430 Mr. W. S. Jevons on the Deficiency of Rain in an 



elastic force of aqueous vapour) has the value T072; that is to 

 say., rather more heat is at first received from the cooling of the 

 air than from the condensation of vapour. When the drop has 

 increased in temperature to 40° F. (/' = 4°-44 C), the value will 

 be -822, or the condensed vapour yields the larger share of heat ; 

 hut even when the drop has the temperature 59°, the value has 

 only diminished to '624. Taking an average of these three de- 

 terminations, we shall find that not more than 55 per cent, of 

 the heat received by the drop will proceed from condensed 

 vapour; consequently we must reduce Sir J. HerschePs first 

 estimate almost to its half. 



21. Again, considering that the temperature of the air in- 

 creases uniformly from the elevation of 12,000 feet, at which 

 Sir J. Herschel supposes the drop to be formed, down to its 

 temperature at the surface, it is truly extravagant to suppose 

 that a rain-drop should fall unaltered through 11,800 or 11,900 

 feet aud then suddenly assume the full temperature of the air in 

 the last 100 or 200 feet. A small drop falling very slowly will 

 take the temperature of the air, or more strictly the temperature 

 of evaporation, all the way down, and its degree of coldness on 

 reaching the lowest stratum of air will be so slight as to produce 

 no appreciable condensation even in perfectly moist air. On the 

 other hand, a large drop falling so rapidly that it has no time to 

 receive heat from the air, will indeed remain of a low tempe- 

 rature, but it will likewise have no time to receive heat from the 

 lowest air. And drops of intermediate size, just in proportion as 

 they fall more quickly and receive less heat from the upper strata 

 of air, will be less able to receive heat from the lowest stratum. 



22. Nor can it be argued that the rain-drop receives heat 

 most freely in the lowest stratum of air because it there meets 

 most vapour. For the humidity of the air invariably increases 

 from the surface of the earth up to the first cloud, as was ob- 

 served by Mr. Welch in each of his four balloon ascents. Even 

 under the most rare or impossible hygrometric conditions the 

 amount of condensation would be quite inappreciable. Under 

 any usual or real conditions, it may be most confidently asserted 

 that a falling drop of rain will either increase uniformly through- 

 out its descent by an extremely minute quantity, or will, as is 

 far more likely, evaporate and decrease by a small quantity. Under 

 no possible conditions will the increase within the last few hundred 

 feet of descent he more than almost infinitesimal. 



23. It is of course perfectly well known and allowed that the 

 temperature of rain is often much lower that that of the air at 

 the surface. I have myself several times observed remarkably 

 cold rain. So M. Boisgiraud* writes to the Paris Academy, 



* Annates de Chimie et de Physique (ser. 2) vol. xxxiii. p. 417. 



