250 Mr. T. Hopkins on the Hourly Alterations of 



which hour condensation is warming the atmospheric mass in 

 the locality. This warmed mass, a mixture of gases and 

 vapour, as it ascends is acted on by the different laws of cool- 

 ing, of condensation, and of expansion by heat, which the 

 constituent parts of the atmosphere obey; and the result is 

 the formation of a buoyant column of cloud, of greater or less 

 thickness, according to the quantity of vapour that has been 

 condensed. The whole local column being thus made lighter 

 by the liberated heat, it presses with less force on the surface 

 of the earth, and consequently on the barometer. This dimi- 

 nished pressure is however effected through the liberated heat 

 driving a portion of the material of the atmosphere, the gases, 

 from the heated part to other parts of the atmospheric space ; 

 and thus we find that the heat just liberated by the conden- 

 sation of vapour counteracts the increased pressui'e of the 

 aqueous matter, which is at the same time passing into the 

 atmosphere in the^ form of vapour. For it is here contended 

 that the vapour that had been produced from four to ten in 

 the morning, is, soon after the last-named hour, not only raised 

 but condensed, — deprived of a part of its heat, and converted 

 into minute particles of water, which float in the gaseous 

 atmosphere as a cloud ; and as such, undoubtedly form a part 

 of the whole atmosphere and contribute to its weight. It is 

 not therefore through a reduction in the quantity of aqueous 

 matter in the local atmosphere, at this period of the day, that 

 the barometer falls ; but that fall is caused by the expanding 

 power of liberated heat driving from the heated vertical co- 

 lumn a part of the ponderable gases which previously existed 

 within it, and, in that way, by removing a part of the material 

 of the atmosphere, causing the remainder to press with less 

 weight on the barometer. 



The quantity of vapour that passes daily into the atmo- 

 sphere while the temperature is rising, and which does not 

 fall as rain, is returned to the earth as dew on its surface ; 

 and thus an equilibrium is established between the production 

 and the condensation of vapour; but this does not take place 

 during the period of which we have been treating, that is to 

 say, from four o'clock in the morning to four in the afternoon. 

 Vapour is not daily abstracted from the atmosphere by the 

 formation of dew on the surface of the earth, until the baro- 

 meter ceases to fall at four o'clock in the afternoon. At pre- 

 sent we have to consider the influence of vapour during two 

 periods of six hours each, — that in which the barometer is 

 rising from four to ten in the morning, and that in which it is 

 sinking from ten to four in the afternoon : and it has been 

 shown that aqueous matter during the whole of this time was 



