cotton spinning will understand
what is meant when it is said that the reeling is effectively a
"doubling" operation, but performed with a variable number of ends, so
as to compensate for the taper of the filaments. In reeling by hand,
as has been said, the size of the silk is judged, as nearly as
possible, by a complex mental operation, taking into account the
number, size, and state of unwinding of the cocoons. It is impossible
to do this mechanically, if for no other reason than this, that the
cocoons must be left free to float and roll about in the water in
order to give off their ends without breaking, and any mechanical
device which touched them would defeat the object of the machine. The
only way in which the thread can be mechanically regulated in silk
reeling is by some kind of actual measurement performed after the
thread has left the cocoons. The conditions are such that no direct
measurement of size can be made, even with very delicate and expensive
apparatus; but Mr. Serrell discovered that, owing to the great
tenacity of the thread in proportion to its size, its almost absolute
elastic uniformity, and from the fact that it could be stretched, two
or three per cent. without injury, it was possible to measure its size
indirectly, but as accurately as could be desired. As this fact is the
starting point of an entirely new and important class of machinery, we
may explain with considerable detail the method in which this
measurement is performed. Bearing in mind that the thread is of
uniform quality, it is evident that it will require more force to
stretch a coarse thread by a given percentage of its length than it
will to stretch one that is finer. Supposing the thread is uniform in
quality but varying in size, the force required to stretch it varies
directly with the size or sectional area of the thread itself. In the
automatic reeling machine this stretch is obtained by causing the
thread to take a turn round a pulley of a given winding speed, and
then, after leaving this pulley, to take a turn around a second pulley
having a somewhat greater winding speed.

[Illustration: Fig. 1 THE MECHANICAL REELING OF SILK.]

By this means the thread which is passing from one pulley to the other
is stretched by an amount equal to the difference of the winding speed
of the two pulleys. In the diagram (Fig. 2) the thread passes, as
shown by the arrows, over the pulley, P, and then over the pulley, P