It consists of a thick disk bobbin of thread, h, fitting loosely in a case constructed in the form of a bivalve, a and d.  This case is furnished with a long beak, usually forming a continuation of the periphery.  The beak is intended to enter and detain the loops of upper thread, and lead them so that they ultimately envelop the shuttle, a motion of the thread which is chiefly due to the oscillation of the shuttle in a vertical plane.  The oscillating movement is to the extent of 180 degs. of the circle, which suffices to cast the loops freely over the shuttle.  The center of oscillation is not coincident with the center of the shuttle; but it is nearly so with the periphery of the thread reel, and exactly coincides with the point where the under thread is drawn from the shuttle, g.  The shuttle thread is thus entirely freed from any tendency to twist, an objection frequently urged against circular or revolving shuttles.  It will be observed, also, that the body of the shuttle is extremely narrow.  Bulging of the thread loops to one side or the other is thus obviated.

But the long beak in this description of shuttle serves an important purpose other than that of seizing the upper thread loops, otherwise a very short beak would be preferable.  It adds so much to the efficiency of the machine that a little further explanation of it appears essential.  In the old fashioned machines the thread required to envelop the shuttle was dragged downward through the cloth, while the needle still remained in the fabric.  This necessitated the use of large needles with deep side channels, to enable the thread to run freely, and as a consequence the punctures that had to be made in the fabric were unnecessarily large, and could not in any case be entirely filled by the thread, a condition which is now recognized as essential in linen stitching and for waterproof boots.

The long beak in both shuttles and hooks offers an immediate solution of the old difficulty experienced with long shuttles.  When the needle begins to rise, the shuttle commences to oscillate, through the loop, the motions so coinciding that the long beak, c, merely detains the loop until the eye of the needle has ascended above the cloth; then, and then only, does the envelopment of the shuttle commence, and the thread required for it flows downward through the puncture.  The envelopment is completed before the needle has attained its highest point, and the consequent loose thread is immediately pulled up by a lever, called a positive take-up, before the needle begins to descend for a fresh stitch.  In this way little or no movement of the thread is required in the cloth while the puncture made is occupied by the needle.  The result is the capability of such apparatus to work with an incredibly fine needle—­indeed, so fine as to be no thicker than the incompressed thread itself.  This would have been considered quite impossible of accomplishment by our earlier machine makers.  The advantage thereby gained in stitching linen goods, and in sewing leather, where every puncture of the needle should be quite filled by the thread, is at once apparent.  Indeed, a rubber or leather sack, stitched in this way, will contain water without leakage—­a very extreme test.