THE TIDES 105 



cannot answer is, What keeps the earth turning on her 

 axis? The experiment of the hoop demonstrates admir- 

 ably that as long as you are content to turn the crank the 

 hoop will not only dutifully rotate, but that it will also 

 remain distended. But it also most emphatically demon- 

 strates that the hoop will infallibly cease doing either of 

 these things the moment you suspend your exertions. 

 That eminent Newtonian, Sir Eobert Ball, veraciously 

 confesses, as we have seen, (p. 90) that the earth has 

 parted company with its engine, but the machinery, he 

 adds, is still attached. In spite of the cranking power 

 having, as Ball imagines, taken an indefinite vacation, 

 the earth does not seem to mind, but goes on expanding 

 herself, or rather keeping herself expanded, from age to 

 age and whirling upward her tides without exhibiting any 

 sign of slackening or weariness. Here, again, observe 

 the want of flinging power discretely sidestepped and its 

 place supplied by the iteration of that euphonious but 

 vacant phrase of Spencer's, "persistence of force." 

 What a saving of fuel there would be if all the fly-wheels 

 in our innumerable factories could be brought to respect 

 this Spencerian philosophy ! You might hunt the lunatic 

 asylums through without being able to discover a poor 

 victim so hopelessly bereft of reason as to imagine that a 

 fly-wheel detached from its engine will continue to run 

 the machinery without coming to rest. Even the perpet- 

 ual motion unfortunates recognize the need of a power 

 source. 



Suppose that, while the hoop is rotating at its highest 

 speed and consequently expanded to the limit, you were 

 sufficiently deft to insert in it a horizontal brace, you 

 could then shut off the driving force without impairing 

 the deformation attained ; otherwise the natural elasticity 

 of the steel will cause the hoop to spring back into its 

 original shape. Again, you might accomplish this dis- 

 tension of the hoop in other ways, as, by setting a heavy 

 weight upon its rim while it stood in an upright position, 

 or by stretching it with your hands, or by compressing it 

 in a vise. And yet again, if the hoop were very large, or 

 relatively very thin, or composed of some softer material, 



