1S76.] 319 



XVIII. — On the Physical Explanation of the Inequality of the two Semi- 

 diurnal Oscillations of Barometric Pressure. — By Hefey F. Blak - - 

 l"OED, JSIeteoroloyist to the Government of India. 



(Received June 22 ; — Read August 2, 1876.) 



There are, perhaps, few phenomena in the domain of terrestrial physics 

 which have received more attention than the diurnal variation of baro- 

 metric pressure ; and on the causes and explanation of which, nevertheless, 

 there is more diversity of opinion even at the present day. Dove, Sabine, 

 Herschell, Espy, Lamont, Kreil, Broun, and many others have in turn 

 engaged in the discussion of this vexed problem ; and, at the present time, 

 Mr. Alexander Buchan is publishing an elaborate and most valuable resume 

 of the existing data in the Transactions of the Royal Society of Edinburgh, 

 as a preliminary to a renewed investigation. 



The general features of the diurnal variation of pressure are familiar 

 enough to every one who has ever observed the rise and fall of the baro- 

 meter for a few days in India, and most other tropical countries. From about 

 3 or 4 in the morning the pressure increases gradually towards sunrise, then 

 more rapidly,' — and culminates generally between 9 and 10 A. M. A fall 

 then sets in, which becomes rapid during the hottest hours of the day, and 

 the pressure reaches its minimum generally between 4 and 5 p. M. The 

 pressure then increases till about 10 p. M. ; but in general does not attain 

 the same height as at the corresponding morning hour. Lastly, a second 

 fall brings it to a second minimum between 3 and 4 a. m., which, except 

 on mountain peaks and at such stations as Simla and Darjiling, is never 

 quite so low as the afternoon minimum.* 



Thus, then, the pressure rises and falls twice in the 24 hours, attain- 

 in°-, in general, its absolute maximum about 9 or 9. 30 a. m., and its absolute 

 minimum between 4 and 5 p. M. 



This may be taken as a general description of the phenomenon as exhi- 

 bited in the tropics ; but it presents many striking variations at different 



* I must correct this statement. I find, on examining the snip's observations for 

 the month of January recorded in the Bay of Bengal, between N. Latitude 20° and the 

 E. C. Light Ship, i. e., between 60 and 100 miles from the coast, that the form of the 

 diurnal barometric curve afforded by them, in this respect, resembles that of hill 

 stations ; the early morning minimum being considerably lower than the afternoon. 

 The relation of this peculiarity to the phenomenon of the diurnal sea-breeze, and the 

 confirmation it affords of the transfer of air from the land to the sea dining the daytime, 

 in the strata above that in which the sea-breeze prevails, which is the main topic of 

 this paper, are obvious. I have not as yet obtained the data for other months.— Note 

 added January 20th, 1877. 



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