METEOROLOGY. 245 
Whether the lightning actually penetrates into the body of the earth on 
striking, or whether it merely becomes diffused over the surface, depends in 
every instance upon the peculiarities and conducting power of the surface, 
and upon the bodies subjacent. Such bodies are oftentimes considerably 
affected, becoming melted, glazed, or otherwise altered. When lightning 
strikes loose sand, it frequently marks its passage by the formation of long 
tubes composed of the melted sand (known as lightning tubes or fulgurites), 
which are found in various regions of the earth. They generally present the 
appearance of a tube of unequal diameter in different parts, contracted 
inferiorly, and then running out into a point. The outer surface is 
generally rough and sandy ; the inside, however, well fused and smooth, and 
of a greenish color. The length amounts to from twenty to thirty-five feet, 
with lateral branches of from an inch to one foot; the diameter from 
three quarters to twenty French lines; and the thickness of the walls from 
one quarter to eleven lines. All these tubes that have been followed to 
any distance appear to lead to water. Even on the surface of hard rocks 
a glazing is sometimes noticed, probably produced by lightning. 
One striking effect of lightning consists in its affecting those magnetic 
needles in whose vicinity it passes, sometimes reversing the poles, and even 
altogether destroying their magnetism. Under the same circumstances 
magnetism may be communicated in greater or less intensity to unmagnet- 
ized bars of iron or steel. These phenomena only confirm the belief in the 
electrical character of lightning. 
A curious action of atmospheric electricity, to which attention has 
recently been called by Professor Joseph Henry, is the effect produced 
upon the magnetic telegraph. Not only. are the wires often struck by 
direct flashes of lightning, and destroyed or injured, but an inductive 
influence is exerted by distant clouds, which sometimes causes an almost 
fearful play of the register. In many instances thunder storms have 
recorded their own approach on the fillet of paper long before there was 
any other evidence of the fact. And, indeed, in some cases the same effect 
has been produced without the agency of electrified clouds, but simply by 
the different electrical conditions of the strata of air through which the 
wires pass. 
Besides the direct stroke of a lightning discharge, we also have the 
returning stroke, by which all the terrible effects of lightning may be 
produced at a considerable distance from the place where the preceding 
direct discharge had taken place. This phenomenon is explicable on the 
theory of electrical induction. The simplest case is that in which a large 
and heavily charged cloud electrified the earth by induction; on the 
discharge of the former the latter again is restored to equilibrium by the 
diffusion of the electricity which had been heaped up, this diffusion, when 
taking place through bad conductors, producing all the mechanical effects 
of ordinary lightning. The returning stroke is on the whole less dangerous 
than the other, and no credible instance has been adduced of its inflaming 
bodies. 
Of all parts of the earth, thunders and lightnings are most abundant in the 
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