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feet in diameter, of the same specific gravity as the earth, should attract with a force one twenty-millionth of the earth's gravity.

In recent times several physicists have succeeded in measuring the attraction of globes of lead having a diameter of a foot, more or less. This measurement is the most delicate and difficult that has ever been made, and the accuracy which seems to have been reached would have been incredible a few years ago. The apparatus used is, in its principle, of the simplest kind. A very light horizontal rod is suspended at its centre by a thread of the finest and most flexible material that can be obtained. This rod is balanced by having a small ball attached to each end. What is measured is the attraction of the globes of lead upon these two balls. The former are placed in such a position as to unite their attraction in giving the rod a slight twisting motion in the horizontal plane. To appreciate the difficulties of the case, we must call to mind that the attraction may not amount to the ten-millionth part of the weight of the little balls. It would be difficult to find any object so light that its weight would not exceed this force. To compare the weight of a fly with it would be like comparing the weight of an ox with that of a dose of medicine. Not only the weight of a mosquito but even of its finest limb might exceed the quantity to be measured. If a mosquito were placed under a microscope an expert operator could cut off from one antenna a piece small enough to express the force measured.

Yet the determination of this force has been made with