Having thus far treated of such general principles and facts connected with practical mechanics as might properly precede, and be of use in, the study of actual manipulation in a workshop, we come next to casting, forging, and finishing, with other details that involve manual as well as mental skill, and to which the term "processes" will apply.

As these shop processes or operations are more or less connected, and run one into the other, it will be necessary at the beginning to give a short summary of them, stating the general object of each, that may serve to render the detailed remarks more intelligible to the reader as he comes to them in their consecutive order.

Designing, or generating the plans of machinery, may be considered the leading element in engineering manufactures or machine construction, that one to which all others are subordinate, [75] both in order and importance, and is that branch to which engineering knowledge is especially directed. Designing should consist, first, in assuming certain results, and, secondly, in conceiving of mechanical agents to produce these results. It comprehends the geometry of movements, the disposition and arrangement of material, the endurance of wearing surfaces, adjustments, symmetry; in short, all the conditions of machine operation and machine construction. This subject will be again treated of at more length in another section.

Draughting, or drawing, as it is more commonly called, is a means by which mental conceptions are conveyed from one person to another; it is the language of mechanics, and takes the place of words, which are insufficient to convey mechanical ideas in an intelligible manner.

Drawings represent and explain the machinery to which they relate as the symbols in algebra represent quantities, and in a degree admit of the same modifications and experiments to which the machinery itself could be subjected if it were already constructed. Drawings are also an important aid in developing designs or conceptions. It is impossible to conceive of, and retain in the mind, all the parts of a complicated machine, and their relation to each other, without some aid to fix the various ideas as they arise, and keep them in sight for comparison; like compiling statistics, the footings must be kept at hand for reference, and to determine the relation that one thing may bear to another.

In the workshop, the objects of drawing are to communicate plans and dimensions to the workmen, and to enable a division of the labour, so that the several parts of a machine may be operated upon by different workmen at the same time—also to enable classification and estimates of cost to be made, and records kept.

Drawings are, in fact, the base of shop system, upon which depends not only the accuracy and uniformity of what is produced, but also, in a great degree, its cost. Complete drawings of whatever is made are now considered indispensable in the best regulated establishments; yet we are not so far removed from a time when most work was made without drawings, but what we may contrast the present system with that which existed but a few years ago, when to construct [76]a new machine was a great undertaking, involving generally many experiments and mistakes.

Pattern-making relates to the construction of duplicate models for the moulded parts of machinery, and involves a knowledge of shrinkage and cooling strains, the manner of moulding and proper position of pieces, when cast, to ensure soundness in particular parts. As a branch of machine manufacture, pattern-making requires a large amount of special knowledge, and a high degree of skill; for in no other department is there so much that must be left to the discretion and judgment of workmen.

Pattern-makers have to thoroughly understand drawings, in order to reproduce them on the trestle boards with allowance for shrinkage, and to determine the cores; they must also understand moulding, casting, fitting, and finishing. Pattern-making as a branch of machine manufacture, should rank next to designing and drafting.

Founding and casting relate to forming parts of m............