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



northern half of square 3 of the North Atlantic, published by the London 

 Meteorological Office. In the former, the double oscillation has almost 

 disappeared, the nocturnal fall of pressure being represented by little more 

 than a halt for some hours between two periods of rising pressure ; and 

 nearly the whole fall of the day takes place between 9 a. m. and 5 p. m. 

 In the case of the Atlantic curve, the day and night oscillations are almost 

 exactly alike ; the night oscillation being only slightly less than that of the 

 day. These characteristic differences are perhaps best expressed by the 

 ratio of the constant co-efficients U' and U" in Bessel's interpolation 

 formula : — 



co = M + V' sin O 6 + O U" sin (n 2 8 + n") + &c. 

 since the magnitude of U' determines the inequality ; and that of Zf", though 

 variable under different conditions of climate, is so to a much less extent 

 than the former term, and chiefly depends on the latitude. The following 

 are the values of V and V" in English inches, and their ratios, for the mean 

 diurnal curves of a few stations (chiefly Asiatic). The arcs u' u" corre- 

 sponding thereto are also given. 



c 







U' 



v! 



U" 



v! 



U' 



U" 



Yarkand 



(9 



months) 



•0348 



4° 33' 



■0215 



161° 59' 



1-6 



1 



Leh 



(September) 



•0517 



343° 9' 



•0254 



143° 19' 



2- 



1 



Lucknow 



(Y 



3ar.) 



•0265 



341° 30' 



•0355 



168° 53' 



075 



1 



Hazaribagh 





11 



•0193 



349° 46' 



•0343 



145° 45' 



0-56 



1 



Calcutta 





11 



•0265 



341° 24' 



•0391 



151° 7' 



0-68 



1 



Bombay 





11 



•0179 



337° 17' 



•0385 



157° 13' 



0-46 



1 



Batavia 





11 



•0240 



24° 7' 



•0369 



159° 34' 



065 



1 



Square 3. 



Atlantic 



•0055 



354° 51' 



•0319 



159° 26' 



017 



1 



As a general rule, the more humid the station and the smaller the 

 range of temperature, the smaller is the value of V ; and hence it has some- 

 times been spoken of as the temperature element of the oscillation ; the 

 double oscillation which is superimposed on it, being referred by Dove, 

 Sabine, and Herschel to the varying tension of water vapour, by Lamont 

 and Broun to some solar influence other than heat, and by Espy .-and Kreil 

 to the oscillation of pressure produced by heat in an elastic fluid expanding 

 and contracting under the influence of gravity. To me it seems that 

 there can hardly be a doubt that the last explanation is the true one,* and 



* True, that is to say, as thus stated in general terms. I do not however fully 

 accept the detailed explanation afforded by any of these authors ; and I am disposed 

 to think that a more probable explanation of the morning oscillation is to be found 

 in the retardation which the transmission of the exalted pressure of the lower to the 

 higher strata must meet with, in the great thickness of highly attenuated but exceed- 

 ingly cold air which constitutes these strata. This pressure cannot be transmitted 



