A Yorkshire Rector of the Eiiihteentli Century. 21 



activity in which Thomson, Rutherford and others ]ia\e 

 actually controlled the velocities of the electric corpuscles by 

 the agency of field of force and have directly counted the 

 numbers of them that are shot out from active matter.'* 



Reference has already been made in the foregoing pages 

 to the ingenuity and inventiveness shown by John Michell in 

 devising and constructing apparatus for the investigation or 

 demonstration of physical problems. By much the most 

 famous of these pieces of mechanism was the Torsion-balance 

 with which he proposed to determine the mean density of the 

 globe. This appears to have been one of his latest inventions : 

 at least, he died before he was able to put it into use. After 

 his death it passed into the hands of his friend, Henry Cavendish, 

 who, after making some modifications in it, put it to the 

 purpose for which its author designed it. The first successful 

 employment of the balance came to be universally known as 

 the ' Cavendish experiment,' but Cavendish was careful to 

 make it clear that the design and construction of the original 

 mechanism were due to the Yorkshire rector. The idea of 

 the inventor was to devise a method whereby it would be 

 possible to measure ' the force of gravitation between two 

 spheres of such small size that thev could be moved by the 

 hand nearer to or further from one another. The essential 

 part of the invention was to contri^•e a balance so delicate as 

 to measure the almost inappreciable tendency of such bodies 

 to unite. Newton had shown that the attraction at the 

 surface of an}' sphere is directly as its radius, which, he ob- 

 served, must always be incomparably smaller than their 

 tendency towards the earth ; that is, their weight. In the 

 largest and heaviest masses with which it had hitherto been 

 found practicable to operate, this tendency amounts to only 

 a very minute fraction of a grain. How could such quantities 

 be accurately estimated, so as not only to leave no doubt of 

 the phenomenon of gravity thus acting on the small scale, 

 but to reduce its amount and hence to ^^'eigh the globe ? ' "j" 



This feat was accomplished by Michell's Torsion-balance 

 in the hands of Cavendish • The result of the Cavendish 

 experiment showed the mean density of the earth to be 5-48 

 times that of water. The experiment has since been several 

 times repeated with increased delicacy of mechanism. The 

 latest repetition was that of Mr. C. V. Boys, F.R.S., who, 

 employing fine quartz fibres instead of metallic wires, found 

 the density to be 5-5270,^ Michell's invention has now become 



* Sir Joseph Larmor's ]\IS. 



t J. D. Forbes, Sixth Dissertation of the Eighth Edition of the Eu- 

 cyclopeedia Britannica, p. 834. 



X Phil. Trans, Vol. 186 (1896). 



1918 Jan. 1. 



