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DESCRIPTION OF THE CAMERA LUCIDA,

AND SPECIMENS.

Sir, I have taken the first opportunity of replying to your correspondent J. O. B.'s query (p. 128), "Does J. understand the use of the pantagraph?" Indeed, Mr. J. O. B., I do understand as much as there is to be understood about it; and, moreover, if you or any of your friends are in want of such an article, I shall be most happy to supply you with a splendid pantagraph of the best workmanship, and in perfect order, at a very cheap rate for the reason why I pronounced the aforesaid instrument unwieldy, &c. was not from mere whim, prejudice, or the want of knowing better, but because I have found it to be so from actual and repeated trials. It is right, however, to state my reasons for using this term. A pantagraph of wood is worth little, because it will warp; and a warped pantagraph is useless, or nearly so. Secondly, this instrument is of little general use, unless it is of what opticians call a "full size" (three feet). Thus the best pantagraph will contain several pounds of solid brass; and though you may undoubtedly have one which is neither large nor unwieldy, it will be nothing better than a kind of child's plaything.

I assure you, Mr. Editor, I could fill one of your pages with a minute detail of the inconveniences attending the use of the pantagraph, but would not, on any account, occupy your valuable room with what I consider so worthless a subject.

I have enclosed sketches of several of the articles in my studio, and hope that they may give to those to whom you may show them, some idea of of what the Camera Lucida is capable.

No. 1 is a portrait. I shall feel pleased if you are able to make out who it is meant for. It is but justice to myself to state, that it is the first attempt I ever made at likenesstaking; and, I assure you, I was literally astonished to find what assistance this unique little instrument gave; and do not hesitate to say, that I will copy the most elaborate piece of sculpture, were it the Lao

coon itself, in the same way with equal facility.

N. B.-It was all, both outline and shading, done by the assistance of the Camera Lucida; and although, as you will easily perceive, it is the work of a mere novice, yet, in justice to the instrument, I must say that I could do much better; but I am rather pressed for time, and it was done in haste.*

No. 2+ I selected as being a very difficult subject. It represents the American double-barrelled air-pump. There is no man in the United Kingdom who could make a correct copy of this complicated instrument without the Camera Lucida; with its assistance, it is nearly as easy as to white-wash a wall; and you will perceive that the perspective is absolutely perfect.

N. B.-This pump is made on an excellent principle, the valves being mechanically opened by the handle itself. It is the most simple, permanent, and accurate pump that is made; and superior even to Cuthbertson's, (vide Pneumatics, Enc. Brit.) which is too complicated, and too apt to go out of order. The two front pillars are left white, being of polished brass. The brass work in general is made light. The dark parts are mahogany or ebony. It exhausts 600 times, and when put in fine order much more. It is altogether the most beautiful instrument I ever beheld. It was made by Jones, 30, Holborn.

No. 3, is a representation of a small model of Vauloue's pile-engine, which goes by an internal moving power.

As to No. 4 (figure of a chair), No. 5 (a sofa), and No. 6 (Jones's best circular-fronted barometer-an exquisite piece of workmanship), they are merely put in with intent to show the great use this instrument might be of to artisans: to put even a chair into correct perspective, is no easy matter; by means of the Camera

Lucida, it may be done in less than five minutes.

J. O. B. says, he wishes I would furnish a drawing and description of the instrument in question, as he cannot find it in the Encyclopedias. I do not know what works bearing that title he may have examined; but had he chanced to stumble upon

the " Supplement" to the Encyclopedia Britannica, which (I hope it will not infringe upon the advertisement act to say so) is by very far the best work of the kind extant; he would have found not a slight notice" of it, but a copious and accurate account of Hooke's Camera Lucida, as improved by Wollaston, illustrated by a quarto engraving executed in the first style of art; and to this I refer him. But as there are, doubtless, many of your numerous readers to whom this instrument would be of extensive use, who may have no opportunity of seeing so expensive a work, I have considered it best to comply with the request of your correspondent, and now proceed to give a drawing and description of

THE CAMERA LUCIDA,

An instrument for drawing objects in true perspective, and for copying, reducing, or enlarging, other drawings.

