Page:Popular Science Monthly Volume 82.djvu/332

328 effect is one of the conservation of rain already produced without the action of the forest, not a case of an increase of rainfall directly due to the forest.

Another effect of conservation may sometimes he seen when, after a rain, the low clouds ("fog") continue to hang over a forest, and may give another light shower there while no more rain falls over the fields. In this case, the drops left hanging on the leaves evaporate; the air over the forest may become very damp; a slight cooling will suffice to produce a second falling of the same water which fell previously. This is clearly not a case of an increase of rainfall. It is pretty safe to say that it would rain somewhat oftener, and a little more heavily, over tropical deserts if the surface were covered with vegetation instead of being sandy and therefore heated to a high degree, although the cause of the rain is far beyond the action of desert or forest. But tropical deserts are sandy deserts because the general condition of the atmospheric circulation makes them so, not because they have been deforested.

There is one effect of trees, often observable during dense fogs, which results in the collection and precipitation of water drops which would otherwise not fall to the surface. This is a mechanical collection by trees, or it may be by telegraph and telephone wires, or by the rigging on board ship, of the fog or cloud particles carried against the object in question by the moving air. When the amount of water thus collected is sufficient, drops fall from the collector as a gentle shower. Thus there is an actual increase in the amount of precipitation, although no increase in the amount of condensation. Many years ago, Sir John Herschel, during his residence at the Cape of Good Hope, called attention to the fact that, when low clouds were closely overhead, a shower of rain might be experienced under the trees on the side of Table Mountain, whereas no rain fell outside. The explanation which he gave was inaccurate, but the fact was important. Recently, Marloth has shown that the collection of water droplets from the clouds on Table Mountain is an important factor in supplying moisture for the swamps and springs. A rain-gauge with a bunch of grass fastened on wires around its rim, so that the collected water drops would run into the gauge, gave from ten to thirty-five times as much "rainfall" as an ordinary gauge. Further, the number of horse-power furnished by a stream coming down the mountain decreased more than one half after a fire had burned off the vegetation on the top of the mountain. Abbe has called attention to the "steady dripping of trees enveloped in cloud-fog" on the windward side of Green Mountain, on the Island of Ascension. This mountain owes its name to the fact that it is always green with verdure. From its summit comes the principal