112 



NATURE 



[January 26, 1922 



mous a rainfall out of the monsoon current of 

 July, and scarcely any out of the indefinite sea- 

 winds of May, is shown by Dr. Simpson to be i 

 twofold— the July winds are both markedly 

 stronger and damper. At Bombay, for example, 

 the mean wind velocity is 7-4 miles per hour in 

 May and 14-2 in July, and the relative humidity 74 

 and 86 per cent, respectively. Consequently, when 

 a wind from the Arabian Sea mounts the Western 

 Ghats, condensation will for two reasons proceed 

 more actively in July than in May. Referring to 

 Figs. I and 2, it will be seen that the pressure- 

 gradient over India is steeper in July than in May, 

 which means stronger winds, and that the power- 

 ful monsoon current of the former month is 

 supplied from the south-east trade wind, with the 

 result that the air which reaches India, after 

 traversing some 2000 miles of sea, is necessarily 

 very humid. In May, on the contrary, the light 

 winds on the west coast blow somewhat north of 

 west and conflict with the south-east trade w^nd 

 over the equatorial part of the Indian Ocean, where 

 rain falls instead of in India. 



ReaHsing how illusory charts of mean monthly 

 meteorological conditions may be as representing 

 actual conditions on , any particular day, Dr. 

 Simpson is able to show that the circulation in 

 the Indian area rarely differs essentially from the 

 mean, and that breaks in the monsoon are asso- 

 ciated with temporary reversions to the conditions 

 typical of May, when clear skies and fierce sunshine 

 are broken only by violent local thunderstorms. 



One cannot but support his conclusion that 

 without the mountains the general rainfall of 

 India would be lighter, if more evenly distributed. 

 Those who argue that in any case the southern 

 portion of the peninsula, below about 18° N., 

 would experience the full effect of the annual 

 northward swing of the equatorial rain-belt forget 

 that the steady equatorial rains depend upon the 

 convergence of air-streams from the northern and 

 southern trade systems, and that where, as on 

 the Benadir coast (Italian Somaliland), such a con- j 

 vergence is prevented by the monsoon regimen I 

 itself, there is found the anomaly of a nearly i 

 rainless strip of coast within 10° N. of the 

 equator. A problem which should engage atten- 

 tion as facilities for travel and research in this 

 part of the world increase is the precise effect of 

 the Himalayas and the high plateau of Tibet upon 

 the strength of the south-west monsoon. The 

 effect of a broad, cold tablelaad 10,000 ft. high is 

 more likely to be negative than positive — that is, 

 to weaken rather than to strengthen the monsoon. 

 The late Prof. Herbertson, whose insight into 

 climatological questions was not perhaps ade- 



quately appreciated by meteorologists, used rcr 

 peatedly to discountenance exaggerated notions 

 concerning the "flue-like" action of Central Asia 

 often entertained by those who rely too much on 

 isothermal maps reduced to sea-level. In any 

 case, there would be a general inflow of air to- 

 wards Asia in summer, as is so well exempUfied in 

 China; but the real controlling centre of the 

 powerful south-west monsoon of India is situated 

 near the mouth of the Persian Gulf, and it is 

 this "cyclonic" centre which guides the air- 

 currents across India to the base of the Hima- 

 layas, which they must perforce mount, and 

 thence on to the highlands of Tibet, 



To summarise, the primary condition of the 

 south-west monsoon is a centre of low barometric 

 pfessure situated to the north-west of India, due 

 to the heated state of this region in summer, and 

 to a certain extent of the Asiatic land-mass as a 

 whole. The special local effect of India itself is 

 quite subsidiary, merely serving to give the isobars 

 and air-currents across the peninsula their final 

 trend. In May the local heating of India, then at 

 its maximum, does not suffice to bring about mon- 

 soonal conditions, but the general Asiatic heating 

 in July does, for reasons discussed. A heavy rain- 

 fall accompanies the monsoon because it is both 

 a humid and a powerful current, and is met more 

 or less at right angles by various high mountain 

 ranges. 



Finally, it is advisable to refer to certain general 

 principles in connection with the theory of the 

 monsoonal circulation, the importance of which is 

 duly emphasised by Dr. Simpson. Whilst it is a 

 conspicuous fact that, broadly speaking, the con- 

 tinents command high pressure in winter and low 

 in summer, and the oceans vice versa, the more de- 

 tailed relationship between pressure and tempera- 

 ture is exceedingly complex, and the precise loca- 

 tion of a centre of high or low pressure depends 

 upon many other factors, such as the rotation of 

 the earth and the configuration of the land. In 

 other words, the atmosphere being a unity of 

 interdependent parts, it is largely a matter of com- 

 promise, as between region and region, what type 

 of circulation shall prevail here and what there. 

 To take but an instance. Nothing is more per- 

 plexing than the drought-producing wind and 

 pressure regimen of the Mediterranean basin in 

 summer with intense insolation and active evap- 

 oration to a meteorologist who conceives of this 

 region as isolated from other regions, and forgets 

 that the Mediterranean circulation has to adapt 

 itself to the great monsoon system of ■ Asia, as 

 well as to the conditions in other parts of the 

 world. 



Helium in Natural Gas. 



By H. B 



'"THE researches of H. P. Cady and D. F. 



j- McFarland in 1905 on some natural gas from 



Kansas led to the interesting discovery of the 



presence of helium in that gas, a fact of which 



NO. 2726, VOL. 109] 



MiLNER. 



advantage was taken afterwards by the United 

 States military authorities during the later stages 

 of the war. In 191 5 the natural gas resources of 

 this country were investigated for a similar purpose 



