Such a succession of ingenious contrivances, introduced by different men to improve upon the first rude instrument, is perfectly analogous to the successive improvements which have been made in the modern clock, since that of Henry de Wyck's was constructed. Large iron wheels, continually exposed to the oxidizing influence of the air, in which unequal and ill-shapen teeth were cut with the inaccuracy of a manual operation, were by no means calculated to transmit the maintaining power with perfect regularity to the balance, supposing it to have been a good regulator; but when it is further remembered that the alternate direct pushes of the escape-wheel against the pallets must have produced jerks, and destroyed, or greatly disturbed, the regularity of this most essential part of the mechanism, great accuracy was not to be expected; even minutes were deemed too small portions of time to be shown by such a machine. The clock was set daily by some person specially appointed to the office, and even then was not to be depended upon, for forty minutes' variation in twenty-four hours was not thought to be an ill performance. The most ancient clocks had no pendulum such as we now see, but had instead a balance vibrating on the top of the clock, as seen in illustration, p. 108, which is an example of ancient clockwork. Upon the invention of springs, in lieu of weights, as the maintaining or motive power in clocks, which was made towards the close of the fifteenth century, it became obvious that time-pieces might be rendered portable, and that the new motive power, a coiled spring, could act independently of position. This discovery was of great importance, and yet to whom we are indebted for it is unknown; the value of the invention became still more apparent when the fusee, or mechanism for equalizing the variable power of a coiled spring, was applied. Berthoud says, 'It was soon perceived that the action of the spring being much greater at the height of its tension than at the end, great variations in the watch resulted therefrom. This was remedied by a mechanism called _stack-freed_, that is, a kind of curve, by means of which the great spring of the barrel acted on a straight spring, which opposed itself to its action, and when this spring was nearly down, acted more feebly.' The word _stack-freed_ was stated to be German, and therefore gave rise to a supposition that the invention was of German origin, but the word is not to be found in a German dictionary, and, if ever German, it was probably strictly technical, and soon became obsolete. Berthoud has given a drawing and description of a portable clock, probably by Jourdain, without a fusee, and some of the modern continental watch-makers have, perhaps, derived their idea from it of making a watch keep time without a fusee. Up to the close of the 15th century the motive power in clocks was always obtained by means of weights; the invention of the coiled spring rendered them portable.