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Proceedings of the Royal Institution of Great Britain.

WEEKLY EVENING MEETINGS.

March 7th.

THE subject of the evening consisted of an account and illustration of some Supplementary remarks on the Reciprocation of Sound. The matter belonged to Mr. Wheatstone, but was delivered by Mr. Faraday. The new information contained in it has been embodied in the account given of the subject of Resonance, at page 175 of this volume.

The tables in the library were furnished as usual with objects of interest.

March 14th.

Mr. Turrell gave the members an account of the chemical menstruums used for etching upon steel plates; he also described and exhibited an instrument, called a perspectograph, for laying down points in perspective.

A very large specimen of native silver from Mexico was placed amongst the other objects upon the library table.

March 21st.

Mr. Millington gave an account of the manufacture and uses of paper, accompanied with models and illustrations of the recent improvements.

Specimens of all kinds of paper were placed in the library, and amongst them, some recently manufactured from straw, by a new process.

March 28th.

Dr. Harwood gave an account of the structure and economy of the Greenland whale, illustrated by numerous fine specimens, preparations, and drawings of the animal and its various parts.

A variety of productions from the East were placed upon the library tables, by favour of Lady Raffles and Mr. Bennet. Some of them were presented by the latter to the Museum. Dr. Boott also gave a specimen of the wax myrtle plant (Myrica Cerifera), and of the myrtle wax from the same plant.

A drawing instrument, invented by Mr. Ronalds, was explained in the library. A pencil and a small bead are so connected together by means of a thread passing over pullies, that if a person, looking through an eye piece, will hold the pencil upon a sheet of paper, and then watching the bead will move his hand, so that the bead shall trace the lines of any object that is selected to be looked at, he will find that, whilst he has been doing this, he has also made a drawing of the subject upon the paper; for the pencil and the

bead describe exactly the same lines, though upon different planes. Thus a drawing is made without ever looking at the paper, but solely at the object.

The meetings, having been suspended during Passion and Easter weeks, were again resumed on the evening of Friday,

April 18th,

when Mr. Ainger described and illustrated the various escapements for timekeepers.

His object, he said, was to explain, by means of very enlarged models, the principles and action of those ingenious and beautiful contrivances which are generally little understood, because of the minuteness of their parts and the rapidity of their motions, which render their mode of operation in machines of the ordinary size too obscure and transient to be understood by mere inspection. To obviate this difficulty, a model was made three feet in diameter, of the wheel called the escape wheel, which in common clocks is not as many inches in diameter, and in watches is of course much less. This model, with its appendages, was made to move with a proportionate degree of slowness, so that the action and reaction of the parts became obvious and intelligible.

A timekeeper may be divided into two parts:

1. The motive part,

2. The regulating part.

The first merely produces motion, the second regulates its velocity. The motive part may be subdivided into

(1) The motive power, which is a weight or spring.

(2) The distributive power, which is a train of wheels increasing the velocity, and of course diminishing the intensity of the force arising from the descent of the weight, or the uncoiling of the spring.

The use of the train of wheels will be understood by considering that the weight or spring barrel of a thirty day clock, will not make, perhaps, more than ten revolutions, while the second's hand of the same clock will, in the same time, make fifty thousand revolutions. The train of wheels, therefore, multiplies the insensible velocity generated by the weight or spring, till it becomes the visible velocity of the second's hand, which is placed on the axis of the last wheel of the train called the escape wheel; this wheel has, therefore, a constant tendency to move in obedience to the impulse communicated through the train from the weight or spring. This tendency to move is what keeps the clock going; but the motion requires to be regulated, and made perfectly uniform, which leads to a consideration of the other essential part of a timekeeper, the regulating part.

This may also be subdivided into two parts:

(1) The pendulum, or balance, which governs the rate.

(2) The escapement, which transmits that government to the train, and in return transmits the impulse of the train to the pendulum. In order to understand properly the escapement, we must have some idea of the properties of the pendulum.

