296 THE POPULAR SCIENCE MONTHLY. 



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 grav- 

 itation. It was accomplished, nevertheless. Cavendish improved 

 Michell's apparatus by applying to it the principle of the famous tor- 

 sion 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 depend- 

 ent 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 kilo- 

 grammes 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 oscil- 

 lated around a new point of equilibrium where the reaction of the tor- 

 sion 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.* 



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 fig- 

 ures differed but little from Maskelyne's (5*44 to 5*58). Baily's experi- 

 ment 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 re- 

 examination 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 experi- 

 ments. " I could not," he says, " rely on these results without repeating the entire com- 

 putation. Still, after a long life spent in abstract researches, being now eighty, and over- 

 whelmed 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. 



