hown must be
adopted. In the diagram the bottom of the slider runs on to a brass
spring between the girder and the base of the appliance, and so gets
jammed; the spiral spring acts merely as an additional guard. The
diagram does not show the lower spring very clearly; it is a mere
strip lying in the groove.

A rod of quartz, with a needle at one end, is prepared as before and
secured in the clamps. During the operation of fastening down the
clamps, there is some danger of breaking the needle, and consequently
it is advisable to soften the latter before and while adjusting the
second clamp.

The process of drawing a thread by this method is exactly similar to
the operation already described in connection with the arrow method.
Though short thick threads form the product generally obtained from
the catapult, it must not be supposed that thin threads cannot be
obtained in this way. If a short length of a very fine needle be
heated, it will be found to yield threads quite fine enough for
ordinary suspension purposes, but naturally not so uniform as those
obtained from the 40-foot lengths obtainable by the bow-and-arrow
method.

It is easy to make spiral quartz springs resembling watch
balance-springs by means of the catapult. All that is necessary is to
see that the quartz is rather unequally heated before the shot is
fired. In the future it is by no means impossible that such springs
may have a real value, for the rigidity of quartz is known to increase
as temperature rises. Hence it is probable that the springs would
become stiffer as temperature rises, even though they work chiefly by
bending, and little or not at all by twisting. As this is the kind of
temperature variation required to compensate an uncompensated watch
balance wheel, it may turn out to have some value.

Sec. 87. Drawing Threads by the Flame alone.

A stick of quartz is drawn down to a fine point, and the tip of this
point is held in the blow-pipe flame in the position shown in Fig.
70.

Fig. 70.

The friction of the flame gases is found to be sufficient to carry
forward the fused quartz and to draw it into threads in spite of the
influence of the capillary forces. If a sheet of paper be suspended
at a distance of two or three feet in front of the blow-pipe flame, it
will be found to be covered with fine threads tangled together into a
cobwebby mass. As this method is an exceedingly simple one of
obtaining threads, I have endeavou