Page:The American Cyclopædia (1879) Volume IV.djvu/715

 CLOCKS AND WATCHES 703 upon the stop S, which has caused it to make a dead beat. The detent, A, is a kind of latch, shown as resting against the pin which is rep- resented opposite the letter A. Attached to its upper end is a light spring, a, projecting beyond its point. The scape wheel, E, is turn- ing in the direction of the arrow by the constant action of the train of wheels. The detent is held against the pin by the spring a' at its lower end. A pin, f, on the verge of the balance wheel, is moving with the wheel against the spring a, and will presently push the detent far enough back to allow the tooth T to pass by the stop, and the tooth T', which moves with greater velocity, to overtake and strike upon the impulse face of the pallet p before it gets beyond its reach, and thus impel the bal- ance, wheel to its full vibration. The fine spring a is designed to allow the pin d to slip by when the balance wheel returns, but to catch it and force the detent back and allow the tooth T to pass the stop when it again moves in the direction of the arrow. The hair spring of the balance wheel is not seen in the drawing, it being placed upon the hidden side of the roller; The balance wheel of a chronometer is made to preserve a uniform time of oscillation upon the same principle as the gridiron pendu- lum ; that is, by a combination of two metals having unequal rates of expansion and contrac- tion by variations of temperature. The rim of the wheel is made of brass and steel, the former outside of the latter, with a number of pins screwed into it which serve as weights. The invention was made by Harrison, but the method of construction which is now generally employed was introduced by Earnshaw about 80 years ago, and consists in soldering a rim of brass around a disk of steel, and then by means of a lathe and other tools cutting away the greater portion of the disk, leaving two arms projecting from the staff, as represented in fig. 13. The rim is then sawed in two places, at its junction with the arms, so that during expansion the other end of the seg- ment of the rim may be free to move to- ward the centre of the wheel, and thus produce sufficient com- pensation to prevent a change in the time of oscillation. The proper adjustment can only be accomplished by trial, which requires the employment of considerable time and skill, and constitutes one of the items of expense in a chronometer or a good watch. In regard to chronometers, little more need be said after giving a description of the escapement, and the principle of the com- pensation balance wheel. The escapement is the most perfect that has ever been devised for a timepiece which is to be kept in one position. 199 VOL. iv. 45 FIG. 13. Compensation Balance. FIG. 14. Chronometer. It has been attempted to use the chronometer escapement in pocket watches, but without success, great irregularities being produced by change of position, and motion consequent on carrying ; but when the timepiece is furnished with a perfectly compensating balance wheel, so that its vibrations are isochronous under any change of temperature to which it may be exposed by the passage of a ship from one zone to another, and it is suspended in gimbals, as represented in fig. 14, so that, whatever an- gle the deck of the ship may form with the horizon, the chro- nometer will always maintain the same po- sition, it becomes one of the most perfect pieces of mechanism. Its great use is in making astronomical obser- vations, and it is especially valuable in deter- mining longitude at sea. The first experiment with a chronometer was made in 1665, in a voyage to the coast of Guinea, by Major Holmes, with a watch made by Huygens. With this the longitude of the island of Togo was obtained with tolerable precision. In 1726 Mr. Harrison made a chronometer which was a marvel for correct timekeeping. In 1736 it was sent to Lisbon, and corrected the reckoning on the voyage as much as a degree and a half. He was paid 500 by the English government to prosecute further experiments, which resulted in his producing an instrument so accurate that it made an error of less than two minutes in a voyage to Jamaica and back, and in obtaining the reward of 20,000 which had been offered. It is not attempted to regu- late chronometers to diurnal time, it being only necessary to ascertain their rate, when the correct time can be calculated. The train of wheels, together with the lever and balance in a modern detached lever escapement, such as is now made in the best watch factories in the United States and in Europe, is repre- sented in fig. 15. It is placed between two plates of brass, the under, called the pillar plate, being an entire circle, while the upper plate, which is removed in the figure, may be either one quarter, one half, three quarters, or a full plate. In many European watches the upper plate is almost entirely replaced by what are called bridges pieces which are screwed to the pillar plate, and have arms which project far enough to receive the arbors of the wheels. The barrel &, which contains the main spring, has the great wheel placed around it, instead of being placed upon a fusee and driven by a chain wound upon the barrel as represented in fig. 9, and which is still the construction in most English watches. Of course the tension of the spring becomes less as it uncoils ; but if the coil is of considerable length the variation need not be great, and by the nice adjustment of the balance is completely counteracted. One