1894.] on the Newtonian Constant of Gravitation. 355 



delicacy (at least so they are called) are found on every table. What 

 precautions are taken to prevent the attractions of the fixed and moving 

 parts from interfering with the result which they are constructed to 

 measure ? None. The attractions are so small, that in no apparatus in 

 use for the measurement of electrical, magnetic, thermal or other con- 

 stants are they ever thought of, or is any provision necessary to prevent 

 their falsifying the result. Nevertheless, the attractions exist, and if 

 only the means are delicate enough they can be detected and measured. 

 The Rev. John Mitchell was the first to devise a successful method. 

 He was the first to invent the torsion balance with which Coulomb 

 made his famous electrical researches, and which bears Coulomb's 

 name. He devised and he made apparatus for this purpose, but he 

 did not live to make any experiments. 



After his death Cavendish remodelled Mitchell's apparatus and 

 performed the famous Cavendish experiment. By means of the ap- 

 paratus, of which for the second time I show a full-size model in this 

 theatre, Cavendish measured the force of attraction between two 

 balls of lead, one 12 and the other 2 inches in diameter, and with 

 their centres 8*85 inches apart. The same experiment has since 

 been made by Reitch, by Baily, and more recently by Cornu and 

 Bailie with greatly superior apparatus of one quarter of the size. 

 All these observers actually determined the attraction between masses 

 which could be weighed and measured, and thus found with different- 

 degrees of accuracy the value of G. 



Let me explain now that this G, the gravitation constant, or as I 

 prefer to call it, for the sake of distinction, the Newtonian Constant 

 of Gravitation, has nothing to do with that other quantity generally 

 written g, which represents the attraction at the earth's surface. This 

 is a purely accidental quantity, which depends not only upon G, but 

 also upon the size of the earth, its mean density, the latitude, the 

 height above the sea, and finally upon the configuration and the com- 

 position of the neighbouring districts, g is eminently of a practical 

 and useful character ; it is the delight of the engineer and the practical 

 man ; it is not the same in all places, but that he does not mind. It is of 

 the earth, arbitrary, incidental and vexatious. Prof. Greenhill should 

 spell his name with a little g. G, on the other hand, represents that 

 mighty principle under the influence of which every star, planet and 

 satellite in the universe pursues its allotted course ; it may possibly 

 also be the mainspring of chemical action. Unlike any other known 

 physical influence, it is independent of medium, it knows no refraction, 

 it cannot cast a shadow. It is a mysterious power, which no man can 

 explain ; of its propagation through space, all men are ignorant. It 

 is in no way dependent on the accidental size or shape of the earth ; 

 if the solar system ceased to exist it would remain unchanged. I 

 cannot contemplate this mvstery, at which we iguorantly wonder, 

 without thinking of the altar on Mars' hill. When will a St. Paul 

 arise able to declare it unto us ? Or is gravitation, like life, a mystery 

 that can never be solved ? 



2 b 2 



