l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



direction, as well as the degree, of its previous force is to be taken 

 from that of the soil on which it previously rested. Hence a wind 

 from the west, if in our hemisphere, will soon be found pursuing 

 a southeasterly course, and crossing successive parallels of latitude. 



The labors of Mr. Espy have been directed to the hypothesis 

 of a central ascending column of rarefied air, and centripetal 

 currents from every side rushing towards its base. Without pur- 

 suing his reasoning, it will be safe to assume that his collection 

 of facts established the existence of a qualified central tendency 

 of the air, in both the general storms and the smaller tornadoes. 

 He presents a theory to account for such motion, which it is not 

 necessary now to examine. Dr. Hare has proposed another method 

 of accounting for tornadoes — a truly brilliant suggestion — of which 

 it is only to be remarked, at present, that it proceeds on the assump- 

 tion of a rush of air from all quarters to a central point. It has 

 been attested also, that at large clearing fires in calm weather, 

 creating centripetal currents, the whirlwind and mimic tornado 

 have been produced. In accounting for the whirlwind motion, 

 therefore, the central tendency of the air will be presupposed. 



In the case of a large fire kindled in an open plain on a calm 

 day, a small circle about the fire is first acted on by the abate- 

 ment of pressure on the side next the fire, and thus receives an 

 impulse toward the common center. As this moves in, the next 

 titer circle loses support and begins to move. Each particle of 

 is moved at first by an impulse towards the center, and during 

 approach to the central region it receives fresh impulses of the 



me direction; and if it comes from some distance its velocity 

 is in this way accelerated, until it reaches the space where the hori- 

 zontal is broken by the upward motion. It is obvious that par- 

 ticles propelled by such impulses would seek the common center 

 in the lines of its radii, and their horizontal forces would be neu- 

 tralized by impact, if no cause for deviation was at hand. But 

 the great law of deflection which affects the course of the winds 

 applies to the movements of these particles. The particles which 

 seek the center from the northern points are deflected west, while 

 those from southern points are deflected east. The whole rush of 

 air from the northern side of the center, coming like a breeze 

 bears west of the center, while an equal breeze from the southern 

 side bears east of the center. The consequence is that the central 

 body of air, including the fire, is acted upon by two forces which 

 combine to make it turn round to the left. These forces are aided 

 by the deviation of the currents from the easterly and westerly 



