1877,] 45 



V. — Note on tTie Variation of tJie Barometric Tides in connection ivith Diur- 

 nal Land- and Sea-Breezes. — By Henet F. Blanford, Meteorological 

 Beporter to the Government of India. 



(Eeceived January 29 :— Eead March 7, 1877). 

 The theory of the diurnal land- and sea-breezes on sea-coasts is perhaps 

 one of the most familiar of meteorological topics, and the subject finds a place 

 in all our handbooks of meteorology and physical geography, as an illustration 

 of winds of convection. It has long been surmised, as a deduction from 

 the theory, that the daily oscillations of pressure over the sea and the land, 

 in the neighbom'hood of coast-lines, must shew marked differences ; the 

 pressure being higher over the land during the night and early morning, 

 over the sea during the afternoon and evening hours. Hitherto, however, 

 as far as I am aware, this inference has never been confirmed by the results 

 of actual observation ; and the following facts, relating to the diurnal vari- 

 ation of pressure over the head of the Bay of Bengal between 60 and 120 

 miles from the coast of the Sunderbans, will therefore have that interest 

 which must always attach to the confirmation of a familiar theory. At 

 the same time, I may point out, the view which I put forward at a recent 

 meeting of the Society, viz., that a considerable transfer of air takes place 

 in the day-time from the land to the sea, also receives confirmation ; and, 

 regarded as a whole, the resulting phenomenon is, to my mind, a very beauti- 

 ful and at the same time simple solution of a problem in meteorological 

 physics. 



The data which have given occasion to this communication have reached 

 me only within the last few weeks. They are the reduced logs, relating to 

 Indian seas, for the month of January, which have been accumulated for 

 many years past by the London Meteorological Office, established by the late 

 Admiral Fitzroy, and now under the direction of Mr. R. H. Scott and 

 Captain H. Toynbee. The observations have all been made with compared 

 instruments and have been corrected to the Kew standard ; and the extracted 

 observations have been carefully sifted in the course of extraction, and all 

 doubtful entries rejected. Those which I shall now discuss are the baro- 

 metric readings of ships north of latitude 20°, approaching and leaving the 

 Sandheads ; and are therefore all taken between distances of 60 and 120 

 miles from the coast. The observations having been made with marine 

 mercurial barometers, it is probable that the amplitude of the range shewn 

 by them may be somewhat less than would be shewn by barometers with 

 large tubes, were it practicable to use such instruments on board ship ; but 

 all the instruments issued by the Marine Meteorological Department (and 

 such only have been employed) have been subjected to a preliminary 



