EUCALYPTUS TEEES. 47 



numerous and vast communities, have demonstrated 

 in sad experiences, not only in times long past, but 

 even in very recent periods. In what manner the 

 forests arrest passing miasmata, or set a limit to the 

 spreading of rust -spores from ruined cornfields; in 

 what manner their humid atmosphere and their feath- 

 ered singers effectually obstruct the march of armies 

 of locusts in the Orient, or hinder the progress of vast 

 masses of acrydia in North America, or oppose the 

 wanderings of other insects elsewhere — all this has 

 been clearly witnessed in our own age. How the for- 

 ests, as slow conductors of heat, lessen the tempera- 

 ture of warm climes, or banish siroccos ; how forests, 

 as ready conductors of electricity, much influence and 

 attract the current of the vapors, or impede the elas- 

 tic flow of the air, with its storms and its humidity, 

 far above the actual height of the trees, and how they 

 condense the moisture of the clouds by lowering the 

 temperature of the atmosphere, has over and over 

 again been ascertained by many a thoughtful observer. 

 In what mode forests shelter the soil- from solar heat, 

 and produce coolness through radiation from the end- 

 lessly-multiplied surfaces of their leaves, and through 

 the process of exhalations ; how, in the spongy stra- 

 tum of decaying vegetable remnants, they retain far 

 more humidity than even cultivated soil ; how they 

 with avidity re-absorb the surplus of moisture from 

 the air, and refresh by a never- wanting d6w all vegc 

 tation within them and in their vicinity, has been 

 explained, not only by natural philosophy, but also 

 often by observations of the plainest kind. How for- 

 est-trees, by the powerful penetration of their roots, 

 decompose the rocks, and force unceasingly from deep 



