[Illustration:  FIG. 4.]

Wilson, in the Wheeler-Wilson machine, had neither of those arrangements, but depended upon the succeeding revolution of the hook to draw up the slack of the preceding stitch.  These devices were all far from perfect in their operation, chiefly because they commenced to act too soon.  In each case the pulling up commenced with the rise of the needle, and the tightening operation subjected the thread to all the friction of rubbing its way through both needle eye and fabric.  Now, an ideal take-up should not commence to 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.  A crank pin is attached to each, and these pins are connected together by a simple link.  An examination of the device itself shows that, while the motion of the main shaft portion is uniform, that of the hook shaft is alternately accelerated and retarded.