Chronometers are portable time-keepers, in which, by the nature of the escapement, and the compensations for heat and cold, mean time is kept with sufficient accuracy to determine the longitude at sea. The relation between time and longitude, will be found explained under the head LONGITUDE. As the principal differences between chronometers and other watches consist in the escapement and the balance, we shall not detain the reader with a descrip tion of the other parts of these machines, but shall proceed to describe more fully the various modes of constructing those grand essentials in all time keepers, the regulator and escapement, noticing particularly those employed in chronometers.
Amongst the various steps by which horological machines were brought to their present state of perfection, the most important is the addition of the pendulum, which furnishes at once the most simple and accurate measure of time that we are yet acquainted with. Various claims have been made for the honour of this grand improvement, but the person to whom mankind is really indebted for bringing it into universal notice, is the celebrated Christian Huygens, of Zuylichein, who, in his excellent treatise De Horologio Oreillaloreo, has described the construction of a pendulum clock, and proved that he made one before the year 1658. His method of supporting the pendulum in the figure on the following page, in which a contrivance is resorted to for the purpose of insuring its isocbronous motion, is ingenious, although it is not quite correct, as has been shown by Mr. Cummings, in his Treatise on Watch-Making. It consists of two c) cloidal checks of brass, forming a curve, in which the silk line by which the pendulum is suspended moves. When the art of clock-making had attained a high degree of perfec tion, and the application of this instrument to astronomical observations rendered the utmost accuracy desirable, it was soon perceived that the varying length of the pendulum rod, in con sequence of its expansion by heat and contraction by cold, was a source of irregularity, which it was deemed difficult to overcome. To diminish as much as possible these variations, we are indebted to that eminent artist, George Graham, for the first application of the principle, which, under various modifications, has since been applied to preserve unchanged the centre of oscillation in a pendulum, and thus to insure the performance of all its vibrations in Ihe same length of time. Those substances which were found to be least alterable by changes of temperature, such for example as wood, and par ticularly deal (pine), were employed in the best clocks. As the different metals are affected by beat in different degrees, Graham conceived the idea that the greater expansion of one might lie employed to counteract the less expansion in another. After a series of trials,
during a period of five or six years, he succeeded perfectly, by attaching to the pendulum rod a vessel containing mercury, which liquid, when the rod was expanded by heat, rose, from the same cause, in the vessel which contained it, so as to compensate for the downward expansion of the rod. This improvement was completed in the year 1721. Five years afterwards John Harrison, a car penter in Barton, in Lincolnshire, subsequently so celebrated for his improve ments in chronometers, invented and applied to a clock of his own manufacture the pendulum, which from its form is called the gridiron pendulum. In this the expansion of the iron rod is corrected by the greater expansion of rods of brass or of zinc, which tend to raise the bob in the same degree in which the expansion of the main rod tends to lower it, and it of course is retained in the same place : in this form the compensation pendulum is, to the present day, most commonly made. The principle upon which these pendulums were con structed has received various modifications in the hands of different artists; Harrison's rods, for example, instead of being arranged in the form of a gridiron, have been inclosed in a tube, and greater elegance and compactness, with a more easy mode of adjustment, have been attained; these, however, we believe, comprise the whole merit of the modern improvements.
The annexed figure represents the mode of compensation proposed by Dr. Fearn. a is the pendulum rod suspended by a flexible spring, in the usual manner, from the cock ; c is a rod or bar of zinc attached to the back plate of the clock, by a screw at its lower end. The head d of this bar works upon a pin, which forms a joint, as repre sented in the drawing. Through this head there is a mortice, which allows one end of the lever e e to pass through it, and within which it may be fixed firmly by means of a tightening screw. The lever e e is attached to the clock-plate by a screw, which is also its fulcrum. Through a slit in the inner end of this lever the suspending spring passes, and is closely embraced by it on its lower side. The operation of this apparatus will readily be conceived ; as the rod a lengthens by heat, or contracts by cold, the rod c will be similarly affected. The expan sion of r will cause the inner end of the lever to descend, and, consequently, to embrace the suspending spring at a lower point, and thus to diminish the effective length of the pendulum. The mode of adjustment is obvious, as a greater or lesser motion may be given to the lever, the upper end of the rod c being made to approach or recede from its fulcrum. To obtain the requisite accuracy, an adjusting screw may be made to act upon the head d.