244 PHYSICS. 
sometimes lasting for several seconds, and occasionally swelling out and 
becoming stronger, and again fainter. In single cases the duration of the 
sound has been estimated at from half to three quarters of a minute. The 
longer duration of the thunder is explained by the time necessary for the 
effect of the lightning in the air to come to the ear, the path of the hghtning 
often being of great length. There is generally a certain interval between 
the flash of lightning and the breaking forth of the thunder ; this is because 
sound requires a longer time to traverse a given space than light, and in 
this way we may calculate the distance at which a visible discharge of 
lightning took place, by noting the time which elapses between the flash and 
the report. Allowing in round numbers 1000 feet for every second, the 
interval in some cases is less than half a second, in others forty to fifty, and 
in an instance cited by Arago seventy-two seconds. 
Lightning without thunder often occurs in nearly clear as well as in 
cloudy weather. This heat lightning is seen in the horizon in low distant 
clouds, whose distance is too great to hear the accompanying thunder. The 
lightning seen at a little distance above the horizon shortly after sunset in a 
clear sky, appears not always to be accompanied by an explosion of thunder, 
but is at any rate an electrical phenomenon. 
Lightning always follows the best conductors in its passage to the earth, 
and especially metals, in whatever way these may be covered by other 
bodies. As a general rule, little injury is produced, except on entering and 
leaving the mass, in which case the surrounding bodies are thrown about, 
torn up, pulverized, &c., the metal itself being partially melted. This is 
strikingly the case with wires which are not thick enough to allow of a_ 
ready passage to the fluid. Besides metals, lightning strikes men and 
animals, which are either killed or rendered senseless: these, next to metals, 
appear to be the best conductors, and after them, moist objects in general. 
Elevated bodies, particularly if they happen to be good conductors, are most 
apt to be struck, as towers, trees, steeples, &c. Sometimes what are called 
magic circles are seen in meadows, circles of three-or four feet in diameter, 
where the grass has been singed. This appears to be owing to the lightning 
having fallen in considerable quantity. The second. crop of grass from 
these circles is generally much fresher and greener than the rest of’ the 
meadow. When bad conductors are interposed in the path of the electric 
discharge, they are torn in pieces and scattered around, exercising not 
unfrequently an enormous power. Thus, in England, in the year 1809, a wall 
consisting of 7000 bricks, and weighiny about twenty-five tons, was displaced 
from its position. In many cases part of the effect produced is to be 
ascribed to the sudden formation of highly, elastic steam from the moisture 
of the conductor. We thus explain the fact that the green living tree, 
containing an abundant supply of sap, is more injured than a dead one, in 
which only a small portion of moisture exists. When combustible bodies 
_are struck, they are generally set on fire; sometimes, however, only carbon- 
ized or shattered, in which latter case we sometimes speak of a cold stroke. 
The stroke is generally acconipanied by a peculiar smell, due probably to 
the sudden formation of ozone. - | 
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