According to one American authority, the preparation is of great value. The priming coat should be thin and well rubbed in, and it is better to use a darker colour than white lead as a base. White lead forms a dense covering to the surface, though it has its disadvantages. When petroleum has formed the first coat, two other coats will suffice, one being the priming coat, and a third coat may be given after the work has stood for a season. It is a very desirable plan to leave the painting, or rather finishing coats, for a time, so that any imperfections in the wood or work may be discovered; it also allows time for any change of colour that may be made. After the priming coat, it is usual in good work to stop all cracks, nail-holes, and other defects with putty; but in the commoner class of paintings, the coats are laid on quickly, the preceding coat has hardly time to dry before the next is put on, and all the defects of wood, bad seasoning, exudation of gum, &c., quickly begin to show themselves through and disfigure the work. A good paint ought to possess body, power of covering, and flow evenly from the brush, and become hard. Though zinc white has less body than white lead, it is more durable, and will stand sulphur acids without blackening. Some colours stand better than others; the ochres, Indian and Venetian reds, burnt and raw umber, are reliable, and may be used without scruple. It is also worthy of notice that salt air acts injuriously on white lead, and zinc white is therefore preferable in situations exposed to the sea-air. (_English Mechanic._) IRON PAINTING.--The decay of iron becomes very marked in certain situations, and weakens the metal in direct proportion to the depth to which it has penetrated; and although where the metal is in quantity this is not very appreciable, it really becomes so when the metal is under ¾ inch in thickness. The natural surface of cast iron is very much harder than the interior, occasioned no doubt by its becoming chilled, or by its containing a large quantity of silica, and this affords an excellent protection. But should this surface be at all broken, rust immediately attacks the metal and soon destroys it. It is very desirable that the casting be protected, and a priming coat of oil or paint should be applied for this purpose; the other coats, though requisite, can be given at leisure.