328 THE POPULAR SCIENCE MONTHLY 



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 atmos- 

 pheric circulation makes them so, not because they have been deforested. 



Influence of Forests in Collecting Moisture from Clouds and 



Fogs 



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 rig- 

 ging on board ship, of the fog or cloud particles carried against the ob- 

 ject in question by the moving air. When the amount of water thus col- 

 lected 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 fas- 

 tened 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 "rain- 

 fall" as an ordinary gauge. Further, the number of horse-power fur- 

 nished 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 



