46 H, F. Blanford — Barometric Tides [No. 1, 



testing for sensitiveness and those only selected which have satisfied the 

 test. It is therefore improbable that any large correction would be required 

 to render the present data strictly comparable with those of a standard 

 barometer. 



The theory of diurnal land- and sea-breezes, as I understand it, is as 

 follows. Under the morning sun, the air resting on a land-surface is more 

 expanded than that resting on the sea; the larger part of the absorbed 

 solar heat being used up, in the former case, in heating the air, while 

 in the latter it is chiefly employed in evaporating water and charging 

 the air with vapour ; and, as I shewed in a former paper, the pressure of a 

 given volume of air, when heated, is raised more than seven times as much 

 as when the same amount of heat is consumed in charging it with vapour. 

 The exact proportion at a temperature of 80° is 7*27. The expansion that 

 follows in the two cases is not, however, quite in the same proportion, be- 

 cause more heat is consumed in work in the one case than in the other. 

 Supposing that the expansion takes place under the same pressure in both 

 cases, the ratios of expansion, for the same absorption of heat, would be 

 5-44 times as great in the case of the heated air as in that of the air 

 charged with vapour, at the assumed temperature. The chief effect of this 



unequal expansion is to tilt the 

 planes of equal pressure (de, 

 fg) somewhat as represented 

 in the accompanying diagram, 

 and to produce a head of 

 pressure at a certain height in 

 the atmosphere over the land ; 

 while at the land- and sea- sur- 

 face the pressure is perhaps but little altered. This process goes on as long 

 as the temperature is rising ; and the result is a current of air, at a certain 

 height in the atmosphere, blowing from the land to the sea. But this 

 transfer of air from the land- to the sea- atmosphere, while tending to restore 

 equilibrium at the higher level, produces an increase of static pressure at 

 the sea-surface, and reduces that at the land-surface ; and therefore, a return 

 current sets in at the lower level, which is the well known sea-breeze. As 

 is well known, the sea-breeze sets in first on the coast-line ; and as the day 

 advances it extends in both directions, coming from further out at sea and 

 penetrating to a greater distance inland. This continues till the equili- 

 brium at the ground- surface is restored, which, however, does not occur until 

 late in the evening. At Calcutta, the anemometer trace shews that, on an 

 average, the retardation of the sea-breeze is such that it does not set in 

 fairly until about 5 or 6 in the afternoon. Its prevalence, for some hours 

 after this, is familiar to all residents in Calcutta in the cool southerly wind 



