IS62 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



of a degree higher when the ball was immersed a little way in the water (neg- 

 lecting those observations in which much of the sides of the pot were exposed to the 

 fire) than when it was tried in steam : at a medium they stood -JgL higher, which is 

 equal to the difference produced by a variation of -f-- of an inch in the baro- 

 meter; so that the boiling point, adjusted at a given height of the barometer, 

 with the ball immersed a little way in the water, will in general agree with that 

 adjusted in steam, when the barometer is ^i- of an inch higher. 



It must be observed, that in all these experiments a piece of flat tin plate was 

 laid loosely on the mouth of the chimney e, so as to leave no more passage for 

 the steam than what was sufficient to prevent the tin plate from being lifted up. 

 In trying the thermometers in steam, this is by no means unnecessary; for, 

 if the cover of the pot does not fit pretty close, the thermometers will imme- 

 diately sink several degrees on removing the tin plate; but when their balls are 

 immersed in the water, the removal of the tin plate has no sensible effect. 



If this cover to the chimney had been heavy, the included steam might have 

 been so much compressed by it, that the water and steam might have acquired a 

 considerably greater heat than they ought to have done; but as this plate lay 

 loose on the chimney, and as its weight was not greater than that of a column 

 of quicksilver, whose base is equal to that of the mouth of the chimney, and 

 whose altitude is -^ of an inch, the excess of the compression of the included 

 steam, above that which it would suffer in an open vessel, could not be greater 

 than that which would be caused by an increase of -l of an inch in the height 

 of the barometer, which is too small to be worth taking notice of; for if the 

 excess of compression was greater than that, the tin plate nmst necessarily be 

 lifted up so much as to afford a sufficient passage for the steam to escape fast 

 enough, though urged by no greater force than that. 



Though in the different trials of the same thermometer in steam, on the same 

 day, and with the same water, so little difference was observed, according to the 

 different manner of trying the experiment; yet there was a very sensible 

 difference between the trials made on different days, even when reduced to the 

 same height of the barometer, though the observations were always made either 

 with rain or distilled water. The difference however never amounted to more 

 than a quarter of a degree, except in one thermometer, in which there were 3 

 observations out of 18 which differed more than that; one of them differed so 

 much as 0.65° from some of the rest. In the observations made with the ball 

 immersed a little way in the water, there was a greater difference between the 

 observations of different days, even negKxding those in which much of the sides 

 of the pot were exposed to the fire. In 2 of the thermometers the different 

 observations differed about -^/^ of a ileiiree from each other; but in the other 

 thermometer they varied y'^. 



