6

ENGINEER AND MACHINIST’S DRAWING-BOOK.

vertical position at any extension ; but this is a doubtful
advantage, especially in compasses designed for drawing
arcs and circles, since each leg being equally removed from
the centre of motion, there must be a tendency on the
part of the one fixed, to tear away from its position.
Mr. Brunei has introduced what are called Tubular Com-
passes, in which the upper part of the legs lengthens out
like the slide of a telescope, thus giving greater extent of
radius when required. The movable legs are double,
having points at one end, and a pencil or pen at the other ;
and they move on pivots, so that the pen or pencil can be
instantly substituted for the points, or vice versa, and that
with the certainty of a perfect adjustment. The design
is very ingenious, and offers many conveniences, but the
instrument is too delicate for ordinary hands. Without
extreme care, it must soon be disarranged and rendered
useless. The Portable or Turn-in Compasses, is a con-
trivance which combines dividers, compasses with movable
legs, and bows, in a pocket instrument, folding up to a
length of not more than three inches. The upper legs are
hollow, and admit either leg of the pen and pencil bows,
which can therefore be substituted for each other. When
closed for the pocket, one leg of each bow slides into the
upper legs, and the other is turned inward towards the
head.

As a concluding remark, we recommend the draughts-
man to choose compasses in which the joint is formed by
a box-screw, that can be tightened or relaxed at pleasure.
The cheaper kinds have merely a common screw, and
these are usually too stiff when first purchased, and in-
conveniently loose after being some time in wear. A slight
turn of the box-screw, by means of the key, keeps the com-
passes in good working order, neither so stiff as to spring,
nor so loose as to render them uncertain and unsteady
in use.

Plain and Double Scales.

Simply-divided Scales.—Scales are measures and sub-
divisions of measures laid down with such accuracy, that
any drawing constructed by them, shall be in exact pro-
portion in all its details. The plain scale is a series of
measures laid down on the face of one small flat ruler, and
is thus distinguished from the sector, or double scales, in
which two similarly divided rulers move on a joint, and
open to a greater or less angle. In the construction of
scales, the subdivison must be carried to as low a denomi-
nation as is likely to be required. Thus, for a drawing of
limited extent, the primary divisions may be feet, and the
subdivisions inches ; but for one of large area, and without
small details, the primaries may be 10 feet, and the sub-

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divisions tenths, or one foot each (Figs. 9 and 10). And
in the case of large surveys, the primaries become miles,
and the lesser divisions furlongs. Indeed the natural size
or extent of the object or area, and the surface to be

occupied by the delineation, must determine the graduation
of the scale. But passing from these general remarks, we
proceed to the plain scales contained in the Drawing Case,
and laid down on the tw.o sides of a flat ivory ruler, six
inches in length.

On one side of the Plain Scale there is usually a series
of simple scales, in which the inch is variously divided,
and the primaries subdivided into tenths and twelfths.
These may be applied to measurements as inches and
tenths, or twelfths; or as feet and tenths, or inches,
according to the nature of the drawing. It may be re-
marked, however, that these small lines of measures are
of only limited use, and that the engineering draughts-
man must usually lay down a scale with special reference
to the work before him; and in all cases it is desirable to
have the scale of construction on the margin of the draw-
ing itself, since the paper contracts or expands with every
atmospheric change, and the measurements will therefore
not agree at all times with a detached scale; and, moreover,
a drawing laid down from such detached scale, of wood or
ivory, will not be uniform throughout, for on a damp day the
measurements will be too short, and on a dry day too long.
Mr. Holtzapffel has sought to remedy this inconvenience,
by the introduction of paper scales; but all kinds of paper
do not contract and expand equally, and the error is
therefore only partially corrected by his ingenious substi-
tution of one material for another.

Diagonal Sccde.—The lines to which we have referred,
give only two denominations, primaries and tenths, or
twelfths; but more minute subdivision is frequently re-
quired ; and this is attained by the diagonal scale, which
consists of a number of primary divisions, one of which is
divided into tenths, and subdivided into hundredths by dia-
gonal lines (Fig. 11). This scale is constructed in the foliow-
iug manner:—Eleven parallel lines are ruled, inclosing ten

(Fig 11.)

A. S S 4 2 B C

equal spaces; the length is set off into ten equal primary
divisions, as A B, B C, C 2, &c.; and diagonals are then
drawn from the subdivisions between A and B, to those
between D and E, as shown in the diagram. Hence it is
evident that at every parallel we get an additional tenth
of the subdivisions, or a hundredth of the primaries, and
can therefore obtain a measurement with great exactness
to three places of figures. To take a measurement of (say)
168, we place one foot of the dividers on the primary 1,
and carry it down to the ninth parallel, and then extend
the other foot to the intersection of the diagonal, which
falls from the subdivision 6, with the parallel that measures
the eight-hundredth part (Fig. 12). More examples or
further explanation would only be tedious. The primaries
may of course be considered as yards, feet, or inches; and
the subdivisions as tenths and hundredths of these respec-
tive denominations The diagonal scale is very useful and