468 Mr. T. Hopkins on the Meteorology of Bombay. 



thereby produced. Thus through solar influence two ca-ises 

 are brought into operation at the same time, which produce 

 opposite effects on the pressure of the atmosphere, — the one 

 tending to make it lighter, the other heavier. 



By a careful registration of the hourly alterations of both 

 the wet and the dry-bulb thermometers, we may obtain, — first, 

 a knowledge of the daily changes of temperature near the 

 surface of the earth ; and secondly, of the extent to which eva- 

 poration keeps down the temperature in the wet-bulb thermo- 

 meter: and from the extent to which temperature is thus 

 kept down in the wet-bulb thermometer, we may infer the 

 energy with which vapour is produced from wet surfaces, 

 and thrown into the air to increase the aggregate atmospheric 

 pressure. If the daily fluctuations of the barometer were pro- 

 duced by these two influences alone, those influences might, 

 without much difficulty, be traced to their results, and their 

 separate as well as their joint effects might be exhibited in 

 the movements of the barometer. 



But in most of the cases where registrations of the two in- 

 struments have been furnished, the barometric fluctuations 

 have not been found to accord with the increase of vapour 

 produced by evaporation. From sunrise until about ten 

 o'clock in the morning evaporation generally proceeds with 

 considerable energy, and a consequent increase of vapour 

 pressure may be traced in a rise of the barometer ; but after 

 that time any increase of vapour pressure on the barometer 

 ceases : yet that evaporation continues active afterwards is 

 shown by the effect that is produced by it on the wet-bulb 

 thermometer. What then becomes of this additional vapour ? 

 Or why does it not further raise the mercury of the barometer ? 



About ten o'clock in the morning the barometer ceases to 

 rise, and shortly afterwards begins to fall, and it continues 

 falling until four in the afternoon, although the cooling of the 

 wet-bulb thermometer by evaporation shows that from ten 

 until one or two o'clock, the time of the highest temperature, 

 vapour must be passing into the atmosphere in increasing 

 quantities ! and the vapour must be presumed to accumulate 

 in the atmosphere until four o'clock, as the evaporation gene- 

 rally continues active up to that hour, and no decline of the 

 dew-point indicates the condensation of any portion of the 

 vapour thus produced. Now if increase of vapour had raised 

 the barometer up to ten o'clock, why should not a further 

 accumulation of vapour after that hour raise the barometer 

 still higher? The fall of the barometer after ten o'clock, it is 

 evident, cannot be accounted for from the operation of the 

 causes named. 



