ABOUT THE 20tH OF DECEMBER, 1836. 157 



more sudden change of temperature. The phenomenon, moreover, of atmo- 

 spheric currents flowing- one over the other in different directions maj be ob- 

 served almost every day of the year. But the fourth cause named, admitting 

 its existence, must be allowed to be by far the most efficient of all. For the 

 reduction of temperature may be conceived to be effected v/ith almost any de- 

 gree of suddenness, and to any extent. Thus, in midsummer, air with a dew 

 point of 80°, being suddenly elevated two or three miles above the earth's sur- 

 face, would, from change of temperature, lose almost the totality of its vapour. 

 By supposing some cause which should continually renew the operation over 

 a limited locality, we could account for the most violent and abundant rains on 

 record. This case, therefore, demands particular consideration. This effect 

 might be produced, 1. By a horizontal current impinging upon the side of a 

 mountain. The current might thus be forced up to an elevation, where it 

 would experience a very great and sudden reduction of temperature, and a por- 

 tion of its vapour be instantly precipitated. 2. A large body of air might be 

 suddenly and mechanically elevated by a volcanic eruption. The direct effects 

 of these two causes must, however, be quite local. If they ever influence dis- 

 tant stations, it must be indirectly, by means of currents here set in motion. 



3. Air may be elevated by a whirlwind; for even in horizontal whirlwinds 

 there is ordinarily, if not always, an upward motion in the centre of the vortex. 



4. When a hot and cold current, moving in opposite directions, meet, the 

 colder, having the greatest specific gravity, will displace the warmer, which is 

 thus suddenly lifted from the surface of the earth, is cooled, and a part of its 

 vapour precipitated. This is a cause which may operate in any locality, and 

 with almost any degree of energy. It is believed, therefore, to be, at least in 

 this latitude, the most common cause of rain. Let us now review the circum- 

 stances of the rain which fell during the period under examination. This oc- 

 curred during the winter, and with a southerly wind. We have here, then, 

 the second cause for the reduction of atmospheric temperature, operating under 

 the most favourable circumstances. Yet the precipitation arising from this 

 cause must begin at the surface of the earth, and proceed very gradually. But 

 in the present case the rain came from a considerable elevation, and poured 

 down in torrents. Although, then, this be admitted as a cause of rain, I can- 

 not regard it as the main cause, nor any thing more than a very feeble aux- 



VII. — 2 p 



