Man and Nature; Or, Physical Geography as Modified by Human Action. George P. Marsh

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СКАЧАТЬ rapidity by radiation. But it must be remarked, that this increased deposition of dew on the neighboring fields is partly due to the forests themselves; for the dense, saturated strata of air which hover over the woods descend in cool, calm evenings, like clouds, to the valley, and in the morning, beads of dew sparkle on the leaves of the grass and the flowers of the field. Forests, in a word, exert, in the interior of continents, an influence like that of the sea on the climate of islands and of coasts: both water the soil and thereby insure its fertility." In a note upon this passage, quoting as authority the Historia de la Conquista de las siete islas de Gran Canaria, de Juan de Abreu Galindo, 1632, p. 47, he adds: "Old historians relate that a celebrated laurel in Ferro formerly furnished drinkable water to the inhabitants of the island. The water flowed from its foliage, uninterruptedly, drop by drop, and was collected in cisterns. Every morning the sea breeze drove a cloud toward the wonderful tree, which attracted it to its huge top," where it was condensed to a liquid form.

      In a number of the Missionary Herald, published at Boston, the date of which I have mislaid, the Rev. Mr. Van Lennep, well known as a competent observer, gives the following remarkable account of a similar fact witnessed by him in an excursion to the east of Tocat in Asia Minor:

      "In this region, some 3,000 feet above the sea, the trees are mostly oak, and attain a large size. I noticed an illustration of the influence of trees in general in collecting moisture. Despite the fog, of a week's duration, the ground was everywhere perfectly dry. The dry oak leaves, however, had gathered the water, and the branches and trunks of the trees were more or less wet. In many cases the water had run down the trunk and moistened the soil around the roots of the tree. In two places, several trees had each furnished a small stream of water, and these, uniting, had run upon the road, so that travellers had to pass through the mud; although, as I said, everywhere else the ground was perfectly dry. Moreover, the collected moisture was not sufficient to drop directly from the leaves, but in every case it ran down the branches and trunk to the ground. Farther on we found a grove, and at the foot of each tree, on the north side, was a lump of ice, the water having frozen as it reached the ground. This is a most striking illustration of the acknowledged influence of trees in collecting moisture; and one cannot for a moment doubt, that the parched regions which commence at Sivas, and extend in one direction to the Persian Gulf, and in another to the Red Sea, were once a fertile garden, teeming with a prosperous population, before the forests which covered the hillsides were cut down—before the cedar and the fir tree were rooted up from the sides of Lebanon.

      "As we now descended the northern side of the watershed, we passed through the grove of walnut, oak, and black mulberry trees, which shade the village of Oktab, whose houses, cattle, and ruddy children were indicative of prosperity."

      Coultas thus argues: "The ocean, winds, and woods may be regarded as the several parts of a grand distillatory apparatus. The sea is the boiler in which vapor is raised by the solar heat, the winds are the guiding tubes which carry the vapor with them to the forests where a lower temperature prevails. This naturally condenses the vapor, and showers of rain are thus distilled from the cloud masses which float in the atmosphere, by the woods beneath them."[181]

      Sir John F. W. Herschel enumerates among "the influences unfavorable to rain," "absence of vegetation in warm climates, and especially of trees. This is, no doubt," continues he, "one of the reasons of the extreme aridity of Spain. The hatred of a Spaniard toward a tree is proverbial. Many districts in France have been materially injured by denudation (Earl of Lovelace on Climate, etc.), and, on the other hand, rain has become more frequent in Egypt since the more vigorous cultivation of the palm tree."

      Hohenstein remarks: "With respect to the temperature in the forest, I have already observed that, at certain times of the day and of the year, it is less than in the open field. Hence the woods may, in the daytime, in summer and toward the end of winter, tend to increase the fall of rain; but it is otherwise in summer nights and at the beginning of winter, when there is a higher temperature in the forest, which is not favorable to that effect. * * * The wood is, further, like the mountain, a mechanical obstruction to the motion of rain clouds, and, as it checks them in their course, it gives them occasion to deposit their water. These considerations render it probable that the forest increases the quantity of rain; but they do not establish the certainty of this conclusion, because we have no positive numerical data to produce on the depression of temperature, and the humidity of the air in the woods."[182]

      Barth presents the following view of the subject: "The ground in the forest, as well as the atmospheric stratum over it, continues humid after the woodless districts have lost their moisture; and the air, charged with the humidity drawn from them, is usually carried away by the winds before it has deposited itself in a condensed form on the earth. Trees constantly transpire through their leaves a great quantity of moisture, which they partly absorb again by the same organs, while the greatest part of their supply is pumped up through their widely ramifying roots from considerable depths in the ground. Thus a constant evaporation is produced, which keeps the forest atmosphere moist even in long droughts, when all other sources of humidity in the forest itself are dried up. * * * Little is required to compel the stratum of air resting upon a wood to give up its moisture, which thus, as rain, fog, or dew, is returned to the forest. * * * The warm, moist currents of air which come from other regions are cooled as they approach the wood by its less heated atmosphere, and obliged to let fall the humidity with which they are charged. The woods contribute to the same effect by mechanically impeding the motion of fog and rain cloud, whose particles are thus accumulated and condensed to rain. The forest thus has a greater power than the open ground to retain within its own limits already existing humidity, and to preserve it, and it attracts and collects that which the wind brings it from elsewhere, and forces it to deposit itself as rain or other precipitation. * * * In consequence of these relations of the forest to humidity, it follows that wooded districts have both more frequent and more abundant rain, and in general are more humid, than woodless regions; for what is true of the woods themselves, in this respect, is true also of their treeless neighborhood, which, in consequence of the ready mobility of the air and its constant changes, receives a share of the characteristics of the forest atmosphere, coolness and moisture. * * * When the districts stripped of trees have long been deprived of rain and dew, * * * and the grass and the fruits of the field are ready to wither, the grounds which are surrounded by woods are green and flourishing. By night they are refreshed with dew, which is never wanting in the moist air of the forest, and in due season they are watered by a beneficent shower, or a mist which rolls slowly over them."[183]

      Asbjörnsen, after adducing the familiar theoretical arguments on this point, adds: "The rainless territories in Peru and North Africa establish this conclusion, and numerous other examples show that woods exert an influence in producing rain, and that rain fails where they are wanting; for many countries have, by the destruction of the forests, been deprived of rain, moisture, springs, and watercourses, which are necessary for vegetable growth. * * * The narratives of travellers show the deplorable consequences of felling the woods in the Island of Trinidad, Martinique, San Domingo, and indeed, in almost the entire West Indian group. * * * In Palestine and many other parts of Asia and Northern Africa, which in ancient times were the granaries of Europe, fertile and populous, similar consequences have been experienced. These lands are now deserts, and it is the destruction of the forests alone which has produced this desolation. * * * In Southern France, many districts have, from the same cause, become barren wastes of stone, and the cultivation of the vine and the olive has suffered severely since the baring of the neighboring mountains. Since the extensive clearings between the Spree and the Oder, the inhabitants complain that the clover crop is much less productive than before. On the other hand, examples of the beneficial influence of planting and restoring the woods are not wanting. In Scotland, where many miles square have been planted with trees, this effect has been manifest, and similar observations have been made in several places in Southern France. In Lower Egypt, both at Cairo and near Alexandria, rain rarely fell in considerable quantity—for example, during the French occupation of Egypt, about 1798, it did not rain for sixteen months—but since Mehemet Aali and Ibrahim Pacha executed their vast plantations (the former alone having planted more than twenty millions of olive СКАЧАТЬ