1821.] Causes of Calorific Capacity, Latent Heal, 8jc. 383 



usually experience, the decrement of recondensation is propor- 

 tional to the whole recondensation and the velocity of the world 

 conjointly, and the recondensation is proportional to the whole 

 evaporation, the apparent increment of evaporation occasioned 

 by a current of air over the surface of an evaporating fluid, is 

 proportional to the velocity of the current and the evaporation 

 conjointly. This conclusion agrees with the views of Mr. Leslie 

 obtained from experiment. 



I have yet been speaking of currents parallel to the surface of 

 the fluid, but mathematically looking at the subject, there seems 

 to be another source of anomaly in evaporation. When the air 

 blows parallel to the surface, the mean collisions of the particles 

 of air against those of the fluid are, perhaps, not increased by the 

 velocity of the current ; but if the air blows obliquely on the 

 surface, it seems reasonable to suppose that the intensity of 

 collision is augmented by a quantity proportional to the cosine 

 of the angle of incidence and the velocity of the current con- 

 junctly. This increase of collision is equivalent to an increase of 

 temperature, and, therefore, ought to produce an increase of 

 evaporation. Assuming what, perhaps, does not in this case 

 materially differ from the truth, that the increment of evapora- 

 tion depending on this cause is proportional to the increment of 

 collision, the increment of evaporation will be equal to the con- 

 tinued product of some constant b, the cosine of the anole of 

 incidence, and the velocity of the wind. But under these 

 circumstances of oblique currents, the incremental evaporation 

 before alluded to is also equal to the continued product of some 

 constant a, the sine of the angle of incidence, and the velocity 

 of the wind. Therefore, the maximum of apparent evaporation 

 will be when the sine of the angle of incidence is to the cosine 

 as a is to b. 



Philosophers who may be engaged in experiments on evapo- 

 ration will not, perhaps, take amiss my recommending to their 

 attention the influence of oblique currents on the apparent acce- 

 leration of evaporation. Experiments of this kind, and a multi- 

 tude of others 1 have in contemplation to make, will, I fear, by 

 me necessarily be subject to lie long on the list of intended 

 consideration. Philosophical experiments, where delicacy and 

 precision are required and united to an almost boundless range 

 of view, and where the ardour of research is damped by unusual 

 discouragement, need more than the efforts of a solitary indivi- 

 dual to compass and to carry into successful execution. Though 

 since my arrival at Cranford no person could have been more 

 fortunate in a part at least of his scientific connexion, yet I 

 •seriously feel the want of those good offices and assistance in 

 my philosophical inquiries the friends I have left in Bristol were 

 ever ready to grant me ; and none do I feel the want of more 

 than those of my respected friend, John Hare, Jim. Esq. a gen- 

 ttamau who, though taking a leading part in the most extensive 



