the ends of a movable lever. There was certainly something novel and bizarre in this idea of attempting to observe the attraction of a ball of lead, which we are accustomed to consider an inert mass—in trying to demonstrate by sight its infinitesimal share in universal gravitation. It was accomplished, nevertheless. Cavendish improved Michell's apparatus by applying to it the principle of the famous torsion balance of Coulomb—the torsion of a wire opposed as a moderate force to the attraction exerted on a lever carried by the wire. His experiments were communicated to the Royal Society of London in 1798. The mode of making the observations is easily described. A horizontal lever of fir-wood was suspended to a metallic wire dependent from the ceiling of a closed chamber. At its two extremities were two small balls and two blades of ivory, marked with divisions. All the movements of the lever were observed from without through lunettes fixed in the walls of the chamber, and directed toward these divisions. Finally, two large globes of lead, each weighing 158 kilogrammes and sustained by a screw-gauge, could be moved toward or from the balls at will, by mechanism worked from the outside. Now, whenever they approached the small balls the latter were seen to obey the attraction of the globes of lead; they were displaced, and oscillated around a new point of equilibrium where the reaction of the torsion wire counterbalanced the attraction of the globes. From these experiments and the ascertained strength of the attraction of the globes in relation to their weight, it is easy to estimate the relation of the mass of the globes to that of the earth, and thence the density of the earth. Cavendish thus found the earth's density to be 5·48, that of water being unity.[1]
Cavendish's experiments were repeated by F. Reich, at Freiberg, in two trials in 1837 and 1849, and also at London in 1842, by Francis Baily, under the auspices of the Astronomical Society. Reich's figures differed but little from Maskelyne's (5·44 to 5·58). Baily's experiment gave a little larger figure (5·67). Baily improved upon the apparatus of Cavendish in several ways: he changed the size and material of the small balls, using balls of platinum, lead, brass, zinc, glass, and ivory. The figure he settled upon was the average of over two thousand tests; still, it is not wholly reliable, his results being affected by certain errors the cause of which was for a long time unknown. The question was of an importance that warranted a reexamination of the data with all the resources of modern science. Two French physicists, A. Cornu and J. Bailie, have recently accom-
- ↑ The considerable difference between this number and that furnished by Maskelyne's observations induced Hutton, then advanced in age, to examine anew Cavendish's experiments. "I could not," he says, "rely on these results without repeating the entire computation. Still, after a long life spent in abstract researches, being now eighty, and overwhelmed with infirmities, I feel that I may be pardoned for shrinking from the task. But I should have no rest were I not myself to undertake the work." Hutton discovered many small errors in calculation, and he found 5·31 to be the measure of the earth's density.
