MIRACLES OF SCIENCE 



the remaining term of which, according to a well- 

 known formula of the mathematician, would be the 

 mass of the earth. 



As the result of a long series of tests made with 

 this apparatus, Cavendish calculated that the aver- 

 age density of our globe is 5.45, the density of water. 



Cavendish's experiment was scarcely improved 

 upon for almost one hundred years though many 

 times' repeated. Then in 1893 Professor Boys of 

 Oxford repeated his tests, with an apparatus of far 

 greater delicacy than any that had been available. 

 The essential improvement introduced by Professor 

 Boys consisted of using a thread of quartz instead 

 of a wire to suspend the balanced balls. The quartz 

 thread is made by shooting an arrow dipped in 

 molten glass, and it constitutes a gossamer filament 

 of almost unbelievable tenuity far stronger than steel 

 and of marvelous elasticity. It has been alleged that 

 a single gram of sand would furnish material for a 

 thousand miles of this quartz filament. Whatever 

 the truth of this estimate, the quartz thread has 

 unique qualities, and the earth-weighing apparatus 

 which Professor Boys constructed with its aid sub- 

 stituted balls one-quarter of an inch in diameter for 

 the two-inch balls of the original Cavendish instru- 

 ment; these tiny balls being suspended on a rod only 

 half an inch in length and deflected in the various 

 experiments by sets of balls 4% and 2% inches in 

 diameter respectively. The apparatus was operated 

 in a partial vacuum, the deflection being noted with 

 a telescope. The adjustment was so delicate that 

 even the tremor caused by distant earthquake was 



