the ends.

Fig. 89. DOUBLE HELICO-VOLUTE.--This form, so far as the outlines are
considered, is the opposite of Fig. 88. A compression spring of this
kind has a very wide range of movement.

Fig. 90. HELICAL SPRING.--This form of coil, uniform from end to end, is
usually made of metal which is square in cross-section, and used where
it is required for heavy purposes.

Fig. 91. SINGLE VOLUTE HELIX-SPRING.--This is also used for compression,
intended where tremendous weights or resistances are to be overcome, and
when the range of movement is small.

Fig. 92. FLAT SPIRAL, OR CONVOLUTE.--This is for small machines. It is
the familiar form used in watches owing to its delicate structure, and
it is admirably adapted to yield to the rocking motion of an arbor.

Fig. 93. ECCENTRIC ROD AND STRAP.--A simple and convenient form of
structure, intended to furnish a reciprocating motion where a crank is
not available. An illustration of its use is shown on certain types of
steam engine to operate the valves.

Fig. 94. ANTI-DEAD CENTER FOR FOOT-LATHES.--A flat, spiral spring (A),
with its coiled end attached to firm support (B), has its other end
pivotally attached to the crank-pin (C), the tension of the spring being
such that when the lathe stops the crack-pin will always be at one side
of the dead-center, thus enabling the operator to start the machine by
merely pressing the foot downwardly on the treadle (D).

CHAPTER VII

PROPERTIES OF MATERIALS

A workman is able to select the right metals because he knows that each
has some peculiar property which is best adapted for his particular use.
These with their meaning will now be explained.

ELASTICITY.--This exists in metals in three distinct ways: First, in the
form of _traction_. Hang a weight on a wire and it will stretch a
certain amount. When the weight is removed the wire shrinks back to its
original length.

Second: If the weight on the wire is rotated, so as to twist it, and the
hand is taken from the weight, it will untwist itself, and go back to
its original position. This is called _torsion_.

Third: A piece of metal may be coiled up like a watch spring, or bent
like a carriage spring, and it will yield when pressure is applied. This
is called _flexure_.

Certain kinds of steel have these qualities in a high degree.

TENACITY.--This is a term used to express the resistance which the body
opposes to the separation of its parts. It is determined by