1877.] 339 



XVIII. — Mcmormuhim on the divrnal Variation of A/mof^pJ/rric Pressure 



at the JSand/ieads, by Chas. HviRDiNG, Esq., with a 'prefatory note by 



Henry F. Elanford. 



In the Society's Journal, Part II, No. 1 of 1877, I publislied a short 

 paper on the Variation of the Barometric Tides in connection with the Diur- 

 nal Land and Sea-breezes, in which the diurnal variation of pressure over 

 the Bay of Bengal between latitude 20° and the Sand-heads, in the month of 

 January, was deduced from the ship observations recorded in the log-books 

 collected by the London Meteorological Office. The method pursued in 

 dealing with these observations was a very rough one, and by no means the 

 best that might have been adopted ; and I assumed (what I now admit 

 was not justified) that the observations dealt with were sufficiently nu- 

 merous to yield mean values for each hour of observation which should be 

 comparable with each other. 



A few weeks ago I received from Captain H. Toynbee, the Superinten- 

 dent of Marine Meteorology in the London Meteorological Office, the 

 following Memorandum, which was drawn up at his request by Mr. Chas. 

 Harding, and as it furnishes an imjDortant correction to the results given 

 in my paper above referred to, I have much satisfaction in communicating 

 it to the Society. 



H. F. B. 



In glancing through Mr. Blanford's paper on the Variation of the 

 Barometric Tides in connection with diurnal land and sea-breezes I was 

 much struck by the diagram on p. 48, which shows the afternoon minimum, 

 deduced from the sea observations, to be considerably less marked than the 

 morning minimum ; and from para. 2, p. 47, I gather that the pith of the 

 paper depends greatly upon the shallowness of this afternoon minimum. 



The discussion of the diurnal range of barometer observations at sea, 

 in the neighbourhood of the equator, in the North Atlantic, (see Official No. 

 20, published by the Meteorological Office) in no way supports this excess 

 of depression in the morning minimum, and the experience derived from that 

 discussion has suggested to me a probable cause of much of the difference 

 exhibited by Mr. Blanford. 



I rather fail to reproduce the mean results given by Mr. Blanford, but 

 I see sufficient to warrant me in inferring that whatever observations he 

 has allowed to enter into the discussion have been summed uj) as follows : 

 if the part of the ocean under discussion yielded sixty-four noon obser- 

 vations, thirty-eight 4 P. M. observations, &c., the sixty-four noon observa- 

 tions were meaned and the mean compared with the mean of the thirty-eight 

 4 P. M. observations. 



I have quite ignored that I might refer to the original logs, and have 

 considered myself restricted to the observations as given in the " Data 

 43 



