between the outer diameter of the cylinder and the measure of the half shell.  It will be seen on measurement that the distance between the center e and the line w is about one-fifteenth part of the outer diameter of the cylinder and consequently with a cylinder which measures 45/1000 of an inch in diameter, now the half shell should measure half of the entire diameter of the cylinder plus one-fifteenth part of such diameter, or 251/2 thousandths of an inch.

After these proportions are understood and the drawing made, the eye will get accustomed to judging pretty near what is required; but much the safer plan is to measure, where we have the proper tools for doing so.  Most workmen have an idea that the depth or distance at which the cylinder is set from the escape wheel is a matter of adjustment; while this is true to a certain extent, still there is really only one position for the center of the cylinder, and that is so that the center of the pivot hole coincides exactly with the center of the chord to the curve of the impulse face of the tooth or the point e, Fig. 130.  Any adjustment or moving back and forth of the chariot to change the depth could only be demanded where there was some fault existing in the cylinder or where it had been moved out of its proper place by some genius as an experiment in cylinder depths.  It will be evident on observing the drawing at Fig. 131 that when the cylinder is performing an arc of vibration, as soon as the entