on the table. It will remain at rest, that is to say, it will be in equilibrium; and not only so, but it will be in stable equilibrium. To prove this, let us try to displace it with our finger, and we shall find that when we remove the pressure the egg will speedily return to its previous position, and will come to rest after one or two oscillations. Furthermore, it has required a sensible expenditure of energy to displace the egg. All this we express by saying that the egg is in stable equilibrium.
Mechanical Instability.
213. And now let us try to balance the egg upon its longer axis. Probably, a sufficient amount of care will enable us to achieve this also. But the operation is a difficult one, and requires great delicacy of touch, and even after we have succeeded we do not know how long our success may last. The slightest impulse from without, the merest breath of air, may be sufficient to overturn the egg, which is now most evidently in unstable equilibrium. If the egg be thus balanced at the very edge of the table, it is quite probable that in a few minutes it may topple over upon the floor; it is what we may call an even chance whether it will do so, or merely fall upon the table. Not that mere chance has anything to do with it, or that its movements are without a cause, but we mean that its movements are decided by some external impulse so exceedingly small as to be utterly beyond our powers of observation. In fact, before making the trial
