328 H. F. Blanford — On the Physical Explanation, Sfc. [No. 4, 



tendency of the wind at night is very prominently exhibited in the curve of 

 diurnal variation ; hut, although of longer duration, it is at no time so in- 

 tense as the westerly tendency in the early afternoon hours. 



In like manner may be explained the difference of epoch of the corre- 

 sponding phases of the semi-diurnal East and West variation at Calcutta and 

 Bombay. The gradient of pressure, in so far as it depends on the semi- 

 diurnal oscillation, will of course be to the West with a rising pressure and 

 to the East with a falling pressure, and this normal tidal gradient is affected 

 by the small difference of amplitude over land and sea, in such manner, that 

 its changes will be accelerated as affecting East coasts and retarded as 

 affecting West coasts. Now if we suppose that the acceleration in the one 

 case and the retardation in the other amount to an hour or an hour and a 

 half, and that the interval between the change in the direction of the 

 gradients, and their effects on the wind as manifested by the anemometer, 

 is also about an hour and a half, we should roughly reproduce the conditions 

 shewn to exist at Calcutta and Bombay respectively. 



According to this view, the local static pressure of the atmosphere, 

 except in so far as it is affected by irregular movements, is shewn by the 

 height of the barometer at the hours of minimum pressure, and the difference 

 of these expresses the weight of the atmosphere removed and restored by 

 the oscillatory movements between land and sea. 



There is much reason to believe that an oscillation of a similar charac- 

 ter takes place between low plains and deep valleys on the one hand and 

 mountain masses on the other, the air being transferred from the low plains 

 and valleys to the hill masses and high plains (such as those of Tibet) in the 

 day time and returned during the night. Thus, it seems to me, are to be 

 explained the very great diurnal oscillations of the Leh barometric curve, the 

 great amplitude of the midday tide at stations in the Assam valley, and the 

 diminished tide at places such as Boorkee and Lahore which Lie near the 

 hills on the margin of broad plains. Also the stormy afternoon winds of 

 the Dipsang, Changchenmo and Biipshu plains, and the fact that at hill 

 stations such as Simla and Darjiling, the night barometric tide exceeds in 

 amplitude that of the day. 



