Here is a startling conclusion, and one, I may say, fully justified by scientific evidence. That electric spark, brilliant as it appears to us, is really a hundred thousand times as bright as it seems to be. We cannot speak with the same precision of a flash of lightning; because its duration has not yet been so exactly determined. But if we suppose that a flash of lightning, in a particular case, lasts for the thousandth of a second, it would follow, from the above experiments, that the flash is a hundred times as bright, in fact, as it appears to the eye. =Various Forms of Lightning.=--The lightning of which I have spoken hitherto is commonly called _forked_ lightning; a name which seems to have been derived from the zig-zag line of light it presents to the eye. But there are other forms under which the electricity of the clouds often makes itself manifest; and to these I would now invite your attention for a few moments. The most common of them all, at least in this country, is that which is familiarly known by the name of _sheet_ lightning. This is, probably, nothing else than the lighting up of the atmosphere, or of the clouds, by forked lightning, which is not itself directly visible. Generally speaking, after a flash of sheet lightning, we hear the rolling of distant thunder. But it sometimes happens, especially in summer time, that the atmosphere is again and again lit up by a sudden glow of light, and yet no thunder is heard. This phenomenon is commonly called _summer_ lightning, or _heat_ lightning. It is probably due, in many cases, to electrical discharges in the higher regions of the atmosphere, where the air is greatly rarified; and, in these cases, it would seem to resemble the discharges obtained by means of an induction coil in glass tubes containing rarified gases. But there is little doubt that in many cases, too, summer lightning, like ordinary sheet lightning, is due to forked lightning, which is so remote that we can neither see the flash itself directly, nor hear the rolling of the thunder.