ON THE EXPANSION OF MOIST AIR. ] 85 



safely conclude then from tins experiment, that the baro- tncal pressure 

 metrical pressure is not counteracted by free vapour, which racted b'^free 

 certainly would be the case, were the hypothesis in question vapour, 

 consonant with the operations of nature? 



After ascertaining the preceding fact, 1 was desirous to The manome- 

 approximate with a greater degree of exactness to the limit able< 

 of the expansion, if a proper instrument could be procured. 

 I say a proper instrument, because the manometer appears 

 to be objectionable on two accounts. In the first place it 

 .would be difficult to graduate a tube of a moderate length 

 so accurately, as to discover the dilatation by it truly to two 

 .or three places of figures. In the next place a manometer 

 of this construction may be made to give different results by 

 a little management, which vvjll be eyident from the follow- 

 ing experiment. 



Exp.<2. A manometer T \ of an inch in diameter was cooled Exp. 2, 

 by water to 35°, and the height of the column of air was 

 then marked on the glass. In the next place the tube was 

 suddenly plunged into water x)f 95°, and the height of the 

 column marked as before. On cooling the instrument again 

 as suddenly to 3o°, the air contracted to its former dimen- 

 sions ; after which the temperature was raised a "second time 

 to 95° in a very gradual manner. The consequence was, 

 that the columj/fell short of its former height by nearly ■£$ 

 of its length. This circumstance determined me, to prefer ^Eolipile pre- 

 an ceolipile to a manometer, the method of using which will feiable. • 

 appear in the following paragraph. 



Exp, 3. What I have called an aeolipile is a copper ves- Exp. 5. 

 Lei of a conical figure and having a flat bottom. The slen- 

 der part of the truncated cone has an aperture ^ of an inch 

 in diameter, which is turned directly downwards when the 

 bottom of the aeolipile is parallel to the horizon. 110 grains 

 of water at the temperature of 64° were put into this ves- 

 sel, which required the addition of 2895 grains of water at 

 the same temperature to fill it. Things being thus prepar- 

 ed, the aeolipile was immersed in a large pan, and suspended 

 free of the sides and bottom by wires. The pan was then 

 heated to 212°, and kept boiling for some time; after which 

 it was reduced to 64° as quickly as possible by pouring cold 



water 



