(249.) The first object of machinery, and the chief cause of its extensive utility, is the cheap production of the articles to which it is applied. Wherever it is required to produce a great multitude of things, all of exactly the same kind, the proper time has arrived for the construction of tools or machines by which they may be manufactured. If only a few pairs of cotton stockings should be required in a country, or in circumstances in which it is impossible to purchase them, it would be an absurd waste of time, and of capital, to construct a stocking-frame to weave them, when, for a few pence, four steel wires can be procured by which they may be knit. If, on the other hand, many thousand pairs were wanted, the time employed, and the expense incurred in constructing a stocking-frame, would be more than repaid by the saving of time in making that large number of stockings. The same principle is applicable to the copying of letters: if only three or four copies are required, the pen and the human hand furnish the cheapest resource; but, if hundreds are called for, lithography may be brought to our assistance, and if hundreds of thousands are wanted, the machinery of a printing establishment is the most economical method of accomplishing the object.

(250.) There are, however, many cases in which machines or tools must be made, where economical production is not the most important object. Whenever it is required to produce a few articles,—parts of machinery, for instance, which must be executed with the most rigid accuracy or be perfectly alike,—it becomes nearly impossible to to fulfil this condition, even with the aid of the most skilful hands.