184 Dr. Apjohn's Formula for inferring the Dew -Point 

 when saturated at tK We will therefore have, on the princi- 



pie already explained, (l- __|-^_) x/'=/'-^^ 



= y^', the tension of vapour at the dew-point. If p = 30, 

 /^' =x /-' - — 



To this solution the following objections may be urged : 



1st, That the air which is cooled by contact with the moist- 

 ened bulb at its stationary temperature is assumed without 

 proof to be saturated with moisture. 



2nd, That the caloric of elasticity of steam is 1 1 29° only 

 at 50°. 



3rd, That the specific heat of air is '267 only under a 

 pressure of 30. 



4th, That the medium which is cooled from t to t' is not 

 pure air, but a mixed atmosphere of air and vapour. 



5th, That the caloric which, at the temperature ^, con- 

 verts the water into vapour, is not derived exclusively from 

 the air by contact, but partly also by radiation from sur- 

 rounding bodies. 



With respect to the first objection, I have only to observe 

 that air, if not an absolute non-conductor, is at least a very 

 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 con- 

 tact be established in the case of every indefinitely thin aerial 

 shell, there can, I conceive, be no doubt that each becomes 

 charged with the full amount of moisture which belongs 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 

 its temperature, and that it is represented by the number 1129 

 only at 50°, a point chosen because of its being nearly the 

 mean temperature of Dublin. In strictness, the number em- 

 ployed should be 967 + 212—^', but it would be easy to show 

 that the uniform use of 1129 cannot give rise to any material 

 error. 



The third objection is one of considerable weight. The spe- 

 cific heat of air varies with the pressure, and in order to secure 

 accuracy of result, a proper correction must undoubtedly be 

 made for this variation. But what is the law which this latter 

 observes ? Upon this point different opinions would appear 

 to be entertained. According, however, to Delaroche and 



