132 
On the Iiifluence of Water 
positing dew will take place in " the interior parts of the soil 
during the day," at the same time that the exterior, or surface of 
the ground, may be projecting both heat and moisture into the 
atmosphere. This process is evidently dependent on the relative 
temperatures and degrees of aqueous repletion of the air and sub- 
soil at a given time ; and independent of the hygrometric power 
of the latter, which is, however, a potent auxiliary to the acquisi- 
tion and retention of atmospheric moisture by soil, particularly in 
its interior parts. Thus, it is apparent that the acquisition of 
moisture by soils in the form of dew, is not limited to the period 
of the night only, nor to the surface of the earth ; and it has been 
shown that the precipitation of dew cannot take place without 
the communication of heat to the recipient substance : hence, the 
importance of sufficient pulverization to permit access and change 
of air to the interior parts of soil. One of the most beneficial 
effects of drainage may be also safely presumed to arise from its 
facilitating the access, and change, of air to the very bottom of 
the bed ; as, in proportion to the escape of water, so will be the 
entrance of the air, which will, j90/7 passu, occupy the place 
vacated by the water. 
Every obseivant farmer must have remarked that the amount 
of dew precipitated cluring the same night varies greatly on 
different soils in fallow, and still more on the leaves of dif- 
ferent plants. Well pulverised soils attract much more dew 
than those which are close and compact, as the radiation of 
heat is effected from many more points in highly comminuted 
than plane surfaces. Sands appear to be powerful attractors, 
and in some countries to depend altogether on the nightly depo- 
sition of moisture for the support of vegetation. An extreme 
example of the derivation of the aqueous element from dew 
alone, and of its highly fertilising qualities, is afforded by the fact 
that, on the sandy plains of Chili, rain is scarcely ever known to 
fall ; yet that soil, which under other circumstances would be 
sterile, is maintained in a productive state by the active forces 
of radiation and absorption. The temperature of the soil is 
moderated during the period of the sun's action by the large 
amount of heat carried off combined with vapour ; wliilst the ex- 
hausted humidity is rej)laced by dew, deposited during the re- 
splendent nights of that tropical region. Instances are also on 
record of the flourishing growth of trees in Africa on sandy dis- 
tricts, never refreshed by rain nor springs, nor by artificial suj)- 
plies of water : whilst soils of another nature, in the same latitude, 
and not far distant, require irrigation to enable them to sustain 
vegetable life. 
It is to the copious dews of our own country that we have in great 
measure to attribute the productiveness of the meadows border- 
