115 
of Edinburgh, Session 1872 - 73 . 
temperate summer heat and winter cold.* As to the first of these 
expectations, I suppose there can be no doubt that it is justified 
by facts; hut it may not he unnecessary to guard against any 
confusion of the first with the second. Not only does the presence 
of growing timber increase and regulate the supply of running 
and spring water independently of any change in the amount of 
rainfall, but, as Boussingault found at Marmato,f denudation of 
forest is sufficient to decrease that supply, even when the rainfall 
has increased instead of diminished in amount. The second and 
third effects stand apart, therefore, from any question as to the 
utility of Mr Milne Home’s important proposal ; they are both, 
perhaps, worthy of discussion at the present time, but I wish to con- 
fine myself in the present paper to the examination of the third alone. 
A wood, then, may he regarded either as a superficies or as a 
solid ; that is, either as a part of the earth’s surface slightly elevated 
above the rest, or as a diffused and heterogeneous body displacing 
a certain portion of free and mobile atmosphere. It is primarily 
in the first character that it attracts our attention, as a radiating 
and absorbing surface, exposed to the sun and the currents of the 
air ; such that, if we imagine a plateau of meadow-land or bare 
earth raised to the mean level of the forest’s exposed leaf-surface, 
we shall have an agent entirely similar in kind, although perhaps 
widely differing in the amount of action. Now, by comparing a 
tract of wood with such a plateau as we have just supposed, we 
shall arrive at a clear idea of the specialties of the former. In the 
first place, then, the mass of foliage may be expected to increase 
the radiating power of each tree. The upper leaves radiate freely 
towards the stars and the cold inter-stellar spaces, while the lower 
ones radiate to those above and receive less heat in return ; con- 
sequently, during the absence of the sun, each tree cools gradually 
downward from top to bottom. Hence we must take into account 
not merely the area of leaf-surface actually exposed to the sky, 
but, to a greater or less extent, the surface of every leaf in the 
whole tree or the whole wood. This is evidently a point in which 
the action of the forest may be expected to differ from that of the 
meadow or naked earth ; for though, of course, inferior strata tend 
* Journal Scot. Met. Soc., New Series, No. xxvi., p. 35. 
t Quoted by Mr Milne Home. 
