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 cumulus is the thunder-storm factory; and when it develops into a cumulo-nimbus, the thunderstorm is probably at hand.

If the air of a warm, moist summer afternoon becomes still and oppressive and if cumulus clouds increase in size, a thunder-storm is very likely to follow; and if a nearby cumulus expands vertically into a thunder-head the storm is pretty certain to follow, somewhere or other in the vicinity. The thunder-head may be visible every where within a radius of 25 miles, but the storm path may be a narrow strip not more than 30 miles in length. The path of the thunder-storm, like that of the tornado, is determined by the circulation of the cyclone in which it is formed. Its forward movement, except in the extreme southern part of the United States, is from a westerly to an easterly direction.

Safeguards Against Lightning.—The destructive effects of lightning in the United States are chiefly loss of life and loss from fire. Loss of life occurs usually when lightning strikes trees under which people and animals have taken shelter. Trees are the objects most frequently struck. Wooden buildings when struck are apt to take fire instantly, but cases are on record which show that wet shingles and weather boards may be ripped off without further damage. Among structures, oil tanks stand first in the likelihood of destruction by lightning. Church spires and large barns are frequently struck, and isolated buildings are regarded as a far greater risk than city buildings; indeed, in the compactly built areas of a city the risk from lightning stroke is negligible.

Lightning rods afford the best protection against lightning. J. Warren Smith of the United States Weather Bureau found that in many thousand insurance risks, the destruction of rodded buildings was negligible. Other authorities regard the safety afforded by lightning rods at from 90 per cent to 97 per cent. The Bureau of Standards points out the necessity of connecting all exposed metal surfaces such as metal roofs, gutters and tanks with the lightning rods. Sir Oliver Lodge recommends iron in preference to copper as a material for lightning rods for the reason that its greater resistance tends to damp the oscillations of a discharge, practically converting them into a one-way current.