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this manner to an equal thickness over its whole area. The operation of casting takes place close to the mouth of the annealing-furnace, into which it is carried immediately it becomes solid. But the large plates of glass are by no means fitted for use when withdrawn from the annealing oven; three processes are yet necessary before they reflect the clear image from the silvered mirrors, or adorn the windows of our mansions. They are first cut by the diamond to the shapes required, an operation requiring no description here. The plates are now ground, to remove the roughness found on the surfaces. This work requires great care. it being necessary to plane off the roughness without scratching the face of the glass. Some powdered flint is therefore spread over the plate, and rubbed along the surface by machinery, which, in the larger glass-houses, is moved by steam. After the flint has removed the larger protuberances, emery powder is applied, first coarse, and then finer, until by successive frictions the plate begins to exhibit a beautiful level.

But all is not yet done; the polishing now fallows. In this operation, pieces of wood covered with numerous folds of cloth, with wool between the folds, are used to bring the finished plate to its last degree of beauty. The friction of these cloth rollers would not, however, be effective without the use of a peculiar substance, called colcoth (the red oxide of iron), used for polishing other hard surfaces besides those of plate-glass. Thus, from the fusion of the Lynn sand, the soda and lime, arises the product, which, having passed through the annealing-oven, the grinding, and the polishing, is now to take its place amongst the highly elaborated productions of art.

The various kinds of glass we have been speaking of, possess different properties, according to the purposes to which they are to be applied. Flint-glass, of which most ornamental articles are made, is rendered softer than the other sorts, and plate-glass, on the contrary, is made of as hard a texture as possible, to prevent its being easily scratched,

We have already spoken of the glass being placed in the annealing furnace which might more appropriately be called an oven, since a low red heat is the highest degree to which it is ever heated. The purpose for which the glass is placed in this furnace, is to allow it to cool gradually down to the temperature of the air, by first placing it in the hottest part of the oven, and afterwards gradually removing it to the mouth.

For some of the larger pieces of plate-glass, this operation will occupy the space of two or three weeks If glass is not properly annealed, the most trifling scratch or blow from a sharp body, or any sudden change from heat to cold, will cause it to break. If suddenly cooled in making, without undergoing the process of annealing, this brittle property is increased to an extreme degree.

Two philosophical toys, one called the Bologna Phial, and the other, Prince Rupert's Drops, or commonly the hand-cracker, are good instances of this.

The Bologna Phial, is merely a wide mouthed bottle of unannealed green glass, extremely thin at the neck and upper half of its sides, and very thick below. A leaden bullet may be dropped into this bottle from the height of several feet without danger, but if a large grain of sand, or, what is better, a small piece of broken gun. flint is allowed to fall into it through the space only of a few inches, the shock produced will break the bottle to pieces. If laid on its side, the thick end may be struck with considerable force with a wooden mallet without danger; but it would be immediately broken, if merely scratched with a piece of sand.

The hand-cracker is a very familiar instance of this property; the thick end of this may be laid on the table, and struck forcibly with the fist without danger, but if it is grasped in the hand, and the smallest portion of the thin end is broken off, the whole of it breaks to pieces, or bursts, with so much violence as to sting slightly the hand that holds it.

The purposes to which this beautiful material have been applied, are as numerous as they are useful; it

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has added materially to the comforts and conveniences of private life; it has, among many other invaluable benefits, assisted the astronomer in his researches, and the philosopher in the detection of the more minute operations of nature among the lower classes of animals; and to it we are indebted for our chief discoveries in electricity. Saturday Magazine, and Sharpe's London Magazine.

MANUFACTURE OF HORSE-NAILS.

THE fabrication of the nails used in shoeing horses is a large and highly important branch of industry, which until lately has resisted all the attempts of inventors to elevate it from a handicraft into a manufacture. The horse-nail must combine many peculiar features. It must be easily flexible, and must bend without any tendency to crack. It must be of small bulk, and so sharp, that notwithstanding its flexibility, it will readily penetrate the hardest hoof. It must be so tough and strong as to withstand, especially at the junction of the head and shaft, all the shocks and friction incidental to travel or to draught. These qualities can only be obtained from charcoal iron of the very finest quality, and have hitherto been obtained only from nails forged by hand from rods. In the course of the present century no less than thirty-one patents or provisional protections have been obtained. for horse-nail machinery, but although many of these patents possess considerable merit, not one of them has ever reached the stage of being actually worked for commercial purposes. In the earlier ones it was usually proposed to punch the nails out of sheets, but sheet iron cannot be obtained of the quality required, and the process of punching is one that would imply considerable waste. Other inventors attempted to substitute rollers for the hammer of the smith, and to roll out the end of the heated rod to the necessary point. The

endeavour failed because the rollers were found to carry a sort of wave or projection of the heated iron before them, and this was apt to crack on cooling and to render the finished nail worthless. Other contrivances fell through for various reasons, often because the inventors knew only the shape of the horse-nail, and were unacquainted with the actual requirements of the farrier, and often, perhaps, for want of the capital necessary to establish them. The horse-nail business has remained in the hands of masters residing chiefly in and about Birmingham, Derby, and Bristol, whose practice it is to give out iron rods to workmen, who forge the nails at their own homes. A skilful workman can make 1,000 nails a day, and is paid 3s. 6d. for this quantity; but, as a matter of fact, few men can continue at this speed of production for many consecutive days, so that the wages paid do not average a guinea a week. This is a very low rate for skilled artizans, and it is said not to be uncommon for the men to sell the fine iron supplied to them by their masters and to forge their nails out of iron of an inferior quality. There are in Great Britain and Ireland about 2,600,000 horses, which represent a demand of about 998 millions of nails (or 5,574 tons) per annum. More than 2,000 tons are made for exportation; and, at an average of £60 per ton, the annual value of the trade is little less than half a million sterling.

Among the more recent patentees of horse-nail ma chinery are the Messrs. Huggett, father and son, the former of whom has been extensively engaged in shoeing horses for many years.

The chief feature of Mr. Huggett's patent is a pair of rollers by which he converts ordinary rod iron into a rod so shaped as to admit of being cut into nail blanks. The upper roller is a simple cylinder; the lower has a series of depressions on its circumference, separated by intervals. Each depression corresponds to two nail heads, each interval to two shanks; and the surface of the roller is so curved in the intervals as to render the middle of each its most prominent part, The actual

roller surface is very narrow, corresponding to the slenderness of the rod; but is bounded on either side by a massive collar, which prevents the smallest lateral spreading of the iron, and limits the alteration of its form to elongation. In order that the iron may yield freely, a very high degree of heat and a rapid motion are necessary. The rods, each two feet in length, are heated in a gas furnace, and are then suffered to run down a shoot to the rollers, which are turning at the rate of 500 revolutions a minute. The lateral collars already mentioned are so contrived as to present the descending rod always in the right direction to the rollers, and it appears almost instantaneously on the other side, still glowing, somewhat contorted, and about trebled in length. It falls into a sort of trough, and is instantly seized with proper tongs by two boys, one at each end, is pulled straight, and laid aside to cool. The rollers are kept constantly lubricated by a stream of coal tar, which at once diminishes friction, and also, by inflaming as each rod is passed through, shields the faces of the rollers by a fine carbonaceous deposit. A single furnace will heat from five to six thousand rods per day of ten hours, a quantity equivalent to over 100,000 nail blanks; and the rollers, which are rather under 7 inches in diameter, could turn out rods at the rate of 900 ft. per minute.

The rod of nail blanks, as it leaves the rollers, may be described as a slender strip of iron, presenting a series of prominences on one side. Fach prominence is about 14 in. long, each interval between the prominences about 3 in., the dimensions varying slightly with the size of the nail that is to be made. From each prominence the rod tapers slightly to the centre of each interval. It is nearly as flexible as lead, and so tough that the most rapid bending to and fro only breaks it with difficulty.

In this state the rod is passed cold through another pair of rollers, so contrived that they compress only the prominences, and give them a nearly square outline in section. It is then taken to a cutting machine and cut

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