as deduced by himself and Dr. Suerman. 263 



this consideration, and the additional hypothesis that the gas 

 so cooled by successive contacts with the moistened bulb is sa- 

 turated with humidity, we can deduce the following equation : 



in which f" ovf have the significations already assigned to 

 them, while m! and m represent, the former the amount of va- 

 pour which would be formed by the caloric evolved from a given 

 bulk of the gas in cooling through t — t' degrees, the latter the 

 maximum quantity which the same volume of such gas could 

 contain at the temperature t'. The correctness of this de- 

 duction is easily shown. For m being the total quantity of 

 moisture in the gas, and m' the quantity introduced into it, 

 7» — m'will be the quantity it already contained ; so that, since, 

 when the temperature and volume of vapour are both given, 

 its elastic force is proportional to its quantity, we shall have 



m : m—iri : :/"' -.f" 

 a proportion which gives, as above. 



In arriving at this conclusion, we have assumed that the air 

 which is cooled by contact with the moistened bulb becomes 

 saturated with humidity. This is the only premiss which we 

 have employed about which a question can be raised. That it 

 is brought, however, into such condition, no one can, I con- 

 ceive, entertain a doubt who reflects upon the very low con- 

 ducting power of air, and the consequent impossibility of its 

 communicating caloric to the moistened bulb unless by actual 

 contact, a condition which can scarcely be fulfilled without 

 the entire of the cooled air being at the same time carried to 

 the maximum degree of humidity. 



In the expression ioxf" given above, y may be considered 

 as known, the corresponding temperature t' being the result 

 of an observation. In order, therefore, to render the formula 

 available it will only be necessary to determine in known terms 

 values of w' and w, which may be done in the following man- 

 ner: 



Let a be the specific heat of the gas under a constant press- 

 ure, and e the caloric of elasticity of aqueous vapour at t\ 

 the stationary temperature of the wet thermometer. It is evi- 

 dent that a grain of the gas in cooling through t — t' — d de- 

 grees gives out the caloric necessary for raising the tempera- 

 ture of a grains of water through the same number of degrees. 

 But the caloric which heats a grains of water d degrees would 



