RELATION OF VAPOUR AND AIR. 575 



deduce certain general laws from Mr. Sharpe's ex- 

 periments; and other persons have offered other 

 rules, as those which govern the force of steam 

 with reference to the temperature: but no rule 

 appears yet to have assumed the character of an 

 established scientific truth. Yet the law of the 

 expansive force of steam is not only required in 

 order that the steam-engine may be employed with 

 safety and to the best advantage ; but must also 

 be an important point in every consistent thermo- 

 tical theory (T A). 



Sect. 5. Consequences of the Doctrine of Evapora- 

 tion. Explanation of Rain, Dew, and Clouds. 



THE discoveries concerning the relations of heat 

 and moisture which were made during the last cen- 

 tury, were principally suggested by meteorological 

 inquiries, and were applied to meteorology as fast 

 as they rose. Still there remains, on many points 

 of this subject, so much doubt and obscurity, that 

 we cannot suppose the doctrines to have assumed 

 their final form, and therefore are not called upon 

 to trace their progress and connexion. The prin- 

 ciples of atmology are pretty well understood ; but 

 the difficulty of observing the conditions under 

 which they produce their effects in the atmosphere 

 is so great, that the precise theory of most mete- 

 orological phenomena is still to be determined. 

 We have already considered the answers given 



