act until the needle has ascended above the fabric, and one
of the most important steps toward perfection in sewing machines was
undoubtedly attained when such a device was actually invented. In effecting
this, the means employed consists of a differential or variable cam,
rotating with the main shaft. This controls the movements of a lever called
the take-up, pivoted to the machine (Fig. 4). Not only has it been
possible by these means to control the tightening of the stitch, but the
paying out of the thread for enveloping the shuttle also, and both the
paying out and pulling up are actually effected after the needle has
ascended above the cloth. The introduction of the positive take-up, the
first forms of which appeared in 1872, not only simplifies the movements of
the shuttle or hook, but for the first time renders the making of the lock
stitch possible, while the needle has a direct up and down motion. Thus, we
find that in most of the swiftest sewing machines, the needle bar is
actuated by a simple crank pin or eccentric, there being no loop dip or
pause in its motion.

The diagram shows a positive take-up in three positions--at the
commencement of the needle's descent, during the detention of the loop by
the beak, and during the casting off of the loop. The dotted lines indicate
the path of the cam to produce these positions. The intermittent movements
of the take-up have thus led to the abandonment of variable motions in both
needle and shuttle, and particularly so in oscillating shuttle machines.

_Wheeler & Wilson's Variable Motion_.--But while the simple and direct
movement is now preferred for shuttles, both oscillating and rotary, the
revolving hooks of Wheeler & Wilson are provided with a differential
motion, and the way it is effected appears sufficiently interesting to call
for a short description. When the rotating hook has seized the loop of
thread, it makes half a revolution with great rapidity; its speed then
slackens, and becomes very slow for the remaining half a revolution. In the
first machines introduced, this was effected by means of a revolving disk,
having slots in which worked pins attached to the main shaft and hook shaft
respectively.

[Illustration: FIG. 5.]

In the later and more improved machines, the variable device is much
simplified (Fig. 5). The main shaft, leading to the rotating hook, is
separated into two portions, the axis of one portion being placed above
that of the other.