the crossing of two threads, one of which lies over, and the other under, the cloth to be sewn.  This crossing point, to insure integrity of the stitch, must occur as nearly as possible in the middle of the thickness of the fabric.  The crossing must also be effected while a certain strain, called tension, is imposed upon both threads.  If the tension of one thread should outweigh that of the other, the locking point becomes displaced.  If the tension be insignificant, the stitches will be loose.  If the tension should vary, as in the long shuttle, there will occur faulty points in the seam.

In the earlier rotating hook the tension depended upon the friction developed between the spool and the hook.  This tension, therefore, varied in proportion to the speed of the latter, and could never be constant.  This was quite apart from the frictional resistance offered to the upper thread in passing over the cavity of the hook.

In the shuttle the tension was obtained by threading through holes in the shell, or beneath a tension plate, as in Howe’s machine.  This tension, so long as the reel ran between spring centers, was never constant.  The variation was chiefly due to the angular strain set up when unwinding from the reel.  This strain varied according to the point of unwinding.  It was light in the middle of the reel and heavy at either extremity.  These drawbacks caused immense anxiety to the first makers of sewing machines, and numerous attempts to overcome them led to little improvement.  With reference to high rates of speed, the older shuttle, requiring a long and noisy reciprocation, had its disadvantages.

The only effective remedy for these drawbacks was a radical one.  It was necessary to substitute depth of reel for length.  Hence, several attempts have been made to construct disk or ring shuttles.  Many forms of those have been tried.  They all depend upon the principle of coiling up the thread in a vertical plane, rather than in horizontal spirals.  Some makers placed the disk in a horizontal plane, and caused it to revolve.  Nothing could be worse, as will be seen, if we follow the course the enveloping loop must take in encircling such a shuttle.  But a complete solution of the difficulty of employing a ring shuttle has been achieved in the oscillating form, invented by Mr. Phil.  Diehl, and known as Singer’s (Fig. 1).  A short examination of it may profitably engage your attention.  The shuttle itself is sufficiently well known, but certain features of it, and to which it owes its efficiency, appear to call for some explanation.  Its introduction dates back some years, during which time it has undergone certain modifications.

[Illustration:  Fig. 1.]