170 



SCIENCE. 



[Vol. XII. No. 297 



other. Vines admits, however, that the currents at the surface of 

 the earth tend to make the barometric depressions converge and 

 unite. 



In accordance with this theory, the upper currents of these two 

 great barometric depressions met and caused the divergence in 

 the two storms, — one to the south and west, the other to the east 

 and north. 



The theory at present favored by the hydrographer, while not 

 denying the possible influence of upper currents, according to that 

 of Vines, takes into consideration only the surface currents as fur- 

 nishing sufficient data to e.xplain the phenomenon. Briefly stated, 

 it is to the effect that an energetic area of high barometer was cen- 

 tral over the Middle Atlantic States during the occurrence of the 

 two cyclones, the second of which caused the extensive southward 

 rush of air from the high area to impinge against the first low, in- 

 stead of encircling it. The natural result to be expected from this 

 would be the forcing of the latter to the southward, which actually 

 took place. In addition to this, a third low area is stated to have 

 passed westward over Jamaica on Sept. 3 ; and this might have 

 tended still more to draw the first low (the great hurricane) to- 

 wards the south, on the principle that adjacent depressions of the 

 barometer on the earth's surface have a tendency to converge, — a 

 theory favored by VifLes, as noted above. 



The results of the labors of the Hydrographic Office promise to 

 be of great interest and value to both meteorologist and mariner, 

 in connection not only with the great storm which has just oc- 

 curred, but with regard to hurricanes in general. 



Trees on Arid Lands. 



Major J. W. Powell, director of the United States Geological 

 Survey, has written the following interesting letter to the Kansas 

 City Times, thus making a valuable contribution to the discussion 

 of the subjects of forest-growth on arid lands, the effects of hot 

 winds, and the extent to which irrigation may change the agricul- 

 tural climate of the plains : — 



" The plains are treeless because they are arid. There is an 

 opinion widely existing in the popular mind, and springing up in 

 the current literature of the West, which is opposite to this, to 

 the effect that the dryness of the climate is the result of the lack 

 of forests. An argument in favor of tree-planting and forest-cul- 

 ture has often been based on this error. The effect of forests upon 

 rainfall has been investigated by many methods, in many countries, 

 and at many times, and the result of all this investigation shows 

 tliat the presence or absence of trees influences the general rainfall 

 or amount of precipitation only to a very limited degree. It is, in fact, 

 not certain that their presence does increase rainfall ; but it is cer- 

 tain, that, if it does, the increase is so slight as to play but an in- 

 significant part as a climatic factor. 



" Yet forests, or abundant trees, exert an influence upon climate 

 in its relations to agriculture. Two ways in which this influence is 

 exerted are worthy of careful consideration. 



" First, While it is not probable that forests diminish or increase 

 the total amount of rainfall in any country, yet it appears that for- 

 ests regulate this rainfall, so that there are fewer fierce storms and 

 more gentle rains. When the rains fall in storms, the water is 

 speedily gathered into streams, and at once passes from the coun- 

 try ; but, when they fall in gentle showers, time is given to moisten 

 the soil and invigorate vegetation. 



" Second, Forests provide against the speedy evaporation of the 

 water by protecting the lands from the fierce rays of the sun, and 

 more especially by protecting the land from the rapid passage of 

 dry winds, which drink up the water from the soil and growing 

 plants with great avidity. 



" It is manifest that the effect of the forests upon the great move- 

 ments of the atmosphere must be very slight when due regard 

 is given to proportions between cause and effect. Forests can af- 

 fect only the winds close to the earth by creating a friction at the 

 surface ; but the soil, and the smaller plants growing therein, may 

 be greatly sheltered by trees. Though the general climate may be 

 scarcely affected, the agricultural climate may be materially modi- 

 fied. 



