112 Meieorology of Bombay , by Colonel Sabine, [No. 31. 



If we now* view together the diurnal variations of the wind and gase- 

 qus pressure, we find a minimum of pressure coinciding with the greatest 

 strength of the sea-breeze, a second minimum of pressure coinciding with 

 the greatest strength of the land-breeze, and a maximum of pressure at 

 each of the periods when a change takes place in the direction of the aerial 

 currents ; or, in other words, we find a decrease of pressure coincident with 

 the increase of strength both of the land and sea-breezes, and an in- 

 crease coincident with their decline in strength. 



The facts thuS stated appear to me to admit of the following explana- 

 tion : — The diminution of pressure which precedes the minimum at 4h is 

 occasioned by the rarefraction and ascent of the column during the heat of 

 the day, and its consequent overflow^n the higher regions of the atmos- 

 phere, which is but partially counter-balanced by the influx of the sea- 

 breeze at the lower part of the column. Shortly after the hottest hour is 

 passed, the overflow above and the supply below become equal in amount, 

 and the diminution of pressure ceases. As the temperature falls towards 

 evening, the column progressively contracts ; when the influx from the 

 sea-breeze more than counterbalances the overflow, and the pressure again 

 increases, until a temporary equilibrium is restored, when the sea-breeze 

 ceases, and the pressure is stationary. As the night advances, the air 

 over the land becomes colder than over the sea; the length of the column 

 over the land contracts, and the air in its lower part becomes denser than 

 in that over the sea ; an interchange then commences between them of 

 an opposite character to that which prevailed during the day. The out- 

 ward flow is now from the lower part of the column, or from the land to- 

 wards the sea, causing the pressure to diminish over the land ; it conti- 

 nues until day-break, when the strength of the land-breeze is greatest, be- 

 cause the air over the land is then coldest in comparison with that over 

 the sea. As the sun gains in latitude, and the temperature of the day ad- 

 vances, the land heats rapidly, the density of the air over the land and 

 sea returns towards an equality, the land-breeze declines in strength, and 

 the drain from the lower part of the column ceases to counterbalance the 

 overflow which the land-column is at the same time receiving in the high- 

 er regions ; the pressure consequently having attained a second minimum 

 at or near the hour of the greatest disproportion of temperature, again in- 

 creases until the temperature and height of the column over the sea and 

 land are the same, and the pressure again becomes stationary. But now 

 the rarefraction of the column over the land continuing its increase in 

 height above the less heated column with which it is in juxtaposition, and 



