68 DISCUSSION. 



over forests than over exposed surfaces, and this is chiefly due to 

 the cooler temperature the forests by their shade induce. A 

 forest therefore, though in a modified extent, occupies much the 

 same position relative to pressure of air as the ocean. The 

 temperature of the sea is always more equable than that of the 

 land, owing to the envelope or interposition of innumerable 

 particles of water that serve to make it cooler in summer and 

 warmer in winter than the land, and this coolness by attracting 

 the overflowing air which I have shown the solar heat has forced 

 upwards from the earth by expansion produces a high pressure on 

 the sea. The same cause produces a higher pressure of atmosphere 

 on forests than on exposed surfaces, for a tree interposes a large 

 shade between the sun and the earth, and therefore lowers the 

 surface temperature by day and induces humidity by preventing 

 the too rapid escape of moisture beneath it, and at the same time 

 by its shade it protects the ground from evaporation. But the 

 atmosphere just above a forest is constantly absorbing moisture 

 therefrom, and therefore it is lighter than the current just above 

 it again, so that it is constantly forced upwards. As these 

 currents ascend they gradually become cooler, and should their 

 aqueous vapour meet a stratum of air already laden to dew point, 

 their additional moisture produces rain. Thus it appears to me 

 that the very occurrence of springs, as noticed by Mr. Abbott after 

 the destruction of the forests and where none apparently existed 

 before, affords pretty conclusive evidence that a condition has 

 been produced by ring-barking unfavourable to rainfall. Again, 

 this lowering of the summer temperature and raising of the winter 

 temperature by forests, is the very cause that induces constant 

 change of currents productive of rainstorms, for it has been proved 

 by the charting of winds with isobarometric lines, that the wind 

 and consequently the rainfall depends on the pressure of the 

 atmosphere, and therefore on its temperature and humidity. It 

 has been shown, also, that winds chiefly blow from places where 

 the pressure of air is high to places where it is low, and that 

 variable winds are affected by local causes, such as the physical 

 features of the country whether level or mountainous, the vicinity 

 of sea or lakes, and lastly such as the prevalence of forest or 

 desert country. We know, too, that given in any locality an 

 excess or decrease of atmospheric pressure, temperature, or 

 moisture, certain atmospheric changes inducing wind take place 

 to restore the equilibrium thus disturbed, which again influences 

 rainfall. Now forests, on account of the aqueous vapor rising from 

 them and the greater pressure of air above them, offer conditions 

 favourable to these atmospheric changes, and consequently 

 favourable to the increase of rainfall. Thus it seems to me that 

 forests must have a direct influence in inducing rainfall, for 

 which reason they should be carefully cultivated. Whilst then I 



