MECHANICS 67 



gravity than would be required to effect this, they 

 would have absolutely no weight and would exercise no 

 pressure whatever. It is the tendency to " fly off" from 

 a horse's back produced by rapidly riding in a circle, 

 which so reduces the weight of a circus rider that he 

 can easily stand on the saddle and perform a number of 

 feats any one of which would be impossible did he ride 

 in a straight line. 



On account of the greater rapidity of motion of the 

 earth's surface towards the equator than towards the 

 poles, the centrifugal force is necessarily greater at 

 the equator, and consequently weight is there slightly 

 diminished, as is easily proved by the vibrations of a 

 pendulum. On account also of the greater mass of the 

 earth's equatorial region, a plumb line does not, in the 

 north, hang absolutely vertical to the earth's surface, 

 but deviates slightly to the south. 



With these various elementary observations we must 

 conclude what we have to say respecting the mechanics 

 of solid bodies, referring the student to other works for 

 the prosecution of his study of that science. 



Passing on now to the consideration of fluid substances, 

 we may first remark that the essential principles of 

 dynamics, apply to them as well as to solids, but the 

 fluid condition calls forth new conceptions, which are 

 treated of as distinct sciences known as hydrostatics, 

 hydrodynamics, and pneumatics. 



All fluids, whether liquid or aeriform, are, of course, 

 no less subject to the action of gravity than are solids, 

 but the commonest observation makes it clear that the 

 internal constitution of their substance must somehow be 

 very different from that of solids. In what precisely that 

 difference consists we do not know, though any specula- 

 tions are useful, provided that (as working hypotheses) 



