The ancients measured time by the regulated flow of water. Striking clocks of some kind were known in Europe as far back as the twelfth or thirteenth century. But they seem to have been unreliable, costly, and rare until the discovery by Galileo of the “isochronism” of pendulums. _Pendulum_ is first found in 1660, in Boyle’s writings, _vibrate_ and _vibration_ in 1667. The new toy seems to have taken hold of Europe’s imagination in the most extraordinary way. Both _clockwork_ and _mainspring_ were used figuratively the first time they are known to have been used in English at all, a sixteenth-century writer even anticipating Paley so far as to write: God’s the main spring, that maketh every way All the small wheels of this great Engin play. We hear talk almost at once of the _springs_[57] of people’s actions. Descartes compared the souls of brutes to watches, and Leibnitz actually compared the souls and bodies of men to _two_ watches! It seems as though the works had started going in our heads. And since then, so far from stopping, they have accelerated, especially during the last century—to what extent it is difficult for us to realize fully, simply because it has all happened so recently. Differences of outlook on such matters as biology and physiology between ourselves and the Middle Ages we readily perceive, though we may not properly understand them; here we stand a long way off, and can often see quite plainly how the old words have altered their meanings. But from the way in which our great grandfathers used such words as _energy_, _midriff_, _motor_, _muscle_, _nerve_, _respiration_, _work_—to take examples only from the passage quoted at the beginning of this chapter—we sometimes find it hard, even when we have traced the history of their meanings up to that date, to feel what different associations they must have called up to the generations which died before Huxley was born. At this time, thirty years after his death, it is only our own imagination, working introspectively on such a phrase as “nervous machinery”, and grasping, as it can do, how the meanings of the two words have been running into one another, which can bring this difference before us. When it has done so, we are again reminded of the simple yet striking truth that all knowledge which has been conveyed by means of speech to the reason has travelled in metaphors taken from man’s own activities and from the solid things which he handles. The present is no different from the past. Only the metaphors get buried deeper and deeper beneath one another; they interact more subtly, and do not always leave any _outward_ trace on the language. It would be interesting, for example, were it possible, to discover just how much of the average man’s idea of blood circulation is due to the invention of that elementary mechanical device, the pump;[58] or how much of the mental image which he has formed of the interaction of