"The relation of forests to humidity, and of prairies and plains 

 to aridity, should be clearly understood. In middle latitudes, and 



under average conditions of relative humidity, low, gnarled forests 

 will be produced with about ten or twelve inches of rainfall ; that 

 is, in the Rocky Mountain region, and generally on the Great 

 Plains, forests of cedar and pinon can be produced with a little 

 more than ten inches of rain annually. Now, this is a well-estab- 

 lished fact. Why, then, are the arid valleys and Great Plains tree- 

 less .' The answer is, that the fires destroy the trees, and prevent 

 their growth. In a region of great humidity, say. of forty inches 

 or more of rainfall, forests are largely protected from fire by such 

 general humidity. In regions of country having from ten to 

 twenty-five inches of rainfall, all forests are destroyed unless pro- 

 tected by art or topographic position. In regions having between 

 twenty-five and forty inches of rainfall, prairie-lands interspersed 

 with timber-lands will usually be found ; that is, in ordinary sea- 

 sons, trees will be protected from destructive Sres by the general 

 humidity, but in excessively dry seasons the trees will be destroyed, 

 now here, now there : so that, by the natural process of tree-prop- 

 agation, the forests will encroach on the prairies, and through the 

 fires of excessively dry seasons the prairies will encroach on the 

 forests ; and so prairie conditions and forest conditions forever con- 

 tend with each other for the possession of a sub-humid land. In 

 the direction in which aridity increases, prairie conditions will more 

 and more prevail ; and, as humidity increases, the forest condition 

 will more and more prevail. In general it may be stated, that, 

 other things being equal, the dryer the climate, the smaller the for- 

 ests ; the wetter the climate, the greater the forests ; for, although 

 the rainfall may be sufficient to grow forests, it may not be suffi- 

 cient to protect them from fires. The Great Plains and the valleys 

 of the Rocky Mountains are all capable of sustaining forests of 

 certain trees adapted to the climatic conditions found therein. 



" It is possible, and in due time it will be practicable, for man to 

 clothe the naked lands of the Great Plains and the arid valleys of 

 the West with forests without artificial irrigation. From this must 

 be excepted certain desert-lands vifest of the lower portions of the 

 Colorado River, where the rainfall is insufficient, and also certain 

 tracts of bad-lands which will always be treeless for reasons that 

 need not here be described. 



" The amount of rainfall necessary to produce forests in any 

 given latitude will depend to some extent upon the character and 

 conditions of the soil, some soils needing more rain than others for 

 this purpose; but the soil condition has narrow limits. 



" If it be true, as has been asserted above, that the arid plains 

 and valleys may all produce forests without artificial irrigation if 

 protected from fires, how are such forests to be planted, in what 

 manner pan they be protected, and how shall the trees adapted to 

 the climate be selected ? These are the practical problems to be 

 solved. 



" Great areas of uninhabited land cannot be redeemed and pro- 

 tected : the protection must come from men living on the land, and 

 utilizing it for agricultural and pasturage purposes. The way in which 

 this can and will be done may be briefly and crudely sketched as 

 follows : — 



" Adown the valleys and across the plains flow many streams of 

 water — brooks, creeks, and rivers — that have their sources in the 

 mountains by which the arid lands are dissected ; and all of these 

 streams can be utilized to irrigate the dry and parched lands that 

 now present the desolation of deserts. By their use many tracts 

 of land scattered far and wide throughout the whole country may 

 be brought under cultivation, and covered with growing fields and 

 luxuriant groves. In this manner populous and prosperous settle- 

 ments may be distributed throughout that land of drjiiig winds and 

 scorching suns. When industrious and thrifty people once get a 

 foothold in this manner, they plant orchards and vineyards, and 

 surround their farms and fields with trees, and plant them by the 

 roadside, and every man devotes a part of his farm to timber-cul- 

 ture, and the naked lands are speedily covered with a rich vegeta- 

 tion. A generation ago the prairie region east of the Missouri 

 River was so destitute of forests that large districts were supposed 

 to be practically uninhabitable; but since that time it has been cov- 

 ered with orchards, vineyards, and groves, and now, from the lands 

 that were once so naked, millions of trees spread their branches to 

 the breezes. In the same manner, by means of artificial irrigation, 

 great numbers of tracts of land will be cultivated throughout the 



