The escapement generally used in our best chronometers, as we shall hereafter see, consists of a toothed wheel at the end of the train, which is prevented from running down by a detent or hook, and of two pallets, a longer and a shorter, fixed upon the verge or axis of the ba lance. These pallets are so placed, that when the face of the longer pallet has just arrived before one of the teeth of the wheel, the shorter pallet strikes out the hook, and allowsthe wheel to push forward the longer pallet with its tooth, during which action, the hook falls again into its place, to catch the succeeding tooth. The balance therefore Proceeds in its vibration, and returns again with out disturbing the train ; because the short pallet does not strike out the hook in its backward course, but only acts on a slender spring, resembling those for merly used in the jacks of harpsichords. In this manner the vibrations are kept up ; and so little do the variations in the maintaining power affect the rate, when all the adjustments are made, that if the main spring be let down to only a small part of its ordinary tension, these time-pieces will keep the same rate for many hours together.
However perfect, practically speaking, the application of the maintaining power may be, yet if the balance and its spring be subject to vicissitudes from heat and cold, it will be in vain to expect accuracy. There are two ways of correcting this compound time-measurer. The first, which was invented by Peter Leroy,con sists in causing the balance to enlarge it self, instead of contracting by heat ; by which means the spring, when in the state of greater rigidity, has more work to do ; and the other acts by lengthening or shortening the spring, when cold or heat may have given it more or less of force. This was invented by Harrison, and depends on the well-known fact, that a short spring is stiffer than a longer; so that by shortening his spring at the time when it was weakened by heat, and the balance enlarged by the same cause, he gave it the stiffness requisite to compen sate for these alterations ; and the same contrivance prodticed the contrary effect in cold temperatures. As we shall more fully exhibit these inventions under the article HonoLocy, it is only necessary tp observe, that Peter Leroy constructed his first time-piece with fluid thermome• ters on the balance, and that he also vented our present expansion balance of brass and steel, soldered or fused togeth er in the rim, which was afterwards in troduced and brought to great perfection by Arnold.
Machines, made upon the principles here cursorily pointed out, have measur ed time to a wonderful degree of perfec tion; and from the immense maritime trade of the British empire, and the sci entific disposition of many wealthy indi viduals, the demand has been so great, as to have produced a very great number of able workmen, fully equal to their construction, at the same time that the prices have been considerably reduced. Most sea commanders of any respecta bility are provided with two or more of them.
Among the other causes of irregularity in time measurers, the resistance of the air has been occasionally considered by authors. But artists seem to suppose, ei
ther that it is a constant quantity, or that its variations are not considerable enough to be brought into the account. The very accurate performance of some chro nometers, and the steady going of astro nornical clocks, seem to give weight to this supposition : but on the other hand it may be remarked,that though the slow motion of heavy pendulums vibrating through:small arcs in astronomical must be subject to very little resistance. indeed from the air, yet it does not fol low that the rapid vibrations of a balance may not be affected by this cause ; and the extreme precision ofsome chronome ters will not, perhaps, be admitted as a very strong argument, when we consider that the changes from barometrical causes may have compensated each other, and that the most perfect machines will vary as much as one second per day, from causes which have not been yet clearly detected, though these are probably re solved into that before us. We are more particularly led to these reflections by a communication from Mr. Manton, of Davies-street, who found by experiment that a chronometer, which was going up on a gaining rate of five seconds per day, did increase its arc of vibration by an ad ditional 50 degrees immediately upon the air being exhausted; and that being kept in vacuo, its rate became 37 seconds per day, the gain being 34 seconds upon the former rate. Hence it follows, that as the difference between the highest and the lowest stations of the barometer indicate a change of about one-fourteenth part in the density of the air, the correspondent change per day, in the rate, may be two seconds and a half, or about one second per inch. Hence it may happen that a capital time-keeper shall indicate a more steady rate from week to week than from day to day.
The causes of imperfection in chrono meters, which still call for farther exer tions of sagacity in our artists, are, 1. The spring gradually tires or falls off from its strength, and neither the law of this vari ation nor its remedy are known. The ef. fects of this change are, that all the ad justments are disturbed by it. 2. There is great reason to apprehend that the ex pansion-bars of brass and steel do change in their relative powers of flexure by their continued action on each other, though it is probable they settle at last. 3. The wear of the acting parts is uncer tain, and will affect the time of striking out the detent and the arc of impulse. 4. No certain rules have been given, or are perhaps known, for making all the vi brations equal in time. If we suppose the long and short vibrations to be at first adjustable, with certainty, to equal times, not only for the extremes, but for all the means or intermediate arcs, it will not fol low that the falling off from wear or from tiring, or from change in the balance, will continue to be accompanied by the same isochronism. 5. The best artists find very great difficulty in adjusting a pocket chronometer for all positions, preserving at the same time the other needful ad justments. See ESCAPEMENT, HOROLOGY, PENDULUM, TRAIN, and the articles thence referred.