40 EFFECTS OF FORESTS ON HUMIDITY. 



— and I hare seen hundreds,— the clay having this property in a pre- 

 eminent degree. 



The walls of houses built of such materials, standing in an atmoS' 

 phere kept humid by over-shadowing trees, or trees growing in close 

 proximity, or even in a proximity not very close, absorb moisture 

 ■with which the atmosphei'e is charged. The moistui'e transmitted or 

 permeating it to the inner surface may be evaporated by the heat of 

 the apartment within, but only to be followed by more and more, 

 keeping the interior of the house constantly humid. Many have 

 sufifered for years in consequence of taking up their abode in a newly 

 built house not thoroughly dried. In the case of such a house as I 

 have referred to, it is never thoroughly dried, the drying process will 

 go on continuously, or be continuously renewed, for a hundred years 

 and more, if the house stand as long. 



Not a little of the dampness may be attributed to the shade and 

 the shelter of the trees, and the consequent want of ventilation ; all 

 which will afterwards come under consideration. "What is brought 

 under consideration at present is the humidity of atmosphere 

 consequent on the copious evaporation of moisture by the leaves 

 of trees. Keference has been made to the hydroscopicity of the 

 walls of houses simply to show that the damp is not produced, but 

 only transmitted by them; and it must be apparent that, in 

 like manner, the damp experienced in connection with want of 

 ventilation with shade and shelter, is damp not produced but 

 only retained by them. The dampness of houses overshadowed by 

 trees, or situated in close proximity to a wood, is indicative of 

 humidity of atmosphere ; and this humidity of atmosphere is 

 occasioned, in part at least, by the copious evaporation proceeding 

 from the leaves of the trees. 



Sect. II. — On Clotich occasionally seen sii^rmounting Woods, while 

 the Atmosphere around is cormparatively Clear. 



The clouds occasionally seen over woods, while the atmosphere 

 around is comparatively clear, are consequent on the condensation of 

 the humidity occasioned by the evapoi'ation from the leaves. 



There is always moisture existing in the atmosphere ; it is reckoned 

 one of its constant constituents, but it vai-ies in quantity. The quantity 

 is minute compared with that of the oxygen and nitrogen of the air ; 

 but it is never absent. There it is, on the highest mountain and in 



