116 MISSOUEI STATE HORTICULTURAL SOCIETY. 



RAINFALL AND THE ECONOMY OF MOISTURE. 



BY WM. FERRELL, KANSAS CITY, MO. 



One of the most important considerations in agriculture and horti- 

 culture is a sufficient supply of rain. Though persons engaged in these 

 pursuits are of course interested most in the prevision and receipt of 

 rain when needed, yet many of an inquiring disposition are undoubtedly 

 interested also in the causes of rain. It was said by Yirgil a long time 

 ago, " Happy is he who is able to know the causes of things." But a 

 knowledge of the cause of rain has also a practical importance, for with 

 this we are better able to judge of the signs which precede rain. 



THE CAUSE OF RAIN. 



The old theory of the cause of rain — namely, that it arises from the 

 contact and intermingling of cold and warm currents of air, approxi- 

 mately or completely saturated with moisture — is now exploded. It was 

 shown by Espy that this is not adequate to give rise to more than a 

 very thin cloud, with perhaps a slight falling mist which can scarcely 

 ever reach the ground. The true theory of rain is that it arises from 

 the condensation of vapor in air, nearly or quite saturated, in ascend- 

 ing currents. As the air ascends it comes under less pressure, and 

 consequently expands, and as it expands it is known from both theory 

 and observation that it must cool and soon lose its capacity for the 

 moisture which it had in it while it was warmer at and near the earth's 

 surface ; and so a part is condensed into cloud and rain. For to each 

 temperature of the air belongs a certain capacity for moisture, which 

 decreases with decrease of temperature, so that as soon as the air, in 

 ascending with a given amount of moisture in it, cools down to the tem- 

 perature at which it can contain no more than it has, which is called the 

 temperature of the dew point, the moisture begins to be condensed into 

 cloud. Espy showed by a few strokes of an air-pump that the air in 

 the receiver, as it expanded, soon became so cooled that it could not 

 contain the vapor in it, and so a cloud was formed. Where the air is 

 not completely saturated when it begins to ascend, it has to ascend 

 until it is cooled down to the dew point before cloud formation takes 

 place, and hence the drier* the air for any given temperature, the higher 

 is the base of the cloud. 



The cloud, which is simply fog, consists of very small particles of 

 water, too small to fall to the earth even where the ascending current 



