190 FOREST INFLUENCES. 



increase the evaporation, and, consequently, also to increase the rainfall. The wind 

 effect, however, will be small because the regions under consideration are in the belt 

 of calms. On the other hand, if deforestation should initially induce a materially 

 increased run- oft", so that there should be left much less water to evaporate, the 

 CA'aporation would be diminished, and conseiiuently also the rainfall would bo 

 diminished ; and when the rainfall had fallen off the streams would fall to their 

 previous quantity of discharge. We have, therefore, a possibility of two opposite 

 tendencies acting at the same time — one to increase the rainfall, the other to decrease 

 it. The question as to which will preponderate, and to what extent, is apparently 

 a question to be ascertained for each separate district, for the result in one region 

 may T)e quite different from what it will be in another. Furthermore, this analysis 

 suggests that a very promising method of investigating the effect of forestation and 

 deforestation in districts of convective rainfall appears to be not by observations of 

 rainfall, but by comparative observations of the discharge of the streams. Finally, 

 in the equatorial belt, which is the only portion of the world having an almost exclu- 

 sively vertical circulation, the rainfall is so much in excess of the needs of vegeta- 

 tion that its possible modification has not the same economic importance as in 

 higher latitudes. There is no motive to increase it, and if diminished no detriment 

 would result. 



Bordering on each side of the equatorial belt are the regions of the trades, which, 

 over the ocean, are almost rainless ; but over intercepting land areas, such as Central 

 America and the Antilles, considerable rainfall occurs. This is frequently difficult 

 to analyze, but it is largely convective and in hilly regions partly orographic. The 

 seasonal distribution shows that the rainfall is intimately related to the annual 

 oscillation of the limits of the trade wind, and that the rainy season requires a special 

 explanation. With the exception of the well-known tropical cyclones of the seas, 

 the distribution of pressure over the trade region is unfavorable to the develop- 

 ment of a cyclonic circulation, and consequently cyclonic rainfall is seldom pre- 

 sented. It is easily seen that the application of this very general statement to 

 the (juestion of the effect of surface conditions requires us to consider some special 

 individual locality. Let us take the island of Barbadoes. This island is 21 miles 

 long, 14 miles across its widest part, and lies in latitude 13" N., longitude 59° 37' W. 

 Its interior is hilly and rises at points to over a thousand feet in height. From an ex- 

 tended series of rainfall records carried on by Governor Rawson the average rain- 

 fall on the coast is found to be 50 inches, and rising 64 inches on the windward side 

 and in the central highlands. During three-fourths of the year the northeast trade 

 wind prevails and the rain comes from that quarter. In October, when the southern 

 limit of the trade reaches the island, the wind turns to the west and the heaviest 

 rains occur, making it the wet season. The distribution of rain over the island, 

 both with the trade wind and the west wind, shows that the rainfall is partly oro-" 

 graphic, but probably the largest part of it must be considered as convective. On 

 ascount of the smallness of the island and the prevailing fresh winds, practically all 

 of the moisture thus precipitated on the island comes from the ocean, and the 

 moisture evaporated from the island itself is carried away to sea. When the 

 island was covered with forests the convective action of the island could not have 

 been greater than at present, because its temperature Avould have been lower, and if 

 a greater evaporation took place there no appreciable amount of the additional 

 vapor could be precipitated on the island itself We have every reason, therefore, 

 to conclude that destruction of forests or any other service change in Barbados is 

 powerless to sensibly increase or decrease its rainfall. 



Passing from the region of trades, we reach latitudes favorable to cyclonic de- 

 velopment. Here convective and cyclonic action are frequently combined. In the 

 warmer latitudes and in the summer season, the equilibrium of the atmosphere be- 

 comes unstable and convection currents are set up which induce an incipient cy- 

 clonic circulation. Then there is a combined convective and cyclonic rainfall. A3 



