portion of hydrogen gas, arising from the decomposition of the water in contact with the red hot iron.

After removing the cover, I placed a grating of wood upon the boiler, consisting of two pieces, each about an inch square, and five or six inches longer than the diameter of the boiler; these were held together at two inches distance from each other, by two other shorter pieces, nailed across them. Whilst the steam was strong, I put my feet alternately on the grating and on the footstool; but after a little stirring, the feet could remain on the grating, the temperature being then lowered.

LXVI.-On the great utility of the Camera Lucida to Travellers. By Captain BASIL HALL, R.N.

E

We have lately seen forty etchings by Lizars, from sketches made by Captain Hall, with the camera lucida, in North America, during the years 1827 and 1828, and which afford a convincing proof of the extraordinary powers of this exceedingly portable instrument.

We believe that we cannot give our readers a better idea of the estimation it was held in by Captain Hall, than by transcribing his own words.

"The following etchings have been selected from a series of sketches made with the camera lucida, in America, during the years 1827 and 1828, and the utmost pains have been taken to adhere to the original drawings. No reduction, enlargement, or embellishment, has been allowed in any instance; but the very lines traced on the spot, have been transferred to the plates, in order to preserve, as far as possible, the character of truth which the mechanical accuracy of the camera lucida communicates to its work, even in hands but little familiar with the management of the pencil.

"This valuable instrument ought to be more generally used by travellers than it now is; for it enables a person of ordinary diligence to make correct outlines of many foreign

scenes to which he might not have leisure, or adequate skill, to do justice in the common way.

"It should be recollected, that in most cases, it is not striking or beautiful views that we require, but merely correct representations as far as form is concerned, of those familiar objects which strike the eye of a traveller every where in his path as characteristic of the country he is visiting.

"If his sketches be further relieved by lights and shadows, another step is made towards the attainment of this purpose; for even a very few such touches, if strictly true to nature, often serve to place new scenes more distinctly before us, than the most elaborate or the most graphic verbal description can ever hope to accomplish.

"This instrument brings both these requisites within our reach; for although it be generally used for outlines alone, there seems no reason why the shading should not be as correctly delineated as the bounding lines of the trees, houses, water, or even the living figures, which are brought within the field of view.

"Artists accustomed to draw in the common way, are sometimes teased with the rigid accuracy, and the confined limits, to which the camera lucida subjects them; while persons altogether ignorant of the subject, are disappointed to find, that for the first day or two, they advance but little. Both parties complain, and not without some reason, that they cannot see the pencil distinctly, or that they lose sight of the object they are drawing just when they wish most to see it; and also, that the apparent motion in the image, caused by the slightest change of motion in the eye, perpetually throws them out. But they may rest assured, that a little perseverance, will put all these difficulties to flight; after which, the wonderful economy of time and trouble will far more than overpay the short labour of instruction.

"It adds greatly to the advantageous and agreeable use of the camera lucida, to have a portable table as part of

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the apparatus. For this purpose, Mr. Dollond, instrument maker, in St. Paul's Church-yard, London, has recently devised a small brass frame, which folds up when not in use, so compactly, as to stow away within the legs of a stand, not larger than a walking-stick. This, together with a camp-stool, of the same slight description, renders the draughtsman quite independent of further assistance, especially if his instrument be furnished with the double movements, and other contrivances, recently adopted by Mr. Dollond. With his sketch-book in one pocket, the camera lucida in the other, and the sticks above mentioned in his hand, the amateur may rove where he pleases, possessed of a magical secret, for recording the features of Nature with ease and fidelity, however complex they may be; while he is happily exempted from the triple misery of perspective, proportion, and form, all responsibility respecting these being thus taken off his hands.

"In short, if Dr. Wollaston, by this invention, have not actually discovered "a royal road to drawing," he has at least succeeded in Macadamising the way already known.”

Edinburgh, July 2, 1929.

LXVII.-On the Graphic Telescope, invented by Mr. CORNELIUS VARLEY; Mechanical Draughtsman, Clarendon-square, Somers-town.

It would not be fair to omit the present opportunity of justly praising the camera lucida, in also putting in a claim in favour of this other excellent drawing-instrument; but which is not, however, by any means so well known as it deserves to be. It may, perhaps, afford our readers some idea of its wonderful powers in assisting correct delineation, when we inform them that it was employed by Mr. Horner in tracing his extraordinary and successful panorama of London, and many miles around, from the lofty situation now occupied by the cross above the dome of St. Paul's;

and which panorama is now exhibiting in the Colosseum in the Regent's Park.

Mr. Varley had furnished Mr. Horner with several of his graphic telescopes; but the fact of one of them at least being actually employed in delineating the panorama of London, was evinced by an accident which befel it; when, in consequence of a high wind, the cabin erected by Mr. Horner was so greatly shook, that the graphic telescope fell out of it; and was so much injured, that it was sent to Mr. Dollond's to be repaired.

It is well known that the camera lucida acts upon the refracting principle; the graphic telescope, on the contrary, acts by reflection; and, on making comparative trials with the two instruments some years since, the Editor thought the preference was due to the graphic telescope, in point of facility in using it. However, they are both highly valuable instruments; and the graphic telescope possesses, besides, the property of being capable of being used as an ordinary telescope.

Mr. Varley, following the example of Dr. Wollaston, secured the benefits to be derived from the sale of his graphic telescope, by patenting it; but both the patents are now expired, and the two instruments of course can be made by every optician possessed of the requisite skill. And, indeed, since the expiration of his patent, Mr. Varley has exerted himself in extending the means of manufacturing his graphic telescopes with greater dispatch, in order to continue to derive that benefit from his invention which he is so well entitled to.

We hope soon to see both these valuable instruments more frequently employed than they have hitherto been.

LXVIII.-On a Portable Filtre. By Mr. J. I. HAWKINS, Engineer.

Mr. HAWKINS has, for many years, been accustomed to the manufacture of filtres, both for purifying water, and the

syrup in sugar refining, by that superior mode adopted by the late Mr. Howard. His water-filtres have long been known, and highly esteemed; they consist of a layer of vegetable charcoal, in powder, of a proper degree of coarseness, namely, about that of the grains of fine gunpowder, interspersed between two earthen plates, pierced full of holes, by means of needles, before the plates are baked, and which plates are also mounted in vessels formed of earthenware, so that they cannot be acted upon by the water; and the Editor can testify to their great utility from his own experience. And, indeed, there can be no better medium than charcoal employed for this purpose, both on account of its great porosity, and its indestructibility by the passage of the water through it. These filtering vessels, however, are not portable ones.

Mr. Hawkins's portable filtre consists of a small conical bag, made of silk velvet, with the pile inwards, and of another bag fitted within it, formed of silk sarcenet; the inside of the velvet being strewed all over with freshly burnt animal charcoal (ivory black), both bags being also secured to rings, formed of wire, made from German silver (kupfer-nickel), which is not liable to oxidation by the action of the water upon it. These conical silk bags are supported within another flat ring (likewise made of German silver), and which has a forked metal branch riveted to its rim, and capable of being turned half-way round, so as either to lie upon and within the compass of the ring, or to be turned into the contrary direction, and thus form a support or handle to the ring. The whole of this apparatus is contained in a round flat varnished paper snuff-box, only four inches in diameter, and an inch in depth, so as at all times to be conveniently carried in the pocket.

With this exceedingly portable apparatus, Mr. Hawkins now always has it in his power to filtre any water he may require, either for drinking at his meals, for making tea, &c. &c.; and its action is sufficiently rapid and effectual; for the power of animal charcoal is well known greatly to