Page:Popular Science Monthly Volume 15.djvu/704

686 from any practical meteorologist, while it is directly controverted by recent investigations into the motion of cirrus clouds, which show beyond 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 Föhn and the Bise, 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 producing a depression, I may cite an investigation by Dr. Hamberg, of Upsala, who has found that in July, 1872, after a prevalence of intensely 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 inquiry, let us proceed to a subject which yields us results of more immediate 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.

The force of the wind depends on differences of atmospherical