Of The Identity Of The Work When It Is Of The Same Kind And Its Accuracy When Of Different Kinds

79. Nothing is more remarkable, and yet less unexpected, than

the perfect identity of things manufactured by the same tool. If

the top of a circular box is to be made to fit over the lower

part, it may be done in the lathe by gradually advancing the tool

of the sliding-rest; the proper degree of tightness between the

box and its lid being found by trial. After this adjustment, if a

thousand boxes are made, no additiona
care is required; the tool

is always carried up to the stop, and each box will be equally

adapted to every lid. The same identity pervades all the arts of

printing; the impressions from the same block, or the same

copperplate, have a similarity which no labour could produce by

hand. The minutest traces are transferred to all the impressions,

and no omission can arise from the inattention or unskilfulness

of the operator. The steel punch, with which the cardwadding for

a fowling-piece is cut, if it once perform its office with

accuracy, constantly reproduces the same exact circle.

80. The accuracy with which machinery executes its work is,

perhaps, one of its most important advantages: it may, however,

be contended, that a considerable portion of this advantage may

be resolved into saving of time; for it generally happens, that

any improvement in tools increases the quantity of work done in a

given time. Without tools, that is, by the mere efforts of the

human hand, there are, undoubtedly, multitudes of things which it

would be impossible to make. Add to the human hand the rudest

cutting instrument, and its powers are enlarged: the fabrication

of many things then becomes easy, and that of others possible

with great labour. Add the saw to the knife or the hatchet, and

other works become possible, and a new course of difficult

operations is brought into view, whilst many of the former are

rendered easy. This observation is applicable even to the most

perfect tools or machines. It would be possible for a very

skilful workman, with files and polishing substances, to form a

cylinder out of a piece of steel; but the time which this would

require would be so considerable, and the number of failures

would probably be so great, that for all practical purposes such

a mode of producing a steel cylinder might be said to be

impossible. The same process by the aid of the lathe and the

sliding-rest is the everyday employment of hundreds of workmen.

81. Of all the operations of mechanical art, that of turning

is the most perfect. If two surfaces are worked against each

other, whatever may have been their figure at the commencement,

there exists a tendency in them both to become portions of

spheres. Either of them may become convex, and the other concave,

with various degrees of curvature. A plane surface is the line of

separation between convexity and concavity, and is most difficult

to hit; it is more easy to make a good circle than to produce a

straight line. A similar difficulty takes place in figuring

specula for telescopes; the parabola is the surface which

separates the hyperbolic from the elliptic figure, and is the

most difficult to form. If a spindle, not cylindrical at its end,

be pressed into a hole not circular, and kept constantly turning,

there is a tendency in these two bodies so situated to become

conical, or to have circular sections. If a triangular-pointed

piece of iron be worked round in a circular hole the edges will

gradually wear, and it will become conical. These facts, if

they do not explain, at least illustrate the principles on

which the excellence of work formed in the lathe depends.