8 TREE CULTURE IN SOUTH AUSTRALIA. 



blown over dry ground heated by the sun, where the air is in conse- 

 quence highly rarified and warmer than the clouds, these dissolve them- 

 selves and vanish ; but should these clouds come in contact with the 

 cooler air above masses of trees they become overcharged with moisture, 

 and rain is the result. 



Arid Tracts would he Improved. — We have seen that the planting of 

 large bodies of trees has the indirect influence of attracting rain-clouds 

 to the sites occupied by them, and that the atmosphere generally about 

 woodlands is in a continual state of moisture by transpiration from the 

 pores of the leaves, and by a certain amount of evaporation caused by 

 the heat of the sun. From this then, it will at once be seen, that by 

 planting arid tracts of land with properly proportioned belts of timber 

 here and there through them, the result is (1) lower temperature, (2) 

 arrest of hot winds, (3) shelter, (4) more frequent raius, and (6) a more 

 humid climate generally, thus making such tracts of country suitable for 

 agricultural purposes. 



We would have a more Humid Clima te. — A humid climate is a result and 

 component part of the whole system of the different influences of trees 

 , upon climate which have already been explaiaed. Water is sucked up 

 from the soil by the roots of the trees, and is exuded again in the form 

 of vapour from the stomachs on the back of the leaves ; this rises into the 

 air and forms itself into clouds, and, if not deposited agaia on the ground 

 as rain by some counterbalancing atmospheric influence, is wafted 

 across the country, cooling the air and keeping up a supply of heavy 

 dews, which refresh and invigorate vegetable life. While agaia, the 

 humidity of the climate is maintained from the simple fact that the 

 green moist foliage of the trees constituting the forests has the well- 

 known tendency of preventing the increase of the sim's rays by radiation, 

 and thus reducing the chances of evaporation. 



The Landscape toould he Improved. — How different the contrast of 

 appearance between a country well stocked with trees and one bare of 

 these. The one looks clothed and picturesque, while the other has that 

 barren unproductive look which wearies the eye. Besides, in a com- 

 mercial point of view alone, there exist strong arguments in favor of 

 trees. In the one case we have luxuriant crops, and fine arborous 

 Tctreats for stock, while in the other both stock and crops are subject to 

 all the changes of the weather, and look stunted and unhealthy. 



Unhealthy Districts tvould be made Hahitahle. — Leaves of trees purify 

 ■the air by absolving the carbonic acid gas thrown into the air by the 

 breath of animals, and from the various forms of decomposition in nature. 

 They separate the carbon from the oxygen, retainins; the first as food for 

 themselves and emitting the other for sustaining animal life. 



In low-lying and swampy parts of a countiy, where rich organic mould 

 has been accumulating for thousands of years back, thereby promoting 

 a rank growth of aquatic vegetation, gases arise which have a very 

 deleterious effect upon animal Hfe, and produce insidious and mortal 

 diseases. This is counteracted by the planting of certain kinds of trees ; 

 notably that of the Tasmanian blue gum (Eu-.alyptus globulus). There 

 IS great room for the researches of science in this matter yet ; but that 



