260 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



pot was exposed to the fire. This difference however was not perceived in the 

 trials of the long thermometer in the deep pot, as there seemed very little 

 difference in the height whether the water boiled fast or slow, or whether more 

 or less of the side of the pot was exposed to the fire. The greatest difference 

 observed in the same thermometer, on the same day and in the same water, 

 according to the different manner of trying the experiment, was half a degree. 



We made some trials with the long thermometer in the deep pot, to deter- 

 mine how much the height of the boiling point was affected by a greater or less 

 depth of water above the ball. By a mean of the experiments it stood .66 of a 

 degree higher when the water rose 14 inches above the ball, than when it was 

 only 3 inches above the ball; so that increasing the depth of water above the 

 ball by 1 1 inches, raised the thermometer .06 of a degree, that is .o6 for each 

 inch. We would by no means infer however from hence, that it is a constant 

 rule, that the height of the boiling point is increased .05 of a degree by the 

 addition of each inch in the depth of the water above the ball; as perhaps the 

 proportion would be found very different in greater depths of water or in wider 

 vessels. 



If this rule be constant, it would show that, when the pressure on that part 

 of the water which surrounds the ball is increased, by increasing the depth of 

 water above the ball, the height of the boiling point is not altered by it more 

 than one half as much as by an equal increase of pressure prod ucetl by an altera- 

 tion in the weight of the atmosphere: for the pressure on that part of the water 

 which surrounds the ball, is as much increased by an alteration of 1 1 inches in the 

 depth of the water above the ball, as by an increase of —j of an inch in the 

 height of the barometer; and such an alteration in the height of tlie barometer 

 is sufficient to raise the boiling point 1°.3. 



It seems as if the height of the boiling point was in some measure increased 

 by having a great depth of water below the ball, as in general the short ther- 

 mometers stood higher when tried in the deep pot than in the short one; this 

 effect however did not always take place. In the former of these cases, the 

 depth of water below the ball was about 18 inches, in the other only 4; but the 

 depth of water above the ball was the same in both cases. It must be observed, 

 that when there was a great depth of water in the vessel, either above or below 

 the ball, the experiments were much more irregular, and the quicksilver in the 

 tube remained much less steady than when it was small. When the depth of 

 water in the vessel is great, it is apt to boil in gusts, which seems to be the 

 cause of this irregularity; though we could not perceive any regular connection 

 between these gusts and the rising of the thermometer. 



In the experiments made with the water not rising so high as the ball, so that 

 the thermometer was exposed only to the steam, we very seiilom found any 



