278 Professor Apsoun on the Theory 
x, 805)* = 2613.88 xt =the quantity of moisture which the air contains when satu- 
ried at ¢. We will therefore have, on the principle already explained (1 a 4 
) x J =f'- ee =f" the tension of vapour at the dew-point. If p = 30, 
ri = <= =f". 
I shall now proceed to state, and subsequently observe upon, the objections which 
may be made to the method of investigation I have pursued. It may be said— 
1°.. That the air which is cooled by contact with the moistened bulb at the sta- 
tionary temperature, is assumed, without proof, to be saturated with moisture. 
2°. That the caloric of elasticity of steam is 1129 only at 50°. 
3°. That the specific heat of air is .267 only under a pressure of 30. 
4°. That the medium which is cooled from ¢ to ¢ is not pure air, but a mixed at- 
mosphere of air and vapour; and 
5°. That the caloric, which at the temperature ¢ converts the water into vapour, 
is not derived exclusively from the air by contact, but partly also by radiation from 
surrounding bodies. 
With respect to the first objection, I have only to observe, that air is an extremely 
bad conductor of heat, and that it is, therefore, very unlikely that the reduction of 
temperature which it experiences in the experiment in question can be effected in‘any 
other way than by actual contact with the moistened bulb. But, if such contact be 
established in the case of every indefinitely thin aerial shell, there can, I conceive, be 
no doubt but that each becomes charged with the full amount of moisture which be- 
longs to its reduced temperature. 
In reference to the second objection, it must of course be admitted that the caloric 
of elasticity of vapour varies with the temperature, and that it is represented by the 
number 1129 only at the temperature of 50°, a point chosen by me as being nearly 
the mean temperature of Dublin. In strictness, the number employed should be 
967 +212—#', but it would be easy to shew that the uniform use of 1129 cannot give 
rise to any material error. 
The third objection is usually considered as one of considerable weight. The spe- 
cific heat of air varies with the pressure, and in order to accuracy of result, a proper 
correction must undoubtedly be made for this variation. But what is the law which 
it observes? Upon this point, different opinions would appear to be entertained. 
According, however, to De la Roche and Berard, (whose views, if not rigorously 
exact, are at least sufficiently so for my present purpose,) for small variations of 
* 10,583 x ro x ,305=the weight of a cubic inch of vapour whose tension is f’ and temperature ¢. 
