, and having set it in the 

 skillet of water over the fire, I found that it ascended gradually as the beat 

 increased, but slower at first, and faster after it was well warm. At length, 

 being arrived at a certain degree of heat, it would then boil with great violence, 

 emitting bubbles, which rising into the neck of the bolt-head, would lift all 

 the incumbent spirit, till they had made their way through. And these suc- 

 ceeding one another very fast, would often raise the spirits to the top of the 

 neck, and spill it; so that I found I could go no farther with this liquor, than 

 to that degree of heat which occasioned this boiling, and which wanted very 

 much of that of boiling water, being almost tolerable to the touch. It was how- 

 ever very remarkable, how exactly this degree of heat was determined by the 

 expansion of the spirits; for in the instant it reached a certain mark on the 

 neck, it began to emit its bubbles; and having been taken out a little to cool 

 and subside, it would certainly and constantly fall a bubbling again, when on a 

 second immersion, it was arrived at the aforesaid mark. During this experi^ 

 ment it appeared, both by the dew on the neck, and by the scent in the room, 

 that though the neck was about 30 inches long, yet the spirits evaporated very 

 fast, considering the smallness of the surface of the liquor. And I have often 

 noted the like evaporations condensed in dew within the head of the ordinary 

 sealed thermometers, in very hot weather. 



This degree of heat, which caused spirits of wine to begin to boil, being deter- 

 mined so nicely, made me conclude, that this might very properly be taken 

 for the limit of the scale of heat in a thermometer; and its effect in the ex- 

 pansion of any other fluid being accurately noted, might be easily transferred to 

 any sort of thermometer whatever. Only it must be observed, that the spirit 

 of wine used to this purpose, be highly rectified or dephlegmated ; for other- 

 wise the different goodness of the spirit will occasion it to boil sooner or later, 

 and so prevent the designed exactness. And by the way give me leave to hint, 



3x2 



