28 THE HARMONIES OF NATURE. 



that cold, and consequently dry, air-currents are perpetually 

 wandering from the poles to the equator to re-establish the 

 equilibrium of the atmosphere, which is constantly disturbed by 

 the mighty action of the tropical sea. The rotation of the 

 earth gradually imparts to these cold polar streams a direction 

 from east to west as they advance towards the lower latitudes, 

 and ultimately changes them into the constant easterly trade 

 winds, the mariner's delight as he crosses the tropical Atlantic 

 on his way to the New World. As during these wanderings 

 their temperature constantly increases, they are able to absorb 

 a constantly increasing quantity of aqueous vapours, until they 

 are completely saturated with moisture. 



In their subsequent progress to the higher latitudes the con- 

 trary tendency takes place, for, as their temperature diminishes, 

 they become incapable of retaining the aqueous vapours they 

 had previously absorbed, and are thus compelled to discharge 

 them under the various forms of rain, snow, hail, or dew, until 

 having lost by far the greater part of their moisture, they 

 finally reach the polar regions, whence, thoroughly desiccated, 

 they begin again their grand circulating tour. 



Thus evaporated and set in motion by the sun, the waters, ever 

 rising and ever falling, migrate through the air. Enormous 

 quantities of water constantly ascend from the ocean, but the 

 countless rivers which they feed are as constantly restoring 

 them to their source. 



Though the atmospherical precipitations are very unequally 

 distributed over the surface of the earth, as they are favoured 

 or impeded by a variety of local causes, such as mountain-chains 

 or forests, both of which act as powerful condensators, the 

 prevalence of dry or moist winds, according to the geographical 

 position, the constitution of the soil, &c., yet, on examining 

 each land in particular, we find a remarkable uniformity in the 

 quantity of rain that annually falls to its share. Thus, though in 

 England some years are more moist than others, yet the dif- 

 ference in the annual quantity of rain only deviates a few inches 

 from the mean average, and the deficiency of one month is 

 generally repaired by the greater abundance of the next. 



The clouds that pass over our heads pay us their regular 

 tribute, and the authority of a higher law reigns even in the 

 apparently free empire of the air. 



