214 TENTH ANNUAL REPORT OF 



water it meets with in its passage to the ground, but the amount 

 thus collected would not he sufficient to account for the difference ob- 

 served. Besides this; the condition does not always exist ; the air 

 near the earth is frequently undersaturated during rain, and in this 

 case a portion of the drop would be evaporated, and its size on reach- 

 ing the earth less than it was above. If the drop is increased by the 

 deposition of new vapor in its descent, then the rain at the bottom 

 ought to be warmer than at the top, on account of the latent heat 

 evolved in the condensation ; on the other hand, if the drop be dimin- 

 ished by evaporization during its fall, then the temperature of th© 

 rain caught at the greater elevation ought to be in excess. That 

 evaporization does sometimes take place during the fall of rain, would 

 appear from the fact that clouds are seen to exhibit the appearance of 

 giving out rain though none falls to the earth, the whole being en- 

 tirely evaporated. That the air should ever he undersaturated during 

 rain is at first sight a very surprising fact ; it may, however, be ac- 

 counted for on the principle of capillarity. The attraction of the sur- 

 face of a spherical portion of water for itself is in proportion to the 

 curvature or the smallness of the quantity, and hence the tendency to 

 evaporate in a rain-drop ought to be much less than in an equal por- 

 tion of a flat surface of water. 



If the diminution of quantity of rain at the upper station depends 

 principally on eddies of wind, then the etfect will be diminished by 

 an increase in the size of the drops, which will give them a greater 

 power of resistance ; and the size of the drop will probably be in- 

 fluenced by the intensity of the electricity of the air, as well as by its 

 dryness. The former, as well as the latter, will tend to increase the 

 evaporation from the surface of the drop. 



It is a well-established fact, which at first sight would appear to be 

 at variance with the results of observations on towers, that a greater 

 amount of rain falls in some cases on high mountains than on the ad- 

 jacent plains. For example, the amount of water which anntially 

 falls at the convent of St. Bernard is very nearly double that which 

 falls at Geneva. This eflect, however, is due to the south wind, loaded 

 with moisture, ascending the slope of the mountain into a colder re- 

 gion, which causes a precipitation of its vapor. From what is here 

 said, it will be evident that the subject of rain is one which involves 

 many considerations, and which still presents a wide field for investi- 

 gation. 



A series of observations have been commenced at this Institution 

 on the quantities of rain at difierent elevations, as well as on gages 

 of different sizes and forms, the result of which will be given in one 

 of the subsequent reports. 



