EXHALATION OF VAPOR BY TREES. 
173 
part of the solid framework of the vegetable, or a component 
of its deciduous products, it is evident that the superfluous 
moisture must somehow be carried off almost as rapidly as it 
flows into the tree.* At the very commencement of vegeta- 
^ We might obtain a contribution to an approximate estimate of the 
quantity of moisture abstracted by forest vegetation from the earth and the 
air, by ascertaining, as nearly as possible, the quantity of wood on a given 
area, the proportion of assimilable matter contained in the fluids of the 
tree at different seasons of the year, the ages of the trees respectively, and 
the quantity of leaf and seed annually shed by them. The results would, 
indeed, be very vague, but they might serve to check or confirm estimates 
arrived at by other processes. The following facts are items too loose 
perhaps to be employed as elements in such a computation. 
Dr. Williams, who wrote when the woods of Northern New England 
were generally in their primitive condition, states the number of trees grow¬ 
ing on an acre at from one hundred and fifty to six hundred and fifty, 
according to their size and the quality of the soil; the quantity of wood, 
at from fifty to two hundred cords, or from 237 to 1,048 cubic yards, but 
adds that on land covered with pines, the quantity of wood would be much 
greater. Whether he means to give the entire solid contents of the tree, 
or, as is usual in ordinary estimates in New England, the marketable wood 
only, the trunks and larger branches, does not appear. Next to the pine, 
the maple would probably yield a larger amount to a given area than any 
of the other trees mentioned by Dr. Williams, but mixed wood, in general, 
measures most. In a good deal of observation on this subject, the largest 
quantity of marketable wood I have ever known cut on an acre of virgin 
forest was one hundred and four cords, or 493 cubic yards, and half that 
amount is considered a very fair yield. The smaller trees, branches, and 
twigs would not increase the quantity more than twenty-five per cent., 
and if we add as much more for the roots, we should have a total of about 
750 cubic yards. I think Dr. Williams’s estimate too large, though it 
would fall much below the product of the great trees of the Mississippi 
Valley, of Oregon, and of California. It should be observed that these 
measurements are those of the wood as it lies when ‘ corded ’ or piled up 
for market, and exceed the real solid contents by not less than fifteen per 
cent. 
“In a soil of medium quality,” says Clave, quoting the estimates of 
Pfeil, for the climate of Prussia, “the volume of a hectare of pines twenty 
years old, would exceed 80 cubic mbtres [42| cubic yards to the acre]; it 
would amount to but 24 in a meagre soil. This tree attains its maximum 
of mean growth at the age of seventy-five years. At that age, in the 
sandy earth of Prussia, it produces annually about 5 cubic metres, with a 
