8 ON THE SEPARATE PRESSURES OF THE AQUEOUS AND 



have opposite tendencies, one, the increase of aqueous matter 

 tending to make the total atmospheric pressure more, and the 

 other, the lightening of the gases by heat, tending to make it 

 less. The former produces the greater effect up to say ten 

 o'clock in the morning, by which time the whole atmosphere 

 has become heavier, and the barometer has been raised; — but 

 after that hour the heating of the gases produces the superior 

 effect, and the barometer falls. Now this fall of the baro- 

 meter, it is contended, is a consequence of a reduction of 

 gaseous pressure, and that reduction must be suiTiciently 

 great, not only to produce the fall of the barometer that 

 occurs, but also to do it while the aqueous matter in the 

 atmosphere is increasing in quantity and weight. 



But these opposite changes in the constituents of the atmo- 

 sphere are not always indicated by the dew-point or tension 

 of vapour near the surface; for it is frequently found that 

 when the aqueous portion of the atmosphere is increasing the 

 most rapidly in quantity and weight, the dew-point is falling, 

 and there is consequently a reduction in the vapour pressure 

 on the surface. 



This curious fact may perhaps be observed the most readily 

 on the sands of a sea shore, which, having been recently 

 covered by a tide, are exposed to the action of the morning 

 sun. That luminary, by raising the temperature of the moist 

 sand, evaporates water from it with superior energy, — fills the 

 lowest stratum of the air with new vapour, which, on account 

 of the obstruction presented by the gases, expands with diffi- 

 culty, and therefore shews great tension and a high dew-point 

 near the surface. Indeed at such times the dew-point is often 

 about the same as the temperature, the air being almost satu- 

 rated with vapour ; and incipient cloud is forming at various 

 elevations. But, under these circumstances, let a thick cu- 

 mulous cloud form above, as it very often does, and then some 

 of the vapour that had saturated the lower air is carried up 

 with the warmed gases which ascend within the cloud, — when 



