EXPERIMENTS ON HEAT AND COLD. J61 



the facls themfelves appear in a fomewhat different point of 

 view from what they are feen in at the firft moment. 



One circumftance is very remarkable, that whether the This rife and 

 mercury rifes or falls in thefe inftances, it is done very rapidly ; fdl are vtry ra- 

 wliereas in the open air, if a thermometer be only two or three 

 degrees above or below the temperature, it moves very flowly. 

 This feems to have fuggefted to every one the idea, that the whence it was 

 elasticity of the glafs bulb of the thermometer has a principal ^oTprtffure'La 

 (hare in producing the effect, by caufing the bulb to yield a the glafs bulb} 

 little to the preflure of the air. It has however been found 

 upon trial that the fame effects take place whether the thermo- 

 meter is fealed or not. My experiments accord with this, hav-butit happens 

 ing made a thermometer and left it un fealed for the exprefs JJ*** ^ r f ^ d or 

 purpofe ; in all the experiments with cond.enfed and rarefied not. 

 air, there was no feniible difference obferved to arife from the 

 inequality of preffure on the external and internal furfaces of 

 the bulbs, the fealed and open thermometers varying the fame 

 in kind and alfo in degree, except from circum fiances to be 

 noticed hereafter. 



It being certain then that a real change of temperature takes Confeq. a real 

 place, it remained to determine the quantity and manner f cha,1 g eof ^ m# 

 that change. Having chofen a fmall and confequently fenfi- p i ac ^. Ur 

 ble thermometer, with a fcale of degrees fuftkiently large to 

 admit of diftinguifhing one tenth of a degree, I proceeded to 

 afcertain feveral facls experimentally. 



EXPERIMENT 1. 



Took a receiver,. the capacity of which was about 120 cu-E xp . I. a very 

 inches, and fufpended the thermometer with its clear bulb frna ! 1 thermome- 

 inthe central part of it ; then letting the whole acquire the receiver fcttby 

 temperature of the room, which was without a fire, I ex- exhauftion, and 

 haufted the air and afterwards reftored it, marking the effeas tS^ffSL 

 upon the thermometer. The medium of feveral trials nearly 

 agreeing with each other was as under : 



The thermometer in the air of the room flood at - 36°. 8 



■ funk upon exhauftion to - - - 34- .7 



' rofe when the air was reftored to 38 .9 



The fuddemefs of the fall and rife puzzled me moft : after 

 refleding upon it for fome time, I conjedured that the real 

 change of temperature of the air or medium was much greater 

 I Vol. III.—November, 1802. M than 



