Of making the Moulds

—1. Each mould ought to present the exact form of its object. 2. It should have such solidity that the melted metal may be poured into it, and fill it entirely without altering its shape in any point. 3. The air which occupies the vacant spaces in it, as well as the carburetted gases generated by the heat, must have a ready vent; for if they are but partially confined, they expand by the heat, and may crack, even blow up the moulds, or at any rate become dispersed through the metal, making it vesicular and unsound. There are three distinct methods of making the moulds :— 1. In green sand; 2. In baked sand; 3. In loam. To enumerate the different means employed to make every sort of mould exceeds the limits prescribed to this work. I shall merely indicate for each species of moulding, what is common to all the operations; and I shall then describe the fabrication of a few such moulds as appear most proper to give general views of this peculiar art.