310 



It is therefore obvious that the oscillations above alluded to would have 

 this effect: — the iipward one woiild take the moist north-west currents, or 

 portions of them, into higher and colder regions ; the barometer falls, and 

 there is a greater probability of rain. Tt would be different when the return 

 or downward oscillation takes place, for then, according to this view, the upper 

 strata of air are brought nearer to the earth's surface, where they gain in heat, 

 and their watery vapour is consequently more likely to be held in suspension ; 

 the barometer rises, and fine weather generally occurs. It is remarkable, 

 however, that some of our heavy rainfalls take place when the wind is south- 

 east, possibly from its flowing under and lifting up moist and rare strata to 

 greater heights, similar to the effect which a range of mountains has if 

 extending across the route of moist winds. The air is compelled to rise to 

 such a height that its watery vapour is quickly condensed, and falls as rain or 

 snow. We have an instance of this in our own West Coast, where the rainfall 

 is considerably more than on the East Coast, and this without any marked 

 difference of pressure. Refen^ing to the second question, why the increased 

 temperature of the air (for the season) often observed before and during stoi-ms, 

 does not keep its vapoiir from precipitating as i-ain 1 This increase is some- 

 times attributed to the latent heat of the aqueous vapour being made sensible 

 when changed into liquid, but this would not account for the increase of heat 

 which is observed before rain actually falls. 



In fine bright weather, and especially under the influence of dry winds, 

 the air must be storing up more aqueous vapour, and Professor Tyndall shows 

 that the power of aqueous vapour to transmit heat rays of high refrangibility, 

 but to absorb the less refrangible and obscure ones (such as terrestrial heat) 

 situate beyond the red end of the light spectrum, is the chief and potent means 

 of preventing the undue dissipation of terrestrial heat from the earth's surface. 

 Moreover, the clouds which form before rain falls, and also when the pressure 

 is low, prevent the radiation of terrestrial heat at night in a very marked 

 degree. It may be therefore inferred, that the lowest stratum of air would 

 rise in temperature as the aqueous vapour accumulated therein. 



Referring now to the third anomaly — of many gases being liquefied by 

 pressure, whereas increased atmospheric pressure genei-ally indicates less 

 probability of rain. The amount of pressure used by Faraday for the lique- 

 faction of gases was sometimes as high as fifty atmospheres, and provision was 

 made for the removal of the heat arising from comjDression. But the fluctua- 

 tions of atmospheric pressure do not often exceed one-twentieth of the whole. 

 Some bodies, however, are only retained in a liquid state by the atmospheric 

 pressure. Alcohol and ether, for instance, if placed under the air-pump, com- 

 mence boiling. Under the ordinary pressure, however, they still pass into the 

 air, but by the slower process of evaporation from their surface. Water, also, 

 if placed in a vacuum, fills it up at once to the point of its greatest density for 



