27 



Let us for a moment consider what the dew point is, a thing frequently mentioned 

 in relation to this. It is the point at which the air is saturated with moisture to the 

 extent of all it can hold at its then existing degree of heat. If it be cooled it must lose 

 moisture. Now our knowledge of dew on the forest leaves is principally given us by a 

 learned gentleman called Meguscher, who says : — 



" Whenever the temperature of the air is above sixty-seven degrees Fahrenheit, the 

 temperature of the tree will be, to the extent of the excess, lower. If the temperature 

 -of the atmosphere be ninety degrees, and the dew-point seventy-five degrees, there will 

 be a copious deposit of dew, and if the lower temperature be the consequence of radiation, 

 the deposit may be expected to take place over the whole of the upper surface of the 

 leaves, in the aggregate, according to Humboldt's measuring, several thousand times the 

 area of the ground they cover." 



Baron Humboldt's statement, to which this refers, is so clear, concise, and yet 

 -elaborate, that I cannot refrain from giving my readers, in full, at least this one small 

 gem from the innumerable brilliancies of this great traveller's writings. He was one of 

 those always successful writers who are successful first that they see, at any cost of travel, 

 all that is possible concerning they mean to tell of, and next, know how to tell it. He 

 says ; — 



"The forest region acts in a threefold manner — by the coolness induced by its shade, 

 by evaporation, and by the cooling process of radiation. Forests uniformly composed in 

 our temperate zone of ' social plants ' belonging to the families of the OonifersB or 

 Amentacese (the oak, beech and birch), and under the tropics composed of plants not 

 living socially, protect the ground from direct insulation, evaporate the fluids they have 

 themselves produced, and cool the contiguous strata of air by the radiation of heat from 

 their leafy appendicular organs. The leaves are by no means all parallel to one 

 another, and present difierent inclinations towards the horizon, and according to the laws 

 established by Leslie and Fourier, the influence of this inclination on the quantity of heat 

 emitted by radiation is such that the radiating power of a given measured surface a, 

 having a given oblique direction, is equal to the radiating power of a leaf of the size of a 

 projected on the horizontal plane. In the initial condition of radiation of all the leaves 

 "which form the summit of a tree, and which partially cover each other, those which are 

 directly presented towards the unclouded sky will be first cooled. 



" This production of cold (or the exhaustion of heat by emission) will be the more 

 -considerable in proportion to the thinness of the leaves. A second stratum of leaves has 

 its upper surface turned to the under surface of the former, and will give out more heat 

 by radiation towards that stratum than it can receive from it. The result of this unequal 

 -exchange will then be a diminution of temperature for the second stratum also. A similar 

 action will extend from stratum to stratum till all the leaves of the tree, by their greater 

 or less radiation, as modified by their difference of position, have passed into a condition 

 of stable equilibrium, of which the law may be deduced by mathematical analysis. In 

 this manner, in the serene and long nights of the equinoctial zone, the forest air which is 

 contained in the interstices between the strata of leaves, becomes cooled by the process of 

 radiation ; for a tree, a horizontal section of whose summit would hardly measure two 

 thousand square feet, would, in consequence of the great number of its appendicular 

 organs (the leaves) produce as great a diminution in the temperature of the air as a space 

 of bare land or turf many thousand times greater than two thousand square feet." 



Taking this and the preceeding paragraphs together, we must be aware that the 

 forest absorbs much moisture n'om dew, which will either fall in drops on the ground, or 

 •be disseminated in the whole atmosphere within and above the forest. All that falls 

 within or stays within is safe from the sun's drying power. So with whatever rain may 



