I 26 



NATURE 



[Dec. 12, 1889 



In point of fact, as may be seen on Dr. Hann's charts 

 for January and July, in the new edition of Berghaus's 

 " Physical Atlas," the diversion of the trade-winds of the 

 Gulf of Mexico, northward up the Mississippi valley 

 takes place only in the summer, and is an effect of the 

 satne agency, viz. the heating of the northern continents, 

 that breaks up the high-pressure zone of the northern tropic 

 into two anticyclones, one in each of the great oceans, 

 and it is the juxtaposition of the Atlantic anticyclone 

 and the Mexican cyclonic depression that determines the 

 course of the winds and the resulting rainfall. To judge 

 from the case of the Western Ghats, we think it may be 

 safely concluded that, if there were no mountain-chain to 

 the west of the Gulf, the results would not be greatly 

 different. All the other instances quoted, illustrative of 

 the diversion of great currents by mountain-chains, ex- 

 cept such as are purely local, appear to us to be really 

 due to other and similar causes. 



In treating of the monsoons, Prof. Ferrel points out 

 with perfect justice that their strength depends on the 

 form of the land, and that they blow strongly only where 

 the interior of the country is high and mountainous. 

 But when he adduces Persia as an illustration of the 

 negative case, we are unable to admit its relevancy. At 

 p. 199 he observes : — 



"In accordance with the preceding view of the prin- 

 cipal cause of monsoons and land and sea breezes, it is 

 seen from observation that all the great monsoons and the 

 strongest land and sea breezes are found — the former in 

 countries and on oceans adjacent to high mountain- 

 ranges, and the latter along coasts with high mountains 

 in the background. Neither the heated interior in sum- 

 mer of the Great Sahara of Northern Africa, nor of 

 Arabia and Persia, which is considered the warmest re- 

 gion on the globe, causes, during this season of the year, 

 any great indraught of air. It is true that at this season 

 the north-westerly winds prevail a little more on the 

 north-west coast of Africa and the ocean adjacent, due, no 

 doubt, to the influence of the highly-heated desert of 

 the Sahara,; but over Arabia and Persia the north-west 

 winds continue to blow almost incessantly, during June 

 and July, away from the interior toward the Arabian 

 Sea. . . . The monsoon influence, therefore, of countries 

 mostly level, without an elevated interior, however highly 

 they may become heated in summer or cooled in winter, 

 is not very great." 



But the interior of Persia is a part of the great table- 

 land of Iran, and, to quote the description of Sir Oliver 

 St. John, " its average height above the sea may be about 

 4000 feet, varying from Sooo or higher in certain of the 

 outer valleys to not more than 500 in the most depressed 

 portions of its centre." Its average elevation is therefore 

 much greater than that of the interior of India, very much 

 greater than that of the Indo-Gangetic plain, which is the 

 goal of the Indian monsoon, and, as a glance at the map 

 will show, it is not deficient in mountains. The explana- 

 tion of the fact that, instead of attracting the monsoon 

 from the Arabian Sea, it is itself swept by north-west and 

 west winds — blowing, not, indeed, towards the Arabian 

 Sea, but towards the lower Indus valley — must then be 

 sought for elsewhere. The true explanation appears to 

 us to lie in a combination of causes. Partly, perhaps, in 

 the latitude, which brings it within the zone of the strong 

 easterly current of extra-tropical regions, which, by its 

 right-handed pressure, must resist any indraught from 



the Arabian Sea ; but chiefly in the fact that any tendency 

 that the heated highlands of Persia may have to create 

 such an indraught is overborne by the stronger set 

 towards India. For the latter country reaches far down 

 into the tropics, and the centre towards which the mon- 

 soon blows must be determined by the resultant of all 

 the temperature gradients of the whole heated region. 

 An eastward direction having been given to the monsoon 

 at the outset, its strength in that direction is greatly in- 

 creased by the energy set free in the Indian monsoon 

 rainfall. 



This question is one of more than theoretical import- 

 ance. These west winds of Persia and Afghanistan are 

 the dry winds of Northern and Western India, and wherk 

 they prevail beyond their normal limits, over the north 

 of the Arabian Sea and a great part of India itself, to the 

 exclusion of the rain-bearing current, they bring the 

 drought and consequent dearth that have made India sck 

 disastrously notorious for its famines. Possibly, the ex- 

 planation of their abnormal extension may be looked for 

 in those oscillations of the great polar cyclonic systems 

 to which Prof. Ferrel alludes at p. 339 of his work. 



Cyclones and tornadoes are treated at great length , 

 each of these subjects occupying more than one hundred 

 pages of the book ; and in connection with the latter is 

 given the author's theory of the formation of hail, a subject 

 which has hitherto been less understood than almost any 

 other phenomenon of the atmosphere. It will be best 

 given in the author's own words : — 



" In the ascending current of a tornado, as in that of 

 the equatorial calm belt, or of a cyclone, the rain-drops 

 are formed down in the cloud region, and carried upward 

 until they become too large to be supported by the current 

 and so fall to the earth. ... In a tornado, however, the 

 ascending current is often so strong that the rain is 

 supported until, by the blending of the small drops by 

 coming in contact, very large drops are formed, and the 

 strong ascending currents often extend so high that these 

 large drops are carried away up into the region of freezing 

 temperature. . . . There they are frozen, and after having 

 been carried up and outward above to a distance from the 

 centre, where the ascending current is not strong enough 

 . . . to keep them up, they slowly descend, and receiving 

 additions of ice as they fall, as long as their temperature 

 remains below zero, . . . they finally fall to the earth as 

 solid hailstones " 



The concentric coatings so commonly observed in large 

 hailstones are explained by these hailstones being carried 

 again and again into the vortex by the strong indraught 

 in the lower part of the storm-cloud, the theory being 

 that every hail-cloud is a tornado, although it may not 

 reach down to the lower atmosphere. The vapour being 

 condensed as water in the lower part of the vortex, which 

 is frozen at a higher level, and as snow in the upper part, 

 each pair of coatings indicate an additional ascent through 

 the storm-cloud. This view, which, even at first sight, 

 seems far more reasonable than any previous theory, has 

 received unexpected confirmation from the experience of 

 more than one adventurous balloonist, more especially 

 that of Mr. John Wise, whose fate it was to be drawn 

 seven times successively into the vortex of a hail-cloud,, 

 and carried up repeatedly until the balloon was thrown 

 out at the top. The account is, unfortunately, too long 

 for extracting. 



