106 FROM NEBULA TO NEBULA 



as lead, its own weight would tend to flatten it. in short, 

 there are numerous ways whereby the hoop may be 

 altered in shape, and by analogy, there may well exist 

 more than one way to account for the oblateness of the 

 earth. 



To ascertain just how much energy is required to dis- 

 tend our hoop to any given degree of elongation, all that 

 is necesssary is to hang it on the hook of a balance scale, 

 lend your weight to the hoop until the desired elongation 

 is obtained, and then read the scale. To effect this 

 elongation by cranking requires power a steady flow of 

 power or energy exactly equal to the elastic strength of 

 the steel, as previously ascertained on the scale, or, what 

 is the same thing, to the sustaining against gravity of a 

 weight of that same magnitude. It not only required a 

 great expenditure of energy on the part of the earth's 

 momentum to whirl the excess water to the equator in the 

 first instance, but it takes a steady supply of power to 

 keep it there. An illustration which may make the matter 

 clearer is that of a horse just able, by putting forth all his 

 strength, to sustain a weight stationary in mid air at the 

 end of a pulley : though the weight does not rise, yet the 

 horse labors hard just to preserve the status. 



Again. Suppose that the hoop instead of being made 

 of spring-steel were made of some material that would 

 not give while rotating, but continue to retain its original 

 circular shape, what then? Which operation, think you, 

 will require the greater expenditure of power; to rotate 

 the rigid hoop at a given number of revolutions per 

 minute, or, the elastic one? The deformation of the 

 latter cannot, of course, be effected without the absorp- 

 tion of power ; shall we then conclude that to rotate it at 

 the same rate of speed as the other will require just this 

 much additional power? Assuredly not! "But," you 

 may say, "then the act of deformation does not absorb 

 energy as at first supposed? How can we explain this 

 apparent paradox?" The answer is plain enough. That 

 part of the centrifugal force which in one case manifests 

 itself in elongating the elastic hoop, that is to say, causes 

 visible distortion, transforms itself into heat in the case 



