EUCALYPTUS TREES. 11 
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 dew all vege- 
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 
