• the Vapour Atmosphere at Bombay. 24-9 



that was produced and discharged into the atmosphere within 

 this period? The quantity passed into the atmosphere in 

 the six hours must have been large, it being the product of 

 an average evaporation of 10°'3 for the whole time; whilst 

 the average for the previous six hours was only 7°*3 when 

 the barometer was rising. Now, can it be supposed that a 

 comparatively small additional quantity of vapour raised the 

 barometer considerably from four to ten in the morning, and 

 a further large addition to that quantity, acting in the same 

 way from ten to four in the afternoon, not only ceased to raise 

 it, but was attended by a fall of that instrument to an extent 

 greater than the previous rise, — without some other cause 

 coming into operation ? It cannot be maintained that the 

 increase of surface thermometric temperature had but little 

 counteracting effect before ten, and great counteracting effect 

 after that time until four o'clock. For the rise of the ther- 

 mometer from four to ten in the morning \vas little less than 

 it was from ten in the morning to two in the afternoon, — the 

 time of the highest temperature, — it having risen so much as 

 3°-5 in the former, and only 3°'8 in the latter period. And it 

 should be remembered that evaporation from ten to four was 

 constantly adding fresh vapour to that which was previously 

 in the atmosphere, where the whole was accumulating, and 

 pressing with its aggregate weight on the barometer. There 

 is, therefore, in the facts presented to us in the tables, no 

 countenance for the supposition that temperature, as measured 

 by the thermometer near the surface of the earth, counteracted 

 the increase of vapour pressure after ten o'clock, and caused 

 the fall of the barometer. 



It is however sufficiently evident, from the facts given, that 

 the vapour produced in the morning constituted the material 

 which supplied the heat that at this time rendered the atmo- 

 sphere warm and light in the locaHty, and caused the baro- 

 meter to fall. 



When the sun advances above the liorizon it warms the 

 surface of the globe, and not only increases evaporation of 

 water, but heals that portion of the atmospheric gases that is 

 near the surface, — which portion rises probably in separate 

 streams or columns, — cooler columns at tiie same time de- 

 scending to the surface and taking the place of the warmer. 

 This process goes on as the sun rises higher, until at some 

 particular time, depending on the locality and season, the 

 ascending columns reach a height sufficient to enable the ex- 

 panchng gases to cool and condense some of the vapour which 

 is intermingled with them. Tiiis ordinarily lakes place from 

 nine to eleven o'clock, or say at ten in the morning, from 



