354 Mr. C. Vernon Boys [June 8, 



consideration of centrifugal force, that the whole sun attracts each ton 

 of the earth with a force equal to a weight here of a little more than 

 one pound, and that if it were not for this, every ton of the earth 

 would continue its journey into space in a straight line for ever, and 

 though we know in the same way that the whole earth attracts each 

 ton in the moon with a force equal to the weight of ten ounces and no 

 more, we cannot tell by any astronomical observation whatever how 

 many tons there are in all. 



Newton showed that to complete his law and to put in the numerical 

 constant (the Newtonian Constant of Gravitation) that would convert 

 his proportion into an equality, two methods are available : we may 

 either make observations on the disturbance of the earth's gravitation 

 by the action of isolated parts of it, we may either find the relative 

 attraction of an isolated mountain or the strata above the bottom of a 

 deep mine, or we may make an artificial planet of our own and find 

 the attraction which it exerts. 



The Newtonian Constant will be known if we know the force of 

 attraction between two bodies which we can completely measure and 

 weigh. Employing the C. G. S. system of measurement, the New- 

 tonian Constant is equal to the force of attraction in dynes between 

 two balls weighing a gramme each, with their centres one centimeter 

 apart. Of course it may be referred to pounds and inches or tons 

 and yards, but as soon as all the quantities but G in Newton's 

 equation 



_. Mass x Mass 

 Force = G Distance2 



are known, no matter in what units the quantities are measured, G is 

 known. The conversion of its numerical value from one system of 

 measurement to another is of course a mere matter of arithmetic. 



Of the first method of finding G, depending on the attraction of a 

 mountain, first attempted by Bouguer at the risk of his life in the 

 hurricanes of snow on Chimborazo, of the experiments of Maskelyne, 

 of Airy and of others, I cannot now find time to speak ; I can only 

 refer to Poynting's essay on the subject. It is the second method 

 with an artificial planet that I have to describe to-night. 



Now let me give some idea of the minuteness of the effect that has 

 to be measured. Is a wall built true by the aid of a plumb-line 

 vertical, or does it lean outwards? Newton's principle shows that 

 the plumb-bob is attracted by the wall, yet it hangs vertically. The 

 attraction is so small that it cannot be detected in this way. Even 

 the attraction of a whole mountain requires the most refined apparatus 

 to make its existence cortain. Do two marbles lying on a level table 

 rush together ? According to Newton's principle they attract one 

 another ; yet if they were a thousand times smoother than they are, 

 no movement of attraction could be detected. 



Leaving matters of common experience, let us go into the physical 

 laboratory where instruments of the highest degree of precision and 



