Mechanics: The Science of Machinery. A. Russell Bond
of the ship east or west by noting how fast or slow the sun was at noon on the particular day on which the observation was taken. This use of the chronometer is common in these days, but up to 150 years ago there was no timepiece sufficiently accurate to permit a navigator to tell with any certainty just where he was.
It was John Harrison, the son of a Yorkshire carpenter, who was the first to build a chronometer worthy of the name. A prize of 10,000 pounds was offered by the British Parliament for anyone who could invent and sell a chronometer which would enable a ship to take a voyage from England to any of the West Indian islands and back and keep track of the longitude within one degree. If this could be defined within two-thirds of a degree, the prize would be 15,000 pounds. Harrison made a bid for this prize, and after years of effort and patient labor, he succeeded in being granted a trial. His son, William, was sent on a ship to Jamaica with the now celebrated chronometer, which was mounted on a large cushion. The instrument was constantly attended by the young man, its position being adjusted from time to time to suit the “lie” of the ship. When the ship was eighteen days out, the vessel was estimated by dead reckoning to be 13° 50′ west of Portsmouth, but the chronometer indicated the position as 15° 19′. The timepiece was immediately condemned as worthless, but William Harrison had not lost faith in the instrument, and insisted that if the ship continued on the same course, a certain island, if properly marked on the chart, would be seen the following day. True to the prediction, the next morning at seven o’clock the island appeared. By means of his chronometer William Harrison was able to predict the appearance of the other islands, and at the end of the voyage, which occupied sixty-one days, the chronometer was only nine seconds slow. When he returned to Portsmouth, after an absence of five months, the error of the chronometer was only one minute and five seconds, giving an error in distance of only eighteen miles, whereas thirty miles was the margin of error allowed by the prize conditions. Such accuracy seemed so incredible that the chronometer had to be tested on a second voyage, during which it was kept under lock and key and when William Harrison had to wind the instrument he was obliged to do so in the presence of two witnesses, lest he move the hand of the chronometer surreptitiously. At the end of the second voyage there was no further doubt that Harrison was fully entitled to the prize. Chronometers soon came to be used extensively, until now they are one of the most perfect of machines made by man, and operate with an accuracy that is almost incredible. Usually a ship is provided with several chronometers, so that one may be used as a check upon another. They are mounted in ball sockets and gimbal joints, so that they are not affected by the roll of the ship, but always lie in a horizontal position.
MARVELOUS PRECISION OF MODERN WATCHES
While we may well marvel at the precision of the chronometer, it is equally marvelous, if not more so, that we may equip ourselves for a few dollars with a timepiece which is so wonderfully accurate as to vary little more than a second per day. If one took the pains to regulate his watch carefully, any of the better makes could be adjusted to such accuracy.
There is nothing very mysterious about the mechanism of a watch. It consists merely of a train of gears which slow down the motions of the mainspring to a convenient speed; and these gears moreover keep the proper relation between the hour and minute and second hands. But when we reflect that a small watch possesses a tiny second hand which travels something like ten miles in a year, and that if carefully regulated it will not vary from that of another watch in the whole journey by more than six or eight inches at the most, we certainly have a reason to marvel. There are 86,400 seconds in a day, and a watch is usually arranged to make five beats per second or 432,000 per day. The interval between beats must be adjusted with such minuteness that one beat must not differ from another by 1/86000 part of a second, else the watch will register more than a second fast or slow at the end of a day. And yet watches capable of such precision are being turned out daily by the thousands. Of course, such perfection would be absolutely impossible without the use of extremely accurate machine tools. It would have been impossible as long as we had to depend upon a watchmaker to make a watch by hand.
THE PACEMAKER OF A WATCH
If we look at the works of a clock the most conspicuous feature is the rapidly oscillating balance wheel which, by the way, is the most important part of the watch, for it governs the release of the power stored up in the spring.
It controls the escapement which brings the whole mechanism of the clock to a standstill five times each second—in fact it is the pacemaker of the watch, for it gives the watch a step-by-step movement and fixes the rate at which the steps are taken.
FIG. 27.—ESCAPEMENT OF A WATCH
The last wheel of the watch train is what is known as an escape wheel. It is formed with teeth of an odd shape, such as shown at A in Figure 27. These teeth are engaged by a pair of pallets B and C, carried by a three-armed lever D. The pallets are usually bits of sapphire or similar hard stone to prevent wear. The third of the lever is slotted at its extremity to engage a sapphire pin E, carried by a disk F, which is mounted on the staff of the balance wheel. The escape wheel A revolves in the direction of the arrow, being impelled by the mainspring acting through the train of gears. One of the teeth of this wheel engages the pallet B, causing the lever D to swing on its axis and push the sapphire pin E toward the left, thereby giving the disk F an impulse in the same direction. Here a delicate coil spring, known as the hairspring, comes into play. Without the hairspring the parts would stand still, the escape wheel being blocked by the pallet B. The hairspring is attached at one end to the shaft or staff of the disk F and the other to the frame of the watch. It tries to hold the disk F in a fixed position, but is disturbed by the action of the escape wheel and is constantly oscillating the disk in its effort to bring it back to its normal position. When the disk swings over to the left the pallet B is clear of the teeth of the escape wheel. This releases the escape wheel and it springs forward in the direction of the arrow, but before it can move through an interval of one tooth it is arrested by the second pallet C, which has been projected into its path by the swing of the lever D. The lever swings back until the pallet C clears the escape wheel and the pallet B engages the next tooth. And so the action continues, the lever swinging back and forth and at each complete oscillation releasing one tooth of the escape wheel.
The hairspring takes up the shock of this intermittent motion and a balance wheel carried by the staff to which F is fastened steadies the oscillatory motion of the lever D. A watch is full of microscopic parts. In a small timepiece there are machine-made screws so small that without the aid of a magnifying glass one cannot see the screw threads cut upon them. But the most marvelous part of the whole watch is the delicate hairspring and the means of adjusting its tension and compensating for its expansion and contraction with changes of temperature.
INANIMATE MATTER IN CONTINUAL MOTION
When working with minute intervals of time many factors must be considered which are not even thought of in machines of grosser proportions. It never occurs to the man in the street that not only the animate world but the inanimate as well is in ceaseless and variable motion. If our eyes were capable of taking in minute microscopic details, we should see that everything is expanding or contracting, swelling or shriveling, twisting and warping in response to the atmospheric changes. Our steel bridges and skyscrapers are in constant motion; solid concrete dams must be provided with expansion joints; the Washington Monument goes through a diurnal gyration in response to the sun’s rays. Of course all this motion is almost immeasurably small. A bar of steel a mile long will expand ⅖ of an inch for every increase of a degree Fahrenheit in temperature. The expansion of a hairspring, which may be nine or ten inches