“Ground employed in agriculture is exposed at almost every season of the year to the full action of the atmosphere; and in the drought and heat of summer, much of its strength is evaporated. In land covered with wood, the case is entirely different, as from the shade afforded by the leaves and branches, very little evaporation takes place. This, then, is another reason that serves in some measure, at least, to explain the seemingly paradoxical fact in question. For, that evaporation has a very powerful tendency to exhaust land, by drawing off and dissipating the more volatile part of the matter which {314} assists in the process of vegetation, there can be no doubt, when we consider that any kind of dung may be deprived of the greater part of its strength by being long exposed to a dry atmosphere. Nor is it merely by preserving its own original substance that land in wood has the advantage of cultivated ground. Whatever is extracted from the latter in the form of vapour, falls again, when condensed, in the shape of rain or dew; but, instead of descending wholly on the same spots from whence it rose, it is, of course, diffused over the whole space which the clouds, containing it, may happen to cover, and woods and moors have as good a chance of receiving it on its return to the earth, as the ground in tillage. The part of it which falls, either on the cultivated fields or the naked wastes, may be again evaporated before it has time to be productive of any benefit; but the portion of it which the woodlands imbibe is retained to enrich the soil; for, the umbrage excluding the rays of the sun, there is no possibility of its being extracted a second time. Land covered with trees, therefore, while it never loses any thing, receives, with every fall of rain, or of dew, a tribute from the riches of the cultivated part of the country. The advantage derived from this source is greater than will be credited by those who are not aware how much {315} of the substances proper for vegetable nutriment are exhaled from the land in a gaseous state during the dry season of the year. “But the principal way in which wood becomes instrumental in enriching land still remains to be noticed. When trees attain a certain size, they attract multitudes of birds, which build their nests and seek shelter among the branches. The dung of these animals is the very richest kind of manure which can be applied to land, and possesses, at least, three times the strength of that commonly used in agriculture. The quantity of it produced during the long series of years which trees require to reach maturity is, especially where large colonies of crows take up their abode, very considerable, and must have a powerful influence in improving and fertilizing the soil.