Page:Popular Science Monthly Volume 37.djvu/558

542 of Ireland, at Valencia. These circumstances are accounted for by the fact that thunder-storms are always associated with great differences of temperature in adjacent masses of air. Such conditions are most likely to occur in hot climates, where the soil gets excessively heated in the daytime, while the air at some distance above it is cool. In cold climates they occur in winter, where a shift of wind from southwest to northwest is sometimes accompanied by a sudden fall of temperature of 15° or even 20°.

We of the British Islands owe our comparative immunity from thunder-storms to our damp climate. The fact is well known that it is comparatively difficult to perform any electrical experiments in these islands, and that all apparatus must be kept constantly in front of a fire in order to prevent moisture being deposited on it. Accordingly, we must suppose that the electrical disturbances which would give rise to explosions and severe storms in France or Germany may pacify themselves comparatively quietly in our atmosphere, and at most only give rise to phenomena of a very moderate character.

I must now say something about the actual lightning flash, which is neither more nor less than a violent electric spark. Three different forms of lightning are generally admitted to exist: (1) The actual flash, or what is commonly called "forked lightning." (2) "Sheet lightning," which usually is the illumination of the sky by a lightning flash which takes place below the horizon. (3) "Globular lightning."

1. As to the term "forked lightning" the representations of it given by artists, which resemble the so-called thunderbolts placed in the hand of Jupiter, are quite absurd. The flash, when photographed, exhibits itself as a line which is continually changing its course, and is described as "intensely crooked" by a very careful observer. It never proceeds for a time in a straight line, and then, turning at a sharp angle, going on farther in an equally straight line, as is represented in pictures. The forking of it is very marked, and this occurs by side flashes passing off from the main track, and eventually losing themselves, like the ramifications of tree-roots. Occasionally the lightning appears to start from a point from which several flashes diverge in different directions.

2. "Sheet Lightning."—Whenever a flash passes from cloud to cloud, or from cloud to earth, the light produced by it illuminates the sky in the neighborhood, and the more intense the flash, the more brilliant and extensive the illumination. At times sheet lightning has been proved to emanate from an ordinary storm distant more than a hundred miles from the point of observation. It is, however, maintained, and apparently with good reason, that occasionally lightning of the "sheet" type, such as what is called