1876.] 



Inequality qftJie tivo Semidiurnal Oscillations. 



321 



oscillation reaches its annual maximum, just at the season when the annual 

 minimum occurs near the sea coasts, even although the general characteris- 

 tics of the atmosphere be substantially the same in both cases." 



I am not at present aware whether Mr. Buchan has been led by these 

 observations to any definite conclusions as to the physical cause of the 

 variation he so clearly summarizes in the passages above cpioted. In the 

 part of his memoir which has reached me, all theoretical discussion is 

 deferred. But these passages afford such remarkable confirmation of an 

 explanation at which I arrived some weeks since, on approaching the subject 

 from an entirely different quarter, that I do not think it necessary to 

 withhold longer the publication of my view. If Mr. Buchan's conclusions 

 are the same as mine, the facts that I have to bring forward will serve to 

 afford independent confirmation of that view. 



Any person glancing over a series of curves illustrating the diurnal rise 

 and fall of the barometer, cannot fail to be struck with the characteristic 

 difference of those places with a continental and those with an insular 

 climate. The case of the Mediterranean, described by Mr. Buchan, seems 

 perhaps to be an exception ; but, as I shall presently shew, it is an exception 

 of such a kind as most strongly to confirm the rule. The accompanying 

 curves are striking, perhaps extreme, examples of this characteristic differ- 

 ence. The first is that of Leh in Ladakh,* situated in the Indus valley 

 (the observatory being 11,538 feet above the sea), and is for the month of 

 September. The climate is characteristically dry, and the summer heat 

 excessive, notwithstanding the elevation. The curve for Yarkand and 

 Kashghar still further north and only 4,000 feet above the sea, is of similar 

 character but smaller amplitude. f The second curve figured is that for the 



* This is computed from the hourly observations recorded during six days by 

 Captain E. Trotter, E. E., and of one day by Dr. J. Scully together with six days' 

 observations by the latter at the hours of 4 and 10 a. m. and p. m. 



t With respect to these curves however, see the final paragraph, page 328. 



