20 WRITINGS OF JOSEPH HENRY. [1855- 



It is now an established truth that certain localities are 

 screened from miasmatic influence b}' the intervention of 

 trees. A more general reco<^nition of this fact might add 

 much to tlie healthfulness of localities in other respects 

 highly desirable. 



The solar rays, in passing through the atmosphere, do not 

 heat it in any considerable degree, but they heat the earth 

 against which they impinge; therefore the temperature of 

 the lower stratum of air is derived, directly or indirectly, 

 from the soil on which it rests ; and this temperature, as 

 has been remarked, will depend upon whether the surface 

 be marshy or dry, clothed with herbage, or covered with 

 sand, clay, or an exposed rock. From this fact it is evident 

 that man has, in this particular also, considerable power in 

 modifying the climate of portions of the earth; and history 

 furnishes us with many examples in which great changes, 

 within human control, have been produced in the course of 

 ages. Nineveh and Babylon, once so celebrated for their 

 advance in civilization and opulence, and Palmyra and Baal- 

 bec, for their magnificence, offer at this da}^ to the traveller 

 the site of ruins which attest their past greatness, in the 

 midst of desolation. Canaan, described in the Bible as a 

 fertile country, "flowing with milk and honey," is now 

 nearly deprived of vegetation, and presents a scene of almost 

 uninterrupted barrenness. The climate of these countries 

 is undoubtedly modified by the present state of the surface, 

 and might again be ameliorated by cultivation, were the 

 encroachments of the sands of the desert stayed by borders 

 of vegetation of a proper character. Many parts, even of 

 our own country, which now exhibit a surface of uninter- 

 rupted sand, may be rendered productive, or covered with 

 trees and herbage. 



A series of observations on the progress of temperature 

 below the surface, in different parts of the country, and even 

 in different fields of the same plantation, would be of value 

 in ascertaining the proper time to introduce the seed, in 

 order that it might not be subjected to decay by premature 

 planting, or lose too much of the necessary influence of 



