the cylindrical surface of the verge. The scapement of the crutch EDF is not accurately a dead beat scapement, but has a very small recoil beyond the angle of impulsion. By this circumstance, the branch A (now at B) is made to press most gently on the cylinder, and keeps the wheel locked, while the balance is going round in the direction BHA. The point A gets moving from A to B by means of a notch in the cylinder, which turns round at the same time by the action of the branch AG on the pallet C; but A does not touch the cylinder during this motion, the notch leaving free room for its passage. When the balance returns from its excursion, the pallet C strikes on the branch A (still at B), and unlocks the wheel. This now acting on the crutch pallet F, causes the branch b of the fork to follow the pallet C, and give it a strong impulse in the direction in which it is then moving, causing the balance to make a semivibration in the direction AHB. The fork is now in the situation A g a, similar to B gb, and the wheel is again locked on the crutch pallet E.

The intelligent reader will admit this to be a very steady and effective scapement. The lockage of the wheel is pro. cured in a very ingenious manner; and the friction on the cylinder, necessary for effecting this, may be made as small as we please, notwithstanding a very strong action of the wheel: For the pressure of the fork on the cylinder depends entirely on the degree of recoil that is formed on the pallets E and F. Pressure on the cylinder is not indispensably necessary, and the crutch scapement might be a real dead beat. But a small recoil, by keeping the fork in contact with the cylinder, gives the most perfect steadiness to the motion. The ingenious inventor, a man of approved integrity and judgment, declares that her Majesty's watch was the best pocket watch he had ever seen. We are not disposed to question its excellency. We saw an experiment watch of this construction, made by a country artist, having a balance so heavy as to vibrate only twice in a second. Every

vibration was sensibly beyond a turn and a half, or 540". The artist assured us, that when its proper balance was in, vibrating somewhat more than five times in a second, the vibrations even exceeded this. He had procured it this great mobility by substituting a roller with fine pivots in place of the simple pallet of Mudge. This great extent of detached vibration is an unquestionable excellence, and is peculiar to those two scapements of this ingenious artist.

Very ingenious scapements have been made by Ernshaw, Howel, Hayley, and other British artists; and many by the artists of Paris and Geneva. But we must conclude the article, having described all that have any difference in principle.

The scapement having been brought to this degree of perfection, we have an opportunity of making experiments on the law of action of springs, which has been too readily assumed. We think it easy to demonstrate, that the figure of a spring, which must have a great extent of rapid motion, will have a considerable influence on the force which it impresses on a balance in actual motion. The accurate determination of this influence is not very difficult in some simple cases. It is the greatest of all in the plane spiral, and the least in the cylindrical; and, in this last form, it is so much less as the diameter is less, the length of the spring being the same. By employing many turns, in order to have the same ultimate force at the extremity of the excursion, this influence is increased. A particular length of spring, therefore, will make it equal to a given quantity; and it may thus compensate for a particular magnitude of friction, and other obstructions. This accounts for the observation of Le Roy, who found that every spring, when applied to a movement, had a certain length, which made the wide and narrow vibrations isochronous. His method of trial was so judicious, that there can be no doubt of the justness of his conclusion. His time-keeper had no fuzee; and when the last revolution of the main wheel was going on, the vi

brations were but of half the extent of those made during the first revolution. Without minding the real rate of going, he only compared the duration of the first and last revolution of the minute hand. An artist of our acquaintance repeated these experiments, and with the same result: But, unfortunately, could derive little benefit from them; because in one state of the oil, or with one balance, he found the lengths of the same spring, which produced isochronous vibrations, were different from those which had this effect in another state of the oil, or with another balance. He also observed another difference in the rate, arising from a difference of position, according as XII, VI, III, or IX, was uppermost; which difference plainly arises from the swagging of the spring by its weight, and, in that state, acting as a pendulum. This unluckily put a stop to his attempts to lessen this hurtful influence by employing a cylindrical spiral of small diameter and great length*.

ers,

It is much to be regretted, that the preceding dissertation is the only one that Dr. Robison has written, upon the very interesting subject of Timekeepwhich he had studied with particular attention. So early as the year 1793 he had projected a magnificent work on the History, Theory, and Practice of Horology; and in a letter addressed to Mr. Thomas Reid of Edinburgh, and containing an account of the plan and object of the work, he requested that this able and experienced artist would co-operate with him in the undertaking. Mr. Reid's occupations did not permit him to agree to this flattering request; and it was probably on this account that Dr. Robison abandoned the work. Many valuable materials respecting the history of Horology, and many profound views respecting its principles, have thus been lost to science. It is fortunate, however, that the practical part which Mr. Reid would have contributed, has been lately published in the article HOROLOGY, which he has written for the EDINBURGH ENCYCLOPEDIA. ED.

SEAMANSHIP.

419. By this word we express that noble art, or, more purely, the qualifications which enable a man to exercise the noble art of working a ship. A SEAMAN, in the language of the profession, is not merely a mariner or labourer on board a ship, but a man who understands the structure of this wonderful machine, and every subordinate part of its mechanism, so as to enable him to employ it to the best advantage for pushing her forward in a particular direction, and for avoiding the numberless dangers to which she is exposed by the violence of the winds and waves. He also knows what courses can be held by the ship, according to the wind that blows, and what cannot, and which of these is most conducive to her progress in her intended voyage: and he must be able to perform every part of the necessary operation with his own hands. As the seamen express it, he must be able ❝ to hand, reef, and steer."

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420. We are justified in calling it a noble art, not only by its importance, which it is quite needless to amplify or embellish, but by its immense extent and difficulty, and the prodigious number and variety of principles on which it is

founded-all of which must be possessed in such a manner that they shall offer themselves without reflection in an instant, otherwise the pretended seaman cannot be trusted on his watch.

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The art is practised by persons without what we call education, and therefore it suffers in the estimation of the careless spectator. It is thought little of, because little attention is paid to it. But if multiplicity, variety, and intricacy of principles, and a systematic knowledge of these principles, entitle any art to the appellation of scientific and liberal, seamanship claims these epithets in an eminent degree.

421. What a pity it is that an art so important, so difficult, and so intimately connected with the invariable laws of mechanical nature, should be so held by its possessors, that it cannot improve, but must die with each individual. Having no advantages of previous education, they cannot arrange their thoughts; they can hardly be said to think. They can far less express or communicate to others the intuitive knowledge which they possess; and their art, acquired by habit alone, is little different from an instinct. We are as little entitled to expect improvement here as in the architecture of the bee or the beaver. Yet a ship is a machine. We know the forces which act on it, and we know the results of its construction-all these are as fixed as the laws of motion. What hinders this to be reduced to a set of practical maxims, as well founded and as logically deduced as the working of a steam engine or a cotton mill? May not the ingenious speculatist in his closet unravel the intricate thread of mechanism which connects all the manual operations with the unchangeable laws of nature, and both furnish the seaman with a better machine, and direct him to a more dexterous use of it?

422. We cannot help thinking that much may be done; nay, we may say that much has been done. We think highly of the progressive labours of Renaud, Pitot, Bouguer, Du Hamel, Groignard, Bernoulli, Euler, Romme, and others';