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 streams 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 image 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, instead 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 electrical 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-
