zio THE POPULAR SCIENCE MONTHLY. 



eter 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 con- 

 dition, and the piaffe 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 posi- 

 tion. 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 move- 

 ment 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 



