378 Mr. Herapath on True Temperature, and ike [Nov. 



mass of vapour and gas at the two temperatures, the elasticities 

 of the compound must be the same at both temperatures. 

 Therefore, and because the proportions of atmosphere and vapour 

 are supposed to be invariably the same, the elasticity of each 

 must be the same at either temperature ; and, consequently, the 

 elasticity of the vapour in the atmosphere is, as our theory deter- 

 mines it, the same as the tension of the vapour corresponding to 

 the temperature at which sensible deposition commences, and 

 not greater or less. It was for want of considering this equality 

 of pressure in the atmosphere, I conceive, that the above two 

 able philosophers, and all who have followed them, have run 

 into errors on this part of the subject. 



A pretty instance presents itself for verifying the truth and 

 consistency of this and the preceding corollaries to those who 

 choose to try it. In the preceding cor. I have said that sensible 

 deposition will commence where the megethmerins of the vapour 

 in air and in vacuo are equal ; that is, when the elasticity proper 

 to the vapour in the atmosphere is equal to the proper tension 

 of the vapour at that temperature. But t' is by this cor. the 

 proper elasticity of the vapour in the atmosphere, which, to make 



it equal to t, must be multiplied by ^ ; that is, to produce sen- 

 sible deposition at the same temperature, the air must be con- 

 densed in the ratio of the tension corresponding to the tempera- 

 ture at which sensible deposition takes place to the tension 

 corresponding to the temperature of the atmosphere. 



We perceive by the present corollary that the apparent humi- 

 dity of the atmosphere does not depend on the absolute quantity 

 alone of vapour it contains, but on the quantity as compared to 

 that which could exist over water in vacuo at the same tempera- 

 ture, or on the absolute quantity of vapour in the atmosphere 

 and the temperature conjointly. Hence the apparent humidity 

 is not a measure of the absolute quantity or proportion of aqueous 

 vapour at different temperatures ; and, consequently, all our 

 hygrometrical instruments which are grounded on the principle 

 of apparent humidity do not in any wise serve to discover the 

 real quantity of humidity, unless a due regard be had to the 

 temperature. Therefore, in all cases where the absolute humi- 

 dity is sought, the temperature must be taken into account. 

 This may be done for all temperatures under the boiling of water 

 by the table computed by Mr. Dalton from his experiments ; and 

 for these and all other temperatures by a simple theorem I shall 

 presently give. 



By this theory, we perceive that a warm atmosphere with the 

 same apparent aridity contains more vapour than a colder ; and 

 hence in any particular place the day air contains more humidity 

 than the night, the evening than the morning, the summer than 

 the winter air,, and the lower regions than the higher. Now 

 philosophers, I believe, tell us, that there is the same proportion 



