Page:Popular Science Monthly Volume 23.djvu/770

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progress of the cyclone proper across the country. They may occur near, or in advance of, the cyclonic storm-center; or they may appear near the outer edge of the general disturbance, hundreds of miles away. They usually develop within the area of highest temperature, and are often preceded by a brassy sky, and hot, gusty winds, sometimes followed by a sultry atmosphere and an ominous calm. They occur most frequently near the close of the day, their general direction being from southwest to northeast. They vary much in size and force, but all have the same general characteristics in regard to appearance and action; being a funnel-shaped cloud, heavily charged with electricity, that goes bounding and whirling along in close proximity to the earth's surface, dealing death and destruction wherever it touches. The generally accepted belief is, that tornado-clouds are formed suddenly, by the meeting of warm and cold currents of air; or, by the union of a positive with a negative cloud, a partial vacuum being formed, constitutes the axis around which the cloud begins to whirl, gathering strength and increased velocity as it goes.

As the tornado now sweeps onward in its course, it rises and falls with a series of bounds, and, with a swaying motion, describes a zigzag course, now forming a chain of loops, and again shooting off on an obtuse angle, varying in the speed of its forward motion, which may be anywhere from ten to thirty miles an hour. At the same time it is rapidly whirling on its axis in the opposite direction from a screw, or the hands of a clock, the air revolving around the vortex necessarily attaining a speed of several hundred miles an hour. First widening, then contracting, now bounding above the tree-tops, and again descending to sweep the earth bare of every object within its reach, the aerial monster surges onward. The largest forest-trees, mere playthings in its grasp, are plucked up by the roots, or snapped off like pipe-stems; substantial buildings are first crushed like egg-shells, then caught up in the vortex and the d├ębris carried sometimes for miles, before it is again thrown off by centrifugal force, and falls by gravitation anywhere, everywhere, as soon as released from the monster's grasp.

It is difficult accurately to describe the tornado's appearance and work, even for those who have been eye-witnesses, or who have personally passed through the horrors its coming brings. While accounts differ as to its appearance and behavior, as witnessed from different points of observation and under different circumstances, all substantially agree that it is cone-shaped, its motion rotary, that its apex resembles fire and smoke, and that vivid lightning and heavy rain-fall usually accompany it. In rare instances, electricity, in the form of St. Elmo's fire, will precede the vortex, and a white, steamy cloud will follow. It will be observed that the form of the tornado-cloud is nicely illustrated by the "proof-plane" used in teaching natural philosophy. The small end of the plane is most heavily charged with electricity, and,