446 THE WORLD MACHINE 



of the earth on the average four thousand miles in length, and 

 that the average density of the earth is five times that of water, 

 it will be seen that this is a comparatively slight pull. 



Even within our planetary system it becomes very much less. 

 Neptune is thirty times more distant from the sun ; the force 

 of gravitation there is, then, nearly a thousand times slighter 

 than upon the earth. A series of telegraph wires spaced forty 

 feet apart would be strong enough to hold Neptune to its 

 accustomed way. Consider, then, how feeble would be the 

 force of gravity at the distance of the nearest star. 



It is easy enough to compute the mutual attraction of alpha 

 Centauri and the sun. They are distant 277,000 times the earth 

 from the sun. The force of alpha Centauri' s attraction upon the 

 earth, then, is something like 75,000 million times less than that 

 of the sun. The mutual attraction of our sun and the nearest 

 neighbour sun, since they have apparently about the same 

 mass, would only be twice this amount. It is inconceivably 

 small. If we were to conceive these two suns as forming a 

 binary system, the one determining the path of the other, it 

 would require millions of times this attracting force to hold 

 them in their orbits, with the suns moving at perhaps ten or 

 fifteen miles per second. 



It does not seem possible, from any conceptions which we 

 can now frame, to think of the sun as a part of a system. If 

 the spacing of the stars that is to say, the suns which we are 

 able to see be at anything like this average distance, we are 

 equally lost in our endeavours to conceive of any unitary stellar 

 system as well. Whence, then, the force which moves the sun, 

 and gives the stars their speed ? This is the celebrated problem 

 proposed by Simon Newcomb, and computed by him for a 

 special instance. 



Since the days of Newton and Leibnitz, the mathematicians 

 play with infinities as deftly as a Japanese juggler with plates 

 and sharp knives. It is not difficult for a mathematician to 

 reckon up what speed a body might attain in falling from infinity 

 towards a known mass with a known attractive force. The 

 case for the stars is stated thuswise by Professor Newcomb : 



The telescope discloses something more than fifty million 

 suns ; suppose that we double this number, and give to each of 

 these suns five times the mass of our own. In other words, 

 let us conceive of a stellar universe equivalent in its mass to 



