Geography of the Air. ^ 153 



ing regard to the vertical distribution of temperature, shows 

 a double oscillation in the annual curve at the level of Leh, which 

 becomes a single one at the height of 4500 meters, while this is 

 substantially the reverse of the oscillation observed below. 



The subject is also treated in another way by Mr. Hill, through 

 analysis of normal monthly means for all India, whereby he suc- 

 ceeds in presenting a formula, the first periodic terms of which 

 represent the two principal factors of the oscillation. 



Mr. Hill has also discussed elaborately the anomalies in the 

 winds of northern India in their relation to the distribution of 

 barometric pressure. The anomalies are: — (1) in the hot season 

 the wind direction frequently shows no relation to the barometric 

 gradient; (2) the winds over the plains show little or no relation 

 to pressure gradients, but an obvious one to temperature, being 

 greatest where the temperature is highest. 



It is pointed out as highly probable that the copious snowfalls 

 of the late winter in the northwest Himalayas not only produce 

 low temperatures on the Himalayan ranges, but subsequently 

 cause dry northwesterly winds over northern and western India, 

 and on this supposition, reliable forecasts of the character of the 

 coming rainy monsoons have been made for a number of years. 

 Convection currents between upper and lower air strata, it is sug- 

 gested by Koppen, explain diurnal variations in wind velocity 

 and direction. At low stations the maximum velocity occurs at 

 the time of the highest temperature, while at high stations the 

 reverse obtains. Hill has examined into an important point con- 

 nected with this subject, that is, the great local differences in the 

 vertical variation of temperature. Hill concludes by saying that 

 high pressures at low levels are the result of low temperatures, 

 and in connection with the fact that wind directions are largely 

 influenced by the irregular distribution of pressure at high levels, 

 it is more important to know the abnormal variations of pressure 

 at the highest hill stations in India than those in the plains. 



Overbeck has lately published a paper on the apparent motions 

 of the atmosphere, in which he clearly and admirably outlines the 

 treatment of the dynamics of the air by his predecessors. He 

 comments on the mode of treatment of Ferrel, as well as those of 

 Guldberg and Mohn. Overbeck then sets forth his own method, 

 and elaborately discusses the influence of the earth's rotation with 

 reference to the resistances which oppose the motion of the at- 

 mosphere. He touches on the effect produced by rapidly moving 



