18

MANUFACTURES.

Manu- the line of separation between convexity and concavity, and is most difficult to hit. A Sect. i.similar difficulty takes place in figuring specula for telescopes ; the parabola is the sur-face which separates the hyperbolic from the elliptic figure, and is the most difficult toform. If a spindle not cylindrical at its end is pressed into a hole not circular, and ifthe spindle be kept constantly turning, there is a tendency in these two bodies sosituated to become conical, or to have circular sections. If a triangular pointed pieceof iron be worked round in a circular hole the edges will gradually wear, and it willbecome conical. These facts, if they do not explain, at least illustrate the principleson which the excellence of work formed in the lathe depends

The operation of printing the Numbers on Bank-notes is one which requires perfectaccuracy; and although each Number differs from all the others, it is accomplished by afew wheels.

OF COPYING.

The two last sources of excellence in the work produced by Machinery depend on aprinciple which pervades a very large portion of all Manufactures, and is one uponwhich the cheapness of the articles produced seems greatly to depend. The principleto which we allude is the system of Copying, taken in its most extensive sense.Almost unlimited pains are, in some instances, bestowed on the original, from whicha series of Copies is to be produced ; and the larger the number of these Copies, themore care and pains can the manufacturer afford to lavish upon the original. It maythus happen, that the instrument, or tool actually producing the work, shall cost five oreven ten thousand times the price of each individual specimen of its power.

As the system of Copying is of so much importance, and of such extensive use in theArts, we shall endeavour to classify a considerable number of those processes in whichit is employed; not, however, pretending to give a complete list, and restricting our-selves to the shortest possible detail which will be consistent with a due regard tomaking the subject intelligible.

Operations of Copying take place under the following circumstances :—by Printing, —by Casting,—by Moulding,—by Stamping,—by Punching,—with elongation,—withaltered dimensions.

OF COPYING BY PRINTING.

Of Copying by Printing. The Art of Printing, in all its numerous departments, isessentially an Art of Copying. Under its two great divisions, Printing from hollowlines, as in copper-plate, and Printing from surface, are comprised numerous Arts.Copper- Copper-plate Printing .—la this instance the Copies are made by transferring by pres-ing. e Pnnt " sure a thick ink from the hollows and lines cut in the copper to sheets of paper. Anartist will sometimes exhaust the labour of one or two years upon engraving a Plate,which will not, in some cases, furnish above five hundred Copies in a state of per-fection.