A New Century

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6. An Essay



OF AN ESSAY, To obtain a Level at Sea.

I have done right in calling these attempts "essays": and if I had said "immature attempts," they would have been better designated. Yet, having promised them to my readers, I cannot now withhold them, although, from want of opportunity of trial, I can do little more than talk of their supposed properties.



The first essay, as shewn in fig. 3 of Plate 45, is a mental deduction from a device which I executed in 1801, and brought before the public at the exhibition then given, by the French government, of the produce of national industrie. It was, nothing more than a pendulum, made with a view to lengthen, considerably, the going of a given clock, without altering the wheels. To that end, the weight or bob, was a heavy bar C D, suspended diagonally on two points A B, placed at a distance from each other, exactly equal to the length of the said bar: and that by the double cross-bars B C and A D, of a length sufficient to make the whole assume a form exactly square: where it may be noted--that were this figure longer than high, the curve of vibration would have two points of inflexion, and the bar would not place itself horizontally at last; and that were it narrower and higher, that curve would assume a form more like, though still distant from, the arc of a circle. In the present case, such was the effect of this disposition of things, that the centre of gravity of the bar described, in vibrating, a curve E C D F, the lower form of which, was so near to a horizontal line, that the times of vibration were immensely prolonged; so much indeed, as to represent a common pendulum of several thousand feet in height; and to give a proportionate slowness to any mechanism with which it should have been connected. In fact, this line is so minutely different from such horizontal line, that it is wholly included in the thickness of the drawn-line C D; nor becomes visible but near it's two ends C D, when it begins to rise, and then rises faster than that described by a short common pendulum.

In fine, this curve itself is formed by continually bisecting the line or bar C D, and drawing lines from it's centre of gravity, thus found in one of it's positions, to the same in another position, till the curve E C D, &c. arises from this process.

It follows, then, from the nature of this curve, (or pair of curves) that the time of vibration of this pendulum is the longer, the shorter the arcs are, in which it vibrates; and that, when the vibrations have attained a certain length, compared with the height to which the centre of gravity rises, the time becomes considerably shorter. I shall not now pursue this idea, because it is at once an abstruse question, and at the same time one of uncertain utility--I mean that it's use is problematical as a pendulum: since the time of a vibration depends on it's length, which cannot easily be determined by any invariable method. I shall, however, add two things on this subject, by way of land mark; the one, that the balance-wheel of a watch has power enough to drive this pendulum, heavy as it is;--and the other, that I have seen it make (for many hours together) vibrations of half a minute's duration! In a word, this is one of the subjects, which untoward circumstances have prevented me from bringing to maturity--but which I owe to my subscribers, and the public, in any, or every state, to which I have brought them.

I therefore, say nothing more of this Instrument as a pendulum: but an inspection of the figure will shew, that it will not be useless as an ELIPSOGRAPH--which it clearly is, since the intersection of the bars A D & B C; describes a true Ellipsis. It may be further shewn, that the ends of the moveable bar C D, are the vibrating foci of a second ellipsis, like the first, which rolls under the other, so that the curve itself is that which the centre of one ellipsis a b c would describe, by rolling on the surface of another e b d. But, into these considerations I cannot now enter, as my "Century of Inventions" is fast becoming due, and time commands dispatch; I beg leave, therefore, to pass to the relation this subject seems to bear to a "Marine Level."

It must, however, be premised, that I scarcely expect either of these methods to be correct enough for astronomical observations; as among other things, they have the nautical top to contend with: but if I am fortunate enough to have suggested useful methods of procuring relative stability on board a rolling ship, so as to suspend the better, a nice instrument of astronomy; or so to counteract the restless ocean, as to assist the victims of sea-sickness, I shall not entirely have lost my labour.

My first idea on this subject, is the following: If we had on ship-board, a simple pendulum of several thousand feet high, it appears certain that the oscillations of the ship would be begun and ended, before any single vibration could have been given to such a length of pendulum--which therefore, would scarcely vibrate at all: and if the natural time of this compound pendulum (for we are not confined to these small dimensions) were made to be much longer than those of the ship on it's meta-centre, this pendulum would scarcely vibrate at all: because it's several tendencies to take motion from the ship, would extinguish each other before they had had time to produce any common effect.



Further, this result would probably be assisted by another property belonging to this mechanism: see fig. 4. This diagonal suspension, as repeated at a b c d, fig. 4, is of such a nature, that when it's centres a b, are placed in any oblique position e f, (say by the rolling of a ship) the suspended bar c d, immediately takes a position of opposite obliquity g h, pointing upward towards i, just as much as the line e b points downward; while the middle line k l remains level--whether caused by the slides k l, or the single slide m.

I dare not assert any thing respecting the form this principle should assume, in order to produce the most useful effects; but it appears that the principal weight of the apparatus should be placed in the centre of gravity of the under bar c d. It would occur, of course, to every mechanician applying this System to real use, that in this fig. 4, we have only provided for one motion of the ship, the rolling motion: and that, in consequence, this System should be suspended in another similar one, acting longitudinally, so as to provide for the pitching motions of the vessel. In a word, I confess, with regret, that I leave much to do, by way of bringing this idea to maturity--it being at this late hour, more than doubtful, whether I shall myself ever be able to resume the subject at sea, where alone it can be duly tried.