The instrument, as represented in the figures, may be used with the small round glass, AB, turned up in front, (fig. 1.) or with the larger glass turned up level underneath the instrument (fig. 3 seen from above). But those who are short-sighted can only use the former, and persons that are long-sighted must use the latter. D is the clamp and screw for fixing the apparatus to the table. Mine (which was made some years ago, and is of course now old-fashioned,") is, in my opinion, much better; being constructed with a solid, square foot of brass, (fig. 4.) and I use a detached clamp. I could give several reasons why I consider this plan preferable, but it would fill much space. I will now give a few of the principal directions from Dollond's conspectus supplied with the instrument.

Fix the instrument to the table and paper, as shown in fig. 1. Turn

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the prism (shown more distinctly in fig. 2.) on its pin till the rectangular face be placed opposite to the object to be delineated. The black eye-piece, E, will then be uppermost; and, on looking through the small aperture thereia, the artist will see the object he wishes to draw, apparently distributed over the paper. The object is shown beautifully, and without the least coloured refraction.

N. B. The eve-hole is to be so placed, that half of it is over the prism, and half over the paper; and as the eye is larger than the eye-hole, the observer sees paper and pencil through the nearer half, and the object at the same time through the further half, and thus has only to trace the lines, as well as his skill, or steadiness of hand, will enable him.

In copying drawings, the copy will be larger or smaller than the original, according as the prism is more or less distant from the paper, than it is from the drawing to be copied. Thus, if the drawing be two feet from the prism, and the paper only one foot, the copy will be half the size of the original. If the drawing be at one foot, and the paper three feet distant, the copy will be three times as large as the original; and so for all other distances.

Though I have studied to be as brief as possible, and have been much more so than I could have wished, yet still this paper has run out to some length. I will, however, observe, that should what I have here stated be the means of exciting J. O. B. (or any other person) to write in opposition to me, and say what can be said in defence of a subject, which I am certain is utterly indefensible, I pledge you my word that I will not reply to it. For, though I would have your readers, as honest Captain Mac-Turk says, "to know and to believe, and to very well consider," that the Camera Lucida will do all that the pantagraph will do, and in a much better style, and also a very, very great deal that the pantagraph can not do; yet I will not be the means of filling your pages with a lengthy and eccentric-chuck

like discussion about a subject which is as evident as the light of day.

To conclude, it is an ill wind that blows nobody good. One man at least is filling his pocket by means of the pantagraph: for, if your readers will look at Mr. J. O. B.'s diagram, and imagine one of the arms behind a transparent screen, and the other connected with the arms of a moveable-jointed doll, they will easily understand the contrivance of Prosopographus, the automaton artist. The patient's head is wedged into a place in the back of a chair, where it is held firm, whilst the artist goes behind and traces the outline of the face (which is thrown upon the screen by a gas light); he afterwards does it over with black. But you will observe, that it can only take profiles; whereas, by means of the Camera Lucida, we can take full, or three-quarter-face likenesses at pleasure: thus leaving the above-mentioned little gentleman far in the back-ground.-We may truly say of it, what Arbuthnot said to Mr. Pope of the genius of Handel, "Conceive the highest of it that you can, and it is much beyond any thing you can conceive."

I had nearly forgotten to observe, that I have added to the sketches sent to you a Camera Lucida sketch of Cary's Map of England and Wales, in order to satisfy your correspondent that it is applicable to this purpose: but I may observe, that map-drawing is one of its lowest efforts. In tracing a map, I will make the pen or pencil run like a snake on the ground, and it shall still be correct. Indeed, to suppose the instrument unfit to copy a mere map or chart, when it will copy the most perfect likenesses of the human face, is like questioning a man's ability for leaping over an ink-stand, when we have seen him with ease clear a five-barred gate. The map is reduced to about 1-12th the original size.

Exeter.

I am, Sir, Your obedient servant,

H. J.

P.S.-Though I am afraid it is rather improbable, yet if any of your correspondents should happen, on business or otherwise, to visit this part,

I will with pleasure give them a practical conviction, that what I have here stated concerning this instrument is not exaggerated. You have my address.

RAIL-ROADS.