A body suspended so as to move without friction, and exposed to no resistance from the air or other causes, being made by means of equal impulses to perform equal vibrations, would perform them in equal times, even if we had no better proof of the fact, than the old metaphysical argument, that there is no sufficient reason to the contrary. So also, though there be friction at the point of suspension, and air resisting the vibration, if that friction, and that resistance of the air, be quite invariable, the vibrations would continue invariable both in quantity and time, for the same want of a sufficient reason otherwise. In all this, however, we have assumed three impossible conditions.

In the first place, it is impossible always to give exactly equal impulses to the pendulum.

In the second, the friction is likely to vary from temperature, and from the changes which the oil, or other lubricating material, undergoes.

In the third, the resistance of the air changes from temperature, moisture, and other circumstances.

These causes (with others) preclude the possibility of making a pendulum perform for any length of time exactly equal vibrations; and since the longer vibrations of a simply suspended pendulum occupy more time than the shorter ones, it becomes an incorrect measure of time. The error produced in this way is very slight, yet enough to be sensible in instruments so perfect as they are now made. The times occupied in describing a complete semi-circle, and the smallest sensible arc, differ only in the proportion of 34 and 29; and the error becomes much less, in proportion as the differences between the arcs of vibration are less.

If, however, as is well known, a pendulum could be made to vibrate, describing the arc called a cycloid, it would perform all its vibrations, whether long or short, in equal times; and it was, therefore, proposed by Huygens, to attach to the upper part of the pendulum an apparatus which should cause it to describe cycloidal arcs,' and thus to free it from the errors arising from changes in the arc of vibration. It was found, however, that this apparatus required an impracticable degree of accuracy in the workmanship; and therefore

it was abandoned for the better plan of endeavouring to make the arcs of vibration as small and as equal as possible. This is the problem, which has for two centuries exercised the ingenuity of the first mathematicians and mechanics of the world, and the best solution of which it is our object to illustrate.

The three difficulties presented by this problem are, as before. stated:

1. The varying friction of the pendulum.

2. The varying resistance of the air.

3. The varying impulse given to the pendulum,

For the first and second no compensation is attempted to be made; they are merely reduced as much as possible, and this has been done so effectually, that a pendulum once set in motion, has vibrated thirty hours without any renewal of the first impulse.

The third cause of error, the varying impulse given to the pendulum, has been the great object of attack. The disturbance which the pendulum is liable to, from its connection with the motive part of the instrument, is of two kinds. In the first place, the pendulum receives a new impulse at each vibration, in order to maintain its motion; that impulse, it is true, always proceeds from the same weight or spring, but that impulse, will vary when the timekeeper is close wound up, and when nearly down; this, however, is trifling, and can be very nearly compensated by other contrivances: but the weight or spring has to operate through the train of wheels, which, at different times, will have different degrees of friction, and will, therefore, transmit different quantities of power to the escape wheel from which the pendulum receives its impulse.

This defect cannot be remedied, that is to say, the motive energy residing in the escape wheel cannot be made uniform. The object sought after has, therefore, been to render the impulse given to the pendulum independent of the force existent in the wheel which gives that impulse; and this has been accomplished by means of successive improvements in the escapement, so perfectly, that increasing the weight attached to a good timekeeper an hundred fold will not increase the impulse given to the pendulum.

In order to understand these gradual improvements in the escapement, we must now observe minutely its mode of operation.

The business of the escapement is to act as a sort of mediator between the escape wheel, (which is always tending to increase its motion by the natural accelerating tendency of the weight or spring,) and the pendulum, which is always tending to diminish its motion from friction, and the air's resistance. The business of the escape

ment is to modify each of these tendencies; to transmit the accelerating tendency of the escape wheel to the decreasing tendency of the pendulum, and vice versa, thus to neutralize the tendency to change, and to make them move together and equally. After the escape wheel has, by means of one part of the escapement, given the pendulum an impulse, it would run on with increased velocity, but that the pendulum, in return, employs another part of the escapement to stop the escape wheel; and this is the conflict which is always going on; the escape wheel impels the pendulum and then escapes (thence its name), but it moves only half the interval between two of its teeth before it is caught by another part of the escapement.

This is the process which takes place in all escapements, though under very different circumstances, and by very different means: it will be rendered intelligible, as regards two of the most common escapements, by the annexed diagram: