That extraordinary man John Harrison having produced the first portable ma chines, which, upon repeated trials, met with success, to the extent required for the great reward offered by the British parliament, must be reckoned the father of modern chronometry; and his long and active career has proved extremely useful, by stimulating with so bright an example other artists to similar endea. yours The principles of Mr. Harrison's watches are well known ; and, as most parts of his construction have been su perseded by more simple contrivances, we shall only mention the principal inven tions of which he appears to be the au thor, and which are still used by the watchmakers of the present day.
The fusee is one among those in ventions which have proved the most ge nerally useful in practice. By this simple mechanism, the main spring, while the watch is going, acts on an intermediate short spring, which Harrison calls the se condary spring, and is constantly kept bent to a certain tension by the former ; and when the watch is winding up, and the principal spring ceases to act, the se condary spring being placed in a ratchet wheel, which is hindered from retrograd ing by a click, continues the motion with out alteration. Other contrivances have been proposed, and executed, to make time-pieces go while winding up ; but none which, like this, combines the ad vantage of simplicity, and the property of providing a supplementary power, which is equal to that of the mainspring when its action ceases. And it is to be ob served, that the utility of the going fu see, which has induced manufacturers to introduce it into all good watches, is pe culiarly important in those time-pieces which have not the power of setting them selves in motion, as is the case with the best modern escapements.
Harrison invented also a compensation for the effects of heat and cold ; which at the time was perfectly new, and has led to the improvements made afterwards in that essential requisite of time-keepers.
The alterations to which the length of the pendulum is liable, by the different degrees of heat and cold, affect the going of clocks with that sort of regulator, (see PvinuLum); and watches, with a balance, are still more subject to irregularity from that source ; because not only the ba lance expands or contracts, according to the rise or fall of the thermometer, but the regulating spring itself, while it suf fers similar changes, becomes weaker or stronger ; so that, from these causes, a time-piece must go slower or faster in too great a proportion to be overlooked or neglected. Graham (Philosophical Tran§
actions, 1726) is the first who thought of applying two metals, of different expansi bility, to correct the errors proceeding from the variation of temperature in a pendulum ; but as he seemed to have had in view to effect it immediately, without the aid of mechanism, he was obliged to fix on steel and mercury, these being the metals which offered to him the greatest difference of dilatation and contraction. Harrison, by multiplying the bars, in creased the total length of the two metals acting on one another, without exceeding the limits of the pendulum ; and thereby produced a sufficient compensation with brass and steel, in the compound or grid iron pendulum, which has been almost universally adopted ever since. This con trivance could not be easily applied to balances ; but Harrison, following still the principle of the different expansibility of metals, applied it in a manner which had not been thought of before, and made it act on the spiral spring, in order to pro duce the desired compensation in the re gulator. This method is described as fol lows : (Principles of Mr_Harrison's Time keeper, p. xii. notes.) " The thermome ter kirb is composed of two thin plates of brass and steel riveted together in seve ral places, which, by the greater expan sion of brass than steel by heat, and con traction by cold, becomes convex on the brass side in hot weather, and convex on the steel side in cold weather ; whence, one end being fixed, the other end obtains a motion corresponding with the changes of heat and cold, and the two pins at the end, between which the balance spring passes, and which it touches alternately as the spring bends and unbends itself, will shorten or lengthen the spring, as the change of heat and cold would otherwise require to be done by the hand, in the manner used for regulating a common watch." This kind of compensation has been since applied in other ways ; but the me thod, in general, is liable to some mate rial objections, on account of its altering the length of the balance spring, and the difficulty, perhaps impossiblity, of ef fecting with it an accurate adjustment.