Sir,-As to my proposition, in your Magazine, of Oct. 24, 1829, No. 324, your correspondent C. H. has mistaken my intention, as to the use or application of the power to propel the carriages along the railway with rollers only. I contemplated the use of horses or stationary engines, as the locomotive engine, I conceive, could not be employed upon a way so constructed without difficulty. In my opinion, however, either horses or stationary engines might be employed; and this for various reasons, which may be demonstrated with as much ease as any of the simplest propositions of Euclid.

I beg, Mr. Editor, you will insert the above in your excellent Magazine, for the purpose of setting your correspondent C. H. right, as he thinks, were horses used, the scheme is worthy a thought, and that all difficulties would be removed. I am sorry he has put a misconstruction on the scheme, as I never intended any other power but that of horses and stationary engines to be applied.

Not only will the expense of such a road be diminished, but you may form an idea how trivial the expense will be of a waggon adapted to travel along a way so constructed, as it will be more of the nature of a sledge, having neither axles nor wheels, but barely a rail on each side of the carriage, either of cast or malleable iron, as you please. If the mode which I have pointed out be found less expensive and equally effectual in practice, it will be a vast saving to the owners of the collieries upon the Tyne and Wear, as well above ground as beneath, and perhaps more so than in any other district in this kingdom.

Jan. 4, 1830.

I am, Sir, Yours, &c.

W. B.

N.B. In my next, I will enter more at length into this subject.

ON THE REMOVAL OF DEPOSITS OF EARTH IN TIDAL RIVERS, AND THE PROBABLE EFFECTS OF REMOVING THE OLD LONDON BRIDGE.

(Concluded from our last Number, p. 339.)

As to the practical construction both of jettees and of these waterturners, so as they will probably have to be used in the Thames, it is better shown in Dover Harbour than in any place in England which I have visited.

The effect of a jettee, whether permanent or temporary, of an apron, or other water-turner, when placed in a river, depends principally upon the velocity of the tidal or other stream, and the breadth of the passage which remains between the projecting head of the jettee and the opposite bank or beach. In many cases, the angle which the jettee makes with the bank from which it protrudes is not of so much consequence.

Fig. 1.

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between BD and BA: yet, if the jettee were posited in the direction BH, it would not produce about BC the same effect that one placed at GB would near BD.

In the case where the purpose of the apron or jettee is to remove a deposit at K, or to gain upon the opposite bank there, the main object is so to contract and divert the stream that it shall strike at K with the greatest force to wear away the obstacle. In those parts of the country where land bounding a stream is valuable, its owners have been sometimes known to increase it by planting osiers or willows upon its banks, which, as their roots protrude, cause a corresponding abrasion into the opposite side of the stream. Then, in a few years, the old trees have been cut down, and new ones planted still closer to the water; and thus by successive, imperceptible thefts upon the opposite neighbours, an estate running for a mile by the side of such a stream has been increased by some.

Let AP be a jettee projecting perpendicely from the bank CD of a river, the stream moving onward in the direction of the arrowhead. The mass of stagnant water CBDA which will result will remain nearly the same, although in the direction of BA, provided that its head remains at B, and that it is posited within the space CBDA. It will only happen that the mass of stagnant waters will diminish on one side of the root or insertion A, and augment on the other, so that the entire mass will posite itself above the jettee, if it be placed in the direction BD; while, on the contrary, it will be found below the jettee or apron, if it be placed in the direction CB. But, if we place the jettee in a direction at GB, out of the mass of stagnant water CBDA, that mass will be augmented below the jettee by the space GBC, while, probably, it will be diminished a little

When the mass of dormant waters, occasioned by a jettee EG, would cover and occupy the whole of a great hole in the bed of a river, which it is proposed to fill; and when the river is not too deep towards the middle of that hole, a single jettee EG is preferable to the two, HI and LM; both because the two would probably cost more in their construction, and because there would be occasioned by these two deposits, O N, PQ, on both sides of them.

If the jettee EG is not competent to produce a quantity of stagnant water, sufficiently extensive to accomplish its operation upon the entire hole, it will be proper, instead of that, to have two smaller ones hi, lm, but