diluted nitric acid is carefully dropped on the plate, and accurately distributed over the surface, another light polishing succeeds, after which, the plate is heated over a spirit lamp, which must be moved beneath by the hand so as to distribute the heat equally; or, which is preferable over charcoal, until its surface is evenly covered with a white appearance like a veil spread over it, when it must be suddenly cooled by laying it on a cold stone or a marble table; after this process, the operation of the acid is repeated three times, but the plates are generally put by after it has been twice applied, that the operation may not be too long delayed; the third and last application must be made immediately before the plate is used. This polishing is the only part of the operation that can be said to be seen: for in all the others, except placing the plate in the Camera, which in the Adelaide Gallery was done out of the room, the day light must be excluded, and the light of a small taper is alone allowable. As soon as the plate is polished, the shutters are closed and the operator places the plate in a close box to undergo the second process-The application of a sensitive coating. This is done by fixing the plate face downwards in a box contrived for the purpose, in the bottom of which stands a cup with Iodine, broken into small pieces and covered with gauze. The fumes of the Iodine rise, and being evenly distributed by the gauze, spread themselves over

the plate, which, within half an hour is covered with a fine coating of a yellow gold colour. The moment it has acquired a sufficient coating of the Iodine, it is removed to a box, and being closed up, the third process is, preparing the Camera Obscura and placing the plate in it. In order to judge of the effect of the object to be represented, the focus is regulated through a powerful lens, on a plate of ground glass occupying the position in which the prepared plate is to be placed. When every thing is properly adjusted, the box containing the plate is introduced and exposed to the focus of the Camera. The time necessary to complete an impression varies according to the power of the sun's rays, sometimes in about twenty-five minutes a representation is formed on the plate, or speaking otherwise, the shadow is then embodied.

The fourth operation is bringing out the image. To do this, the board with the plate is removed from the box and adjusted face downwards, at an angle of forty-five, in an iron box contrived for the purpose, in the bottom of which is a cup of mercury which is heated by a spirit lamp placed beneath; after it has remained here some time, it is replaced in the case with folding doors until the fifth and last process (removing the sensitive coating) is performed. This operation is to remove a portion of the Iodine when a solution of common salt is made use of. The plate is first dipped in common

water, and is then plunged into the salt and water and carefully moved round with a copper hoop; when the yellow colour has departed, the plate is placed on a desk at an angle of forty-five, and distilled water warm, but not boiling, is carefully poured over the surface. As soon as the moisture has evaporated, the operation is completed, and the drawing may be safely exposed to the full light of day. The appearance of these drawings is very peculiar. The shadows are a dull grey, varying until they become almost black, and though the pictures they delineate are accurate in the extreme, they are not pleasing. They appear unnatural and look somewhat like a moonlight scene. The Daguerreotype, with all its necessary apparatus, is manufactured and sold in Paris, for about £20. In Bombay, where the sun is always powerful, pictures of scenery could daily be produced. And one great advantage is, that for copying, their fidelity to nature will strongly recommend them. We cannot take leave of the Adelaide Gallery without expressing our admiration of the usefulness of such an institution. There is a powerful microscope and lectures upon several subjects are delivered, and for all this, the admission is only one shilling.

POLYTECHNIC INSTITUTION.-We often went to see this Institution, 309, Regent Street, it was established in 1838, upon the same principle as the Gallery of Practical Science, in the

Lowther Arcade, but having had the advantage of seeing the working of that institution, it started with peculiar advantages over its predecessor, and we are quite sure that there is not in any country to be procured so much intellectual amusement for a shilling, as is to be had at the Polytechnic Institution; for you can be constantly amused and your mind improved, from half-past ten in the morning until half-past four, and only pay one shilling, or you can go from seven until nearly eleven at night, for a similar sum. A band of music plays daily from three to five o'clock, and in the evening from about eight until the exhibition closes. There are thirty different rooms connected with the institution. The whole building is three hundred and twenty feet in length.

The first apartment you enter, is a hall fortyfive feet long and forty feet wide, devoted to machinery and manufactures. All the machines in this hall are worked by steam power. There is a lathe for turning ivory, hard woods, brass, &c., with all the apparatus, for the most beautiful of ornamental turning. A loom for weaving ribbons, a loom for weaving checks, a braiding machine, a twisting machine, a warping mill, and a beautiful four horse power steam engine. On the opposite side is a gem engraver at work, a copper plate printing press at work, steel and copper plates engraving, also a manufactory for optical instruments,

and for grinding and preparing lenses for telescopes and glasses for spectacles, &c.

You then proceed to the great hall, one hundred and twenty-feet long, forty feet wide, and forty feet high; in the centre are two canals with a surface of seven hundred feet of water, attached to which are all the appurtenances of a dock yard, constructed by the government engineers, the models deposited by permission of the lords of the admiralty, with a great many locks to keep up a head of water upon canals, and a series of water wheels in motion, to illustrate lectures on naval architecture and hydrostatics. At the end of the canal is a deep reservoir of water into which a diving bell capable of containing four or five persons is lowered to a considerable depth under water, air being supplied by two powerful air pumps, so that visitors may descend with convenience, and whilst we were there we saw several persons go down, among whom were some ladies, the only inconvenience experienced whilst under the water is a great pressure upon the inside of the ears, which to stout persons of a full habit of body becomes very painfully troublesome. We know several persons who have descended and they have felt no ill effects from it. A diver, clothed in a patent water and air tight diving dress, goes down a ladder to the bottom of the reservoir of water, being supplied from the air pump with air through a tube that enters into his