50 METEOROLOGY. 



the opinion that the moon is the principal cause of the changes of the 

 weather under discussion, but go directly in favor of that opinion. 



4. In order to estimate correctly the lunar influence, we must not 

 forget that our atmosphere is subject also to the action of the sun's at- 

 traction and the powerful influence of his rays. 



Independently of the influence of both these bodies, the atmos- 

 phere would revolve uniformly with the earth, and be subject to no 

 change whatever. There would be a general stagnation — a continual 

 sameness — none of the vicissitudes which at present characterize the 

 weather. 



Now suppose the solar influence alone to exist : what change would 

 this produce ? We should see the great system of atmospheric circula- 

 tion at present existing, at once springing into being. This may be con- 

 cisely explained as follows : The absorption of the sun's rays by the sur- 

 face of the earth, by which the air immediately resting upon it is heated 

 and rarefied, will produce diurnal changes of great extent. The unequal 

 absorption by different portions of that surface will produce local dis- 

 turbances in density and pressure resulting in local currents or winds. 

 And the greater ahsorplion within the tropics and low latitudes, will 

 cause a determination of the lower mass of the atmosphere in both 

 hemispheres towards the equator, and of the upper mass towards the 

 poles. The lower or polar current, which, by continually passing over 

 portions of the surface of the earth whose velocity of rotation is great- 

 er than its own, is constantly left behind them by the diflerence of their 

 several velocities, is gradually turned in a curve westward, and in the 

 latitude of about 30° becomes the north-eastern trade wind, on the 

 northern, and the south-eastern trade on the southern side of the equa- 

 tor. The upjier currents, having a greater velocity of rotation than 

 those portions of air through which they successively pass in their de- 

 parture from the region of the trades, are constantly tending more and 

 more eastward, until, in the latitude of about 50°, they become almost 

 east. These currents, leaving the torrid zone with a high temperature, 

 and consequently holding a lai-ge quantity of watery vapor in solution, 

 gradually lose their elevated temperature, and deposit their moisture in 

 their progress towards the poles. To these currents, then, we ought to 

 look as the great treasuries of cloud, rain and snow ; and their course 

 ought to be the great track of our extended storms. 



But in naming cloud, rain and storm, we have insensibly passed be- 

 yond the immediate efiect of the solar influence. If the sun were the 

 sole disturber of the atmospheric equilibrium, it is but fair to assert, that 

 every day would be nearly alike in reference to cloud and rain ; for the 



