the same time that it is rubbed round with a piece of folded cloth greased on the surface. Joints in cisterns, etc., are generally made by scraping the edges clean with a steel scraper, and applying some lamp-black and size by means of a brush to the parts beyond, leaving a bright space of an inch or so on each side of the joint, a ladle of melted solder is then gradually poured on the joint and rubbed down with a piece of greased cloth, the lamp-black and size preventing the solder adhering to any part but that left bright, and in this way a straight neat joint is produced. THE SMELTING OF METALS. All metals are got from the earth where they exist in the form of “ores” (in reality metals combined with other matters), and “smelting” is the process of getting rid of these other matters, the chief of which are sulphur and oxygen. The ores when dug from the mine are generally stamped into powder, then “roasted,” that is, made hot and kept so for some time to drive off water, sulphur, or arsenic, which would prevent the “fluxes” acting properly. The fluxes are substances which will mix with, melt, and separate the matters to be got rid of, the chief being charcoal, coke, and limestone. The ore is then mixed with the flux and the whole raised to a great heat; as the metal is separated it melts, runs to the bottom of the “smelting furnace” and is drawn off into moulds made of sand; it is thus cast into short thick bars called “pigs,” so we hear of pig-iron, pig-lead, &c. Iron is smelted from “ironstone,” which is mixed with coke and limestone. The heat required to smelt iron is so very great, that a steam-engine is now always employed to blow the furnace (before the invention of the steam-engine, water-mills were used for the same purpose). The smelting is conducted in what is called a blast furnace. When the metal has all been “reduced” or smelted, and run down to the bottom of the furnace, a hole is made, out of which it runs into the moulds; this is called “tapping the furnace.” Smelting is often confounded with melting, as the names are somewhat alike, but the processes are entirely different; in melting, the metal is simply liquified, in smelting the metal has to be produced from ores which often have no appearance of containing any, as in the case of iron-stone, which looks like brown clay. By way of experiment let the reader take a small portion of “litharge,” which is a reddish powder, mix it with a drop of oil into a thick paste and place it on the end of a flat piece of charcoal or wood, and direct the flame of a candle upon it by means of a blow-pipe; a slight hissing noise will be heard, and in a moment or two a small, bright globule of lead will make its appearance. FOUNDING. Illustration: GENERAL FOUNDRY, WOOLWICH ARSENAL.