GREAT FIRES AND RAIN-STORMS. 209 



have a very delicate test for the character of the electricity in the body 

 which we bring into the neighborhood of the needle. But, the needle 

 being only about two inches in length, slight movements in it can 

 hardly be detected. How is this to be remedied*? We are going to 

 deal with delicate impulses, and it is necessary to have some means of 

 observing them. Standing at this window, through which the sun 

 sti*eams brightly, with a little mirror, we can, as any school-boy knows, 

 throw the image of the sun in almost any direction that we. please. 

 Now it rests upon the brick wall across the street, a hundred feet or 

 more distant from us. Let us turn the glass slightly : see how small a 

 movement suffices to make the sun's imao-e on the wall dart over at least 

 twenty feet ! Why can we not attach a little mirror, which shall not 

 weigh more than a feather, just above our needle, and let it reflect, in- 

 stead of the sun, a little point of light from a kerosene-lamp upon yonder 

 wall which is four feet from it ? We have done so. We allow the light 

 of the lamp to stream through a small opening in a screen of blackened 

 paper, and to fall upon the mirror. Upon presenting this metallic 

 plate, which has been charged with positive electricity, the spot of light 

 darts along the scale pasted on the wall ; it has gone over nearly six 

 inches of the scale, while the motion of the needle and the mirror was 

 hardly perceptible. We have now an extremely delicate test for the 

 presence of electricity so delicate that even the small charge ever 

 present in our bodies is sufficient when we approach the instrument 

 to make the spot of light dart to and fro. It is only necessary now to 

 have some convenient means of presenting the body to be examined 

 to the needle ; for it will be seen that all movements of the air in its 

 neighborhood must be avoided. To accomplish this end, we surround 

 the needle with four plates of brass, which are carefully separated from 

 the needle. They are in the form of sectors of a circle and lie in an 

 horizontal plane, the suspension fibres of the needle going through a 

 round hole in the centre of the circle of which the sectors form a part. 

 These sectors are separated from each other at first ; the opposite pairs 

 can, however, be connected at will. It is not necessary to dwell upon 

 their peculiar construction : their object is to prevent the charge, led 

 to them by these copper wires, running to any part of the room, and 

 thereby to influence the needle. It will be seen that this instrument,, 

 of which we have explained only the principal features, is wonderfully 

 delicate, and far superior to the old electrometers which showed elec- 

 trical attraction and repulsion by the divergence of two suspended 

 gold leaves. 



It remains now to describe the " water-dropper." This consists 

 merely of a tin vessel carefully insulated at the base, with a long glass 

 tube projecting from an orifice near the bottom. The water runs 

 through this tube and issues in a fine stream from its end, breaking 

 into drops about fourteen inches below it. A collecting-plate connected 

 with one of the brass plates which we have described in the electrom- 



TOL. II. 14 



