686 THE POPULAR SCIENCE MONTHLY. 



from any practical meteorologist, while it is directly controverted by 

 recent investigations into the motion of cirrus clouds, which show be- 

 yond a doubt that the motion of the upper currents of air over a cyclone 

 is outward, and not inward, as the descending theory would demand. 



Moreover, some of our readers may have noticed, in " Nature " of 

 January 16th, a notice, copied from the " Times," of the formation on 

 the Lake of Geneva, on January 2d, of a veritable small water-spout, 

 forty feet high and ten yards in circumference, by the meeting of two 

 winds, known locally as the Fohn and the JBise, on the surface of the 

 lake. Here the water-spout was raised, and did not descend from the 

 clouds. 



4. The last theory we shall notice is that of the late Mr. Thomas 

 Belt, who seeks for the origin of the disturbance on the ground, and, 

 like M. Faye, assigns the same explanation to the smallest dust-whirl 

 eddies and the largest storms which sweep over the earth. 



This theory assumes as the first cause the heat of the sun. The 

 heat-rays pass through the atmosphere without warming the upper 

 strata, and so Mr. Belt supposed that over a sandy soil a mass of air 

 close to the ground might rise in temperature much higher than the 

 superincumbent layers of the atmosphere. The lower strata would 

 therefore become lighter, and a condition of unstable equilibrium 

 would arise. This, however, could not last for ever, and, sooner or 

 later, the heated lower air would burst up, and the ascending column 

 thus produced would be the nucleus of the nascent cyclone. 



The difficulty in accepting this explanation is, that we should like 

 some ocular evidence of such a sequence of conditions. The supporters 

 of the theory, however, point to accredited instances of the formation 

 of whirlwinds over volcanoes like Santorin, and over extensive fires 

 like those of Carolina canebrakes. 



In confirmation of these views of the effect of solar heat in pro- 

 ducing a depression, I may cite an investigation by Dr. Hamberg, of 

 Upsala, who has found that in July, 1872, after a prevalence of in- 

 tensely warm weather in southern Sweden, pressure gave way over 

 the heated area, the isobaric lines following the trend of the coast ; 

 and a rotatory movement was thereby generated in the atmosphere 

 above it, resulting in a perfectly formed cyclone which passed on over 

 northern Finland. It would appear, therefore, that the production of 

 a cyclonic disturbance may be attributable to more than one agency, 

 as all the theories mentioned have some facts in their favor. 



Leaving, then, this abstruse and imperfectly understood line of in- 

 quiry, let us proceed to a subject which yields us results of more im- 

 mediate practical utility : the character and history of the storms when 

 they have once started on their travels. I shall commence by saying 

 that a greater mistake can not be made than to assert that all storms 

 are distinctly connected with cyclonic disturbances. 



TJie force of the wind depends on differences of atmospherical pres- 



