the Meteorology of Bombay. 31 



causing the pressure to diminish over the land ; it continues to do 

 so until towards daybreak, when the strength of the land breeze is 

 greatest, because the air over the land is then coldest in comparison 

 with that over the sea. As the sun gains in altitude and the tempe- 

 rature of the day advances, 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 higher regions ; the pressure 

 consequently having attained a second minimum at or near the hour 

 of the greatest disproportion of temperature, again increases 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 

 rarefaction of the column over the land continuing, its increase in 

 height above the less heated column with which it is in juxtaposi- 

 tion, and its consequent overflow, occasion the pressure to decrease 

 until the minimum at 4 o'clock is reached. 



We have thus therefore at Bombay a double progression of tlie 

 diurnal variation of the gaseous pressure ; the principal minimum oc- 

 curring at 4 o'clock in the afternoon, occasioned by an overflow from 

 the column in the higher regions of the atmosphere ; and the second 

 minimum occurring at 18 h , occasioned by an efflux from the lower 

 part of the column. The first minimum is similar to that which has 

 been shown to take place at Toronto, Prague and Greenwich, and is 

 similarly explained : the second minimum, which does not take place 

 at the three above-named stations, is owing to the juxtaposition of 

 the columns of air over the sea and land, which differ in tempera- 

 ture, and therefore in density and height, in consequence of their 

 resting respectively on surfaces which are differently affected by heat. 



Plate It shows the curve of the gaseous pressure, and the curve 

 of the elastic force of the vapour ; and between them is placed a dia- 

 gram illustrating the hours of prevalence and of the greatest strength 

 of the land and sea breezes. At Toronto and at Greenwich the di- 

 urnal curve of the vapour is a single progression, having its maxi- 

 mum at or near the hottest hour of the day, and its minimum at or 

 near the coldest hour. We perceive in the Plate which represents 

 the phenomena at Bombay, the modification which takes place in 

 consequence of the supply of vapour brought in by the sea breeze 

 continuing until a late hour in the evening, and prolonging the pe- 

 riod during which the tension is at or near its maximum. The mi- 

 nimum occurs as usual at or near the hour of the coldest tempera- 

 ture. 



If, then, the explanation which I have offered to the Section, of the 

 physical causes which produce the diurnal variation of the gaseous 

 pressure at Bombay, be correct, the diurnal variation of the barome- 

 tric pressure occurring there is also explained, since it is simply the 

 combination of the two clastic forces of the air and of the vapour. 



The solution of the problem of the diurnal variation of the baro- 

 meter is therefore obtained by the resolution of the barometric pres- 



