An article showing that this subject is not devoid of interest or obsolete is given below from the _Bicycling World_, in which I think the law of whirling bodies will apply. “The Rochester wheelmen debated the question, ‘Why does a bicycle stand up while rolling and fall down as soon as onward motion ceases?’ The answer decided to be correct was, that ‘the bottom of the wheel can have no side motion because it rests on the ground; and since the bottom is constantly becoming the top and the top the bottom, if the upper part of the wheel gets any lateral motion, it is checked by being brought round upon the ground again before the motion has too much influence.’” I do not suppose this ingenious decision, rendered by the high and mighty Solons of the Rochester Club, was a serious one; however, we do find that just such logic is quite common. It is not plain whether the question discussed was that of a bicycle with or without a man upon it, but I take it to be the latter. Some of the gentlemen had no doubt noticed that to give the machine a shove it would keep upright for a longer time running than when standing unsupported. This is purely a case of the law that whirling things tend to keep their own plane, as illustrated in the gyroscope and the spinning top. In the running bicycle without a man upon it to constantly rectify its position, the principle is simply one of the parallelogram of rotations. If the wheel from any external force starts to fall over, or, in other words, to revolve around a horizontal line normal to its geometric axis, then, since the wheel is already revolving about its axis in the axle, the resultant of these two rotations will be a rotation about an axis inclined to the former axis of the wheel, which means that the wheel will begin to circle around a centre at some distance from the wheel on the side towards which it starts to fall. This new axis about which the wheel revolves will of course be in a plane perpendicular to the new plane of the wheel, and will be inclined downward from the horizontal plane through its centre, so that the wheel is no longer running in a vertical plane. The rotation about the centre outside of the wheel, towards which centre the wheel leans, brings into play a centrifugal force acting to upright the wheel; that is, to bring it back to a vertical plane. Now, if the wheel be run along a straight groove, so that circling around a centre is prevented, then it will fall as quickly as when standing still; or if, in the bicycle, the steering-wheel be locked so that it will not turn out of the plane of the two wheels, there would be no uprighting resultant, and the machine, according to Newton’s law of independent forces, would fall. SOME QUESTIONS OF POTENTIAL ENERGY, MOMENTUM, AND HILL-CLIMBING.