54 2 THE POPULAR SCIENCE MONTHLY. 



west 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 dis- 

 tance 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 thunder- 

 bolts 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 pass- 

 ing 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 dis- 

 tant 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 



