Page:Popular Science Monthly Volume 2.djvu/222

 stands under this stream of water. Now the drops of water in their fall upon the plate remove by their impact the charge which the plate has by itself—for all bodies have a greater or less electrical condition, and the plate then takes the electrical condition of the air in which it is immersed.

Let us place our water-dropper on the window-sill with its tube projecting into the open air; and, having placed the collecting-plate so that the drops of water may strike upon it, let us notice our little spot of light. It is a clear day in early summer; there are no clouds to be seen, save a rift away on the horizon in the west. The spot of light moves gradually over the scale, indicating that there is a slight positive charge of electricity in the atmosphere. Now it is stationary, and we are about to record the reading of the scale, when the spot of light gives a quick jump and then returns to nearly its original position. Perhaps some movement of ours has deranged the instrument. We look at it carefully, and return to our position of observation. A low rumble, as if of distant thunder, is heard. We do not mind this at first; presently the spot of light darts again along the scale, and again returns to its original position. We stand in silence, waiting for further developments. Now, we hear again a rumble, and a low muttering, as if of thunder in the west. Can this movement of the spot of light have any connection with the distant lightning? At least five minutes must have elapsed between the time of the movement of the spot of light and the moment when the thunder was heard. Again the spot moves, again follows a low peal of thunder; again and again the same phenomenon is observed. There can be no doubt of it: the electrical discharges of the approaching storm, yet miles away, are registered by this little instrument in our laboratory. Now the storm approaches nearer. We hear the wind in the trees; a few drops fall upon the tinned roof; the lightning darts hither and thither, and the spot of light leaps responsively to it.

Such, then, is the delicacy of our instrument. By allowing the spot of light to fall upon sensitive paper, which moves along by clock-work, we shall have all of its motions recorded by photography. This registration has been accomplished by Sir William Thomson, to whom we owe so many beautiful electrical instruments. It would be well if our Signal Service should make contemporaneous observations in different parts of the country by means of these instruments. Such observations could not fail to throw light upon the connection of electrical storms with rain-storms, and extend our knowledge of meteorology.

Let us now, having proved our instruments, approach the question of the influence of flames upon the electrical state of the atmosphere. Our observations must be made in the laboratory, and are necessarily of a somewhat general character from the nature of the subject. It is a cold, clear day in early winter, with the wind blowing freshly from