130 BOARD OF AGRICULTURE. 



from the equator, 85 inches ; 20°, 10 inches ; 30°, 40 inches ; 40°, 

 30 inches ; 50°, 25 inches ; 60°, 20 inches. For our present pur- 

 pose, we need not trace the rainfall into higher latitudes. 



Now, were there no circumstance to modify the distribution of 

 rain other than that of latitude, there would be a direct ratio be- 

 tween the quantity of rainfall and the latitude, which is not strictly 

 the case, since there are certain modifying conditions which come 

 in to control the distribution of rain, other than the condition of 

 latitude. Before treating, however, of those modifying circum- 

 stances, permit me to state, by way of comparison, the quantity 

 of rainfall in the diiierent zones of the world, contrasting the old 

 world with the new. 



Within the tropics of the old world the annual rainfall is '16 

 inches; within the tropics of the new world, 115 inches. The 

 mean for the torrid zone is about 96 inches. The annual rainfall 

 in the north temperate zone of the old world is 34 inches ; in the 

 jiew world, 39 inches ; the mean being about 361 inches. The 

 annual rainfall ibr the frigid zone is estimated to be about 15 

 inches. You will remember that the amount of evaporation there 

 is but two inches ; the rainfall thei-efoi-e is very much in excess of 

 the evaporation, and hence the vapor which supplies it must be 

 •drawn from lower latitudes. 



The first modifying circumstance of the general law which has 

 been given, is that of the prevailing winds ; that is, rainfall is 

 influenced by the direction and character of the prevailing winds. 



We have here a globe representing the earth, which, for the 

 moment, we "will suppose to be stationary. Now, while the sun 

 is pouring down his warm rays upon the equatorial parts of the 

 earth, the temperature -here is raised higher than the temperature 

 at any other portion of the globe. The result is an ascensional 

 amount of hot air is produced in the region of the equator, which 

 ascends and falls back towards the pole, one part going toward the 

 north pole, the other toward the south pole. If the earth were a 

 stationary body, at the same time with the rising and falling back 

 of the heated air, cold currents would come in from the north and 

 IVoin the south to supply its place, and were it not for the con- 

 stituents which intercept and deflect these currents, the only 

 winds we should experience upon the globe would be the surface 

 winds from the north and from tlie south. 



Now, the earth has a motion upon its axis from west to east. At 

 the equator that motion is something more than a thousand miles 



