NEWTON 's THEORY OF PLANETARY MOTIONS 89 



not to investigate it. Let us, for once, discuss it openly 

 and frankly without let or hindrance. 



In the first place, what is the construction of the 

 pendulum, and what its principle of operation? 



It consists of three essentials : (1) a point of support 

 against gravity, (2) a pendent string or wire, and (3) a 

 bob. When the latter is raised an arc and then gently re- 

 leased, it descends to the lowest point and rises thence, 

 without stopping, to an equal height (provided there be 

 no air or other friction) on the other side. The principle 

 is quite plain : gravity causes the bob's descent during one 

 unit of time, and stops its ascent during the next unit. 

 Without gravity acting, the bob would stay wherever it 

 might be placed in the pendular circle. Now, where, I 

 demand, is the source of this gravity of which mention 

 has just been made ivithin or without the pendular 

 circle! Without it, of course. But where is this source 

 in the earth-moon system, for example? It is within at 

 the very center of the "pendular" orbit. And where is 

 the "point of support"! There is none. 



Where, I ask the fair-minded reader, is, then, the 

 analogy! 



Let us see now if we cannot convert the illustration 

 of the pendulum into a true parallel. Substitute in place 

 of the supporting hook some imaginary fixed object in 

 free space, and conceive to be attached to this an elastic 

 cord in lieu of the original common string or wire; and 

 at the loose end of this elastic cord, again, affix the erst- 

 while bob. Now conceive yourself as taking hold of the 

 bob and pulling it away until the string is at high tension 

 and then flinging the weight, tangentially, as hard as you 

 have a mind to, and ask yourself whether it would con- 

 tinue revolving around the "fixed point" forever, main- 

 taining throughout the initial tension on the cord? 



Eeverting to my theory: The earth constitutes the 

 fixed point; her attraction for the moon, the elastic cord; 

 the moon herself the bob and the stellar resultant the 

 extraneous source of gravity. A true pendulum, this, 

 throughout every degree of the circle. 



