some of which, as has been before observed, are advantageously used in peculiar situations. None of them, however, are thought, at the present time, likely to become a general substitute for the lock.

Another contrivance, called the balance-lock, consists in floating the boat into a case or vessel, at the termination of one canal, and moving it vertically, by machinery, to the other canal. There are two of these cases suspended over the same pullies, on an axle, so that they at all times balance each other; and that, whether one or both contain boats; because they are so executed, that a boat, on entering, expels from the case a quantity of water equal to its own weight, and the same quantity returns into the case when the boat is passed out. But one of the most ingenious machines which has been invented to avoid the loss of water, is described in the Repertory of Arts, vol. I. page 81, first series. It may perhaps be considered as a modification of the balance-locks previously invented; still it exhibits great originality and inventive powers in its authors. It is necessary in this, as in other balance-locks, that the two canals terminate in the same vertical plane; the end of the upper canal being closed by gates at its termination. Things being in this state, a well or pit is sunk at the head of the lower canal, of a depth somewhat exceeding the difference of elevation between the two levels. This pit is filled with water, and a diving chest, or buoy, sufficiently strong to bear a heavy external pressure is then made, and put afloat over the pit. On the top of this chest several strong posts are erected, high enough to reach the bottom of the upper canal. These posts support a cradle, which is open above, and nearly filled with water, and having gates at both ends, through which the boat may pass in and out. The specific gravity of the buoy must be so much less than that of its surrounding water, as to be just able to support the load, consisting of the pillars and cradle, which are fixed to it, together with the canal boat, and a sufficient quantity of water to keep it afloat in the cradle. When so

loaded, it should be just oovered by the water in the pit, where it can move up and down on the application of a very small force. To let down a boat from the upper level, the end of the cradle is fixed by screws to the gateway of the canal, the gates of which, as also those of the cradle at the end next to the boat, are then opened; the boat enters from the canal into the cradle, displacing a quantity of water just equal to its weight, consequently the burden on the buoy is not altered. All the gates are then closed, and the fastenings, which confined the cradle to the gateway, taken off; when on the application of a trifling force to the mass, the buoy descends to the bottom of the pit, bringing the boat to the plane of the lower level. The gates at the end of the cradle are then opened, and the boat passing out, its place is supplied by water from the lower canal.

There is yet another contrivance (Rep. vol. II. page 235) differing considerably from the above, although of the same class. It is a caisson, or diving trunk, made so as to be perfectly tight when the gates at its ends are closed. This floats in a lock between the two canals; the canal boat is received into it, and it is made to descend through the water of the lock, to the plane of the lower canal; when by opening the end of the caisson, and corresponding gates in the lower part of the wall of the lock, the boat is passed out. The ascending motion is obtained by pumping water out of the caisson, which operation is reversed to obtain a descent. In this, and the preceding invention, the manner of passing an ascending boat will be understood, from the operation of descending, which has been described.

In another, and distinct class of contrivances, it is proposed to fill the common lock, by elevating water from a sort of cistern, made near it. To accomplish this, different kinds of plungers are to be used, some of which are very ingenious, particularly those of Betancourt, Steevens, Busby, and Bogaerts. In these the plungers are so counterbalanced, as to be always in equilibrio with the water

in the lock, at whatever height it may be; consequently, the application of a very small force, destroying the equilibrium, produces the rise or fall of the water, as may be required.

(To be continued.)

XI.-On the futility of the attempts to construct Perpetual Motions. By Dr. THOMAS P. JONES, Editor of the "Journal of the Franklin Institution."

It will not be expected, by those conversant with the inquiry, that any thing really new can be offered on a subject which has been so frequently and so ably treated as the inquiry into the possibility of constructing a machine which has within itself a principle of continued motion. There is something extremely fascinating in the pursuit of this object, as is evinced not only by the attempts of a host of tyros in mechanics, but by the persevering efforts of some men of genius and science, who, although they have professed faith in the admitted laws of motion, have yet proved by their works that their faith was not perfect. Whilst there is nothing in the known laws which govern the material world, upon which to found the idea of being able to construct a perpetual motion, the time might not be mispent which should be devoted to an investigation of the causes which operate upon the mind in exciting and keeping alive the expectation that such a machine will some day be discovered; but if we possessed the ability fully to prosecute this investigation, it would belong more to a work devoted to moral, than to mechanical, philosophy.

Some of our readers may be ready to exclaim, "but we have always understood, that all philosophers denied the possibility of any such thing." We believe that all who really merit the name of mechanical philosophers, do unite in such a denial; but if this be the fact, the corps is but a small one; for our own observations, together with the

numerous facts upon record which might be called as evidence, go to prove most clearly, that there are but few persons who admit this truth as they admit an axiom; there appears, in general, to exist some mental reservation; some apprehension that if they declare the thing impossible, it may nevertheless happen, that some lucky wight may "hit upon it," and thus ruin their reputation as accurate philosophers!

The subject of mechanics is one which, of necessity, occupies a large portion of the attention of mankind; all the moving powers which we can command, are called to our aid; but into the actual employment and adaptation, there enters much more of practice than of principle. A great proportion of our mechanics are men of observation, intelligence, and experience; and many of them have paid praiseworthy attention to science. But their very pursuits and occupations, although greatly aided by the scientific knowledge they have acquired, forbid their carrying such investigations to a great extent. And we ought not, therefore, to be much astonished, if some of them are occasionally engaged in this fruitless pursuit. To their credit, however, this is now a rare occurrence, as the observations founded upon correct practice must necessarily lead to the same general results as does a correct theory. The constant employment or notice of the various machines which are daily seen in operation, induces almost every man to conclude that he knows something of mechanics. many of these machines, the cause of their motion is very obscure; whilst the motion itself is not only evident, but so uniform and continuous, as may well lead the ordinary mind astray, and cause the conclusion that the step from some of them to an actual perpetual motion, is but a short one.

In

There are but few terms in our language which are less definitive than the term science; it embraces almost every department of human knowledge, whether natural, moral, or physical; and it happens, unfortunately, that when

philosophers and men of science are mentioned, the world are not very discriminative; and the opinion of the adept in natural history, or in chemistry, will carry an undue weight in subjects to which he has not attended; and of which, although he is a man of science, he is nearly or completely ignorant. It is in this way certainly, and in this only, that the votaries of science, and the believers in a mechanical perpetual motion, have been identified. On which side were the scientific of Philadelphia arranged, when Redheffer's machine was exhibited at Chesnut Hill? Those who recollect the period, will find no difficulty in answering the question. We believe, that nineteen-twentieths of those who were so esteemed, were either avowed believers; or, as the politicians would say, upon the fence. We know one gentleman who professed, and was believed, to be a man of great mechanical knowledge, who delayed in completing a patent, lest Redheffer's machine should be found to be genuine. We are of opinion, that there is scarcely any other subject so familiarly spoken of, and so little understood, as the principles of mechanics; and no one, therefore, in which quackery is more certain of success.

Let not our readers expect, that, because we have thus freely spoken our sentiments, we are about to demonstrate, that a mechanical perpetual motion is an impossibility; we should indeed be willing to take any particular machine, which might be pointed out to us as such, and undertake to show the fallacy of its claim; but to give a general negative demonstration, is a task which we cannot undertake. It belongs to those who advocate its possibility, to establish a principle upon which it may be made to act; the general practice, however, has been to exhibit a complication of levers, weights, or other powers, which serve to obscure the action of the individual parts, and to claim, for the whole, effects to which these individual parts, taken alone, have no power to contribute.

It has been, we think, truly observed, that to produce a perpetual motion, we must find a body which is, at the