steam jet or other exhauster, so that air could be withdrawn from the box or let into it by a small hand- tap, as desired. This, then, was the whole apparatus which constituted the invention; it was extremely inexpensive, and once made, almost indestructible.

This device took the place of the stone table under the ordinary polishing machine. Its operation may be described as follows:-- The sheet of glass to be polished is gently slid upon the table, and covers all the small holes in the slate bed; and if then the small tap which connects the underside of the slate table with the steam jet or other exhaust apparatus is turned on, a partial vacuum is formed beneath the sheet of glass, and it becomes in an instant immovably fixed and adherent to the slate bed on which it rests. There is no plaster employed, and consequently none of the mess or labour attending its mixture and chipping off; there is no delay in the use of the polishing machine while the plaster is becoming hard, or when it is being cleared away. The plate of glass is an absolute fixture in less than a quarter of a minute after the exhaust is turned on; and it is as rapidly released by reversing the tap and readmitting the air to the box. The plate is then turned over, and the tap being opened to the exhaust, it instantly becomes re-fixed to the slate surface; there is no cost of plaster, there is no labour, and no risk of snapping off the corner of the plate to release it. Take a moderate sized plate of 6 ft. by 10 ft. (60 square feet), and take a very low exhaust, say, 2 lb. per square inch, equal to 288 lb. per foot or 17,280 lb. of atmospheric pressure, holding it immovably fixed. Every schoolboy who has seen how powerfully the glass bell of an air-pump is held in place by atmospheric pressure, must understand this simple, effective, cleanly, inexpensive, safe and rapid way of holding down a plate of glass.

This invention, which formed one item of the many improvements in the plate-glass manufacture which I did not carry out, has been available for the free and unrestricted use of the public for nearly fifty years, and yet no plate-glass works in this, or any other, country has taken advantage of it. The simple fact is that an invention must be nursed and tended as a mother nurses her baby, or it inevitably perishes. Nor is this almost incredible indifference to their interest the result of the invention being unknown to the public; for I exhibited a polishing table so constructed, among many other things, in the International Exhibition of 1851, where it became one of the most attractive of my exhibits. I well remember that on one occasion I was requested to be present two hours prior to the opening of the Exhibition to the public, and had the honour of showing and explaining the device to Her Most Gracious Majesty, who was on that occasion accompanied by His Royal Highness the Prince of Wales and other distinguished persons.

The slate table was about 4 ft. by 3 ft., and I used a plate of polished glass a little less than a square yard, weighing about 36 lb. The firm way in which it was held was most easily demonstrated by placing one side of the plate about four or five inches on the edge of the slate bed, and allowing the remainder to project. Not only did the atmospheric pressure sustain the plate overhanging in this way, but no one could lift it up or force it down. I was also able to illustrate a fact but little known, viz., that a plate of perfectly flat ground glass lying in absolute contact with a true plane surface cannot be smashed by a blow from a wooden mallet with a curved face. I have struck my yard-square sheet of glass at the Exhibition, when held down by atmospheric pressure, dozens of times before the public, as hard as I could strike it with a wooden mallet, and never broke a single sheet in doing so. If it lay hollow and not in absolute contact with the table, a child could fracture it in a dozen places, but when in contact all over its surface, no amount of force less than that at which glass crushes to powder will crack a properly supported sheet. From what I have said I think I have shown that, however self-evident an invention may be, or however advantageous it might be to a manufacturer, if it is public property he will not touch it.

I have already so far trespassed on the patience of my readers in reference to the manufacture of glass that I must bring these remarks to a close. But there is just one little point that I may be excused for mentioning; it has reference to the silvering of glass, which everyone knows was effected by the amalgamation with mercury of a large sheet of thin tinfoil, the amalgam adhering to the surface and remaining on the side next the glass, a beautifully-polished and highly-reflecting surface. But it had a bluish or leaden hue that was most unfavourable to the fair sex, and spoiled the best complexion. I thought much over this defect, and at last succeeded in greatly improving the whiteness of the reflection. This I effected