REVIEWS. 
289 
MEASUKING MACHINES.* 
I T is hardly too much to say that the power of producing true surfaces lies 
at the bottom of ail machine construction. When W att first constructed 
his steam-engine one of his great difficulties consisted in making the piston 
fit true to the cylinder. Such extreme precision has, however, since been 
obtained that we are now able to detect the difference of fit in two pistons 
though they may not differ by more than the ten-thousandth of an inch 
in diameter. This great advance in the construction of machinery of preci- 
sion is due mainly to the exertions and ingenuity of Sir Joseph Whitworth, 
who has spared no pains to effect improvements in this direction. His steel 
u surface-plates ” are indeed the nearest approach to absolutely true planes 
which human ingenuity has yet contrived. So true, in fact, are these surfaces 
that if apiece of gold-leaf be rubbed between them it entirely disappears, its 
molecules being forced into the pores of the steel. Sir Joseph’s standard 
gauges are likewise marvels of mechanical skill. Professors Goodeve and 
Shelley have therefore rendered no small service to students of mechanics 
by putting before them clear descriptions of these instruments of precision, 
but above all by describing the beautiful measuring machine with which 
the name of Whitworth will always be associated. 
There are two ways in which minute linear magnitudes may be gauged ; 
one being the old method of measurement by means of the micrometer 
and microscope ; the other being the method of “end measurement,” which 
relies upon truth of surface and delicacy of touch. The latter is the prin- 
ciple on which the Whitworth machine is based. At first sight it may seem 
strange that minute differences of magnitude should be more readily detected 
by the sense of feeling than by the eye aided by the microscope. Yet Sir 
J. Whitworth has shown beyond doubt that this is the case. Workmen in 
securing a good mechanical fit usually depend on touch, and the u feeling 
piece ” attached to the Whitworth machine can be manipulated with 
extreme delicacy. 
It must be remembered that Sir Joseph’s measuring machine is not 
intended so much for measuring the actual length of a bar as for determining 
very minute differences in the lengths of specially prepared bars : hence it 
is peculiarly adapted for multiplying copies of standards of length. For 
use in the workshop a Whitworth machine is constructed to indicate a differ- 
ence of one ten- thousandth of an inch ; but the great triumph is the far more 
delicate instrument by which a difference of a millionth of an inch may be 
detected. To understand the construction of this beautiful instrument the 
reader must refer to the series of plates attached to the work of Professors 
Goodeve and Shelley. 
* The Whitworth Measuring Machine, including Descriptions of the 
Surface Plates, Gauges, and other Measuring Instruments made by Sir 
Joseph Whitworth, Bart., &c. By T. M. Goodeve, M.A., &c., and C. P. B. 
Shelley, C.E., &c. With four plates and 44 woodcuts. 8vo. London: 
Longmans. 1877. 
