1844.] fr: a paper read before the British Association 1845. 113 



its consequent overflow, occasion the pressure to decrease until the mini- 

 mum at four o'clock. 



We have thus, therefore, at Bombay, a double progression of the diur- 

 nal variation of the gaseous pressure ; the principal minimum occurring 

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

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

 mum 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 shewn to 

 take place at Toronto, Prague, and Greenwich, and 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 temperature, and therefore in density and height, 

 in consequence of their resting respectively on surfaces which are differ- 

 ently affected by heat. 



Th^ mean range of the diurnal variation of the temperature in the clear 

 •season is 7-8°, and in the clouded season 3-7°; of the gaseous pressure 

 0 196 !n . in the clear, and O082 in . in the clouded, season; the mean diur- 

 nal range both of temperature and of pressure in the clear season is there- 

 fore more than twice as great in the clouded season, affording a further 

 illustration of the connexion subsisting between the variations of temper- 

 ature and of pressure. 



It does not appear necessary to dwell on the dependence of the diurnal 

 variation in the elastic force of the vapour on that of the temperature. 

 The dependence is an obvious one, and the facts are correspondent. 



If, then, the explanation which I have offered to the section of the phy- 

 sical causes which produce the diurnal variation of the gaseous pressure 

 at Bombay be correct, the diurnal variation of the Barometric pressure is 

 also explained, since it is simply the combination of the two elastic forces 

 of the air and of the vapour. 



The solution of the problem of the diurnal variation of the barometer 

 is therefore obtained by the resolution of the barometric pressure into its 

 constituent pressures of vapour and air, since the physical cause of the 

 diurnal variation of its component pressures have been respectively traced 

 to the variations of temperature produced in the twenty-four hours by 

 the earth's revolution on its axis, and to the different properties possess- 

 ed by the material bodies at the surface of the globe in respect to the re- 

 ception, conveyance, and radiation of heat. 



Annual variation. — If we now proceed to the annual variations, which are 

 shewn in the subjoined table, we perceive that the leading features of the 



15 



