This phenomenon may be easily explained. It consists in a gradual and comparatively silent electrical discharge between the earth and the cloud; and generally, but not always, it has the effect of preventing such an accumulation of electricity as would be necessary to produce a flash of lightning. I can illustrate this kind of discharge with the aid of our machine. If I hold a pointed metal rod toward the large conductor, you can see, when the machine is worked and the room darkened, how the point of the rod becomes luminous and shines like a faint blue star. I substitute for the pointed rod the blunt handles of a pair of pliers, and a tuft of blue light is at once developed at the end of each handle, and seems to stream away with a hissing noise. I now put aside the pliers, and open out my hand under the conductor--and observe how I can set up, at pleasure, a luminous tuft at the tips of my fingers. Now and then a spark passes, giving me a smart shock, and showing how the electricity may sometimes accumulate so fast that it cannot be sufficiently discharged by the luminous tuft. Lastly, I present a small bushy branch of a tree to the conductor, and all its leaves and twigs are aglow with bluish light, which ceases for a moment when a spark escapes, to be again renewed when electricity is again developed by the working of the machine. [Illustration: THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE.] Now, if you put a thundercloud in the place of that conductor, you can easily realize how, through its influence, the lance and bayonet of the soldier, the alpenstock of the traveler, the pointed spire of a church, the masts of a ship at sea, the trees of a forest, can all be made to glow with a silent electrical discharge which may or may not, according to circumstances, culminate at intervals in a genuine flash of lightning. =Origin of Lightning.=--When we seek to account for the origin of lightning, we are confronted at once with two questions of great interest and importance--first, What are the sources from which the electricity of the thundercloud is derived? and, secondly, How does this electricity come to be developed in a form which so far transcends in power the electricity of our machines? These questions have long engaged the attention of scientific men, but I cannot say that they have yet received a perfectly satisfactory solution. Nevertheless, some facts of great scientific value have been established, and some speculations have been put forward, which are well deserving of consideration.