U" 



u" 



•0415 



151° 34' 



•0423 



146° 48' 



•0437 



146° 44' 



•0425 



146° 38' 



•0385 



148° 13' 



■0336 



146° 23' 



•0396 



150° 30' 



0372 



144° 29' 



•0400 



151° 25' 



•0393 



160° 59' 



0399 



164° 22' 



0411 



158° 55' 



326 H. F. Blanford— On the Physical Explanation of the [No. 4, 



regard to this latter point, it has been shewn by Lamont and Kreil's in- 

 vestigations, that between clear and cloudy days there is a difference of this 

 kind ; and that it is manifested, not only in the greater magnitude of the 

 diurnal coefficient u, but also, although to a much less degree, in that of the 

 semidiurnal coefficient it" of the barometric formula. Further evidence of 

 the same kind is afforded by the values of these coefficients for the several 

 months at Calcutta. 



V u' 



January, '0287 330° 18' 



February, -0319 327° 12' 



March, "0343 329° 27' 



April, -0361 336° 53' 



May, '0325 344° 43' 



June, "0218 357° 28' 



July, -0192 2° 6' 



August, -0218 0° 5' 



September, "0232 354° 41' 



October, -0234 343° 12' 



November, -0250 337° 38' 



December, -0270 335° 18' 



The driest months in Northern India being March and April, Avhile 

 July is the wettest and most cloudy. 



On Espy and Kreil's hypothesis of the cause of the double oscillation, 

 there is no apparent reason why the evening maximum, arising from con- 

 traction and dynamic pressure, should be equal to the morning maximum ; 

 which seems unquestionably due to the increased tension of the lower at- 

 mosphere, in consequence of heating and the introduction of vapour ; and 

 any inequality will of course appear in the value of u 1 or of the coefficients 

 of other terms of odd periodicity. But the fact established by the anemo- 

 meter, that an outflow of air from a heated land area takes place during 

 the day time, at once assigns a cause for the greater part of the equality, 

 viz. an alteration of the static pressure. This is not an overflow in the 

 upper regions of the atmosphere, but an outflow of the lower strata* or a 

 tendency in that direction. It does not, of course, follow that, to produce 

 a reduction in the mass of air over a contiment, there should be an actual 

 motion of the air outwards in all directions. The very small forces in 

 action will be manifested even more in retarding inflowing currents than 

 in accelerating efflux ; and it is only in very dry and highly heated region 

 such as India, that they produce well marked diurnal surface winds blowing 

 outwards towards the sea ; winds of elastic expansion, such as are the hot 



* Excepting of course in the immediate neighbourhood of the coast, when the sea 

 breeze of the lowest stratum is a secondary effect of the outflow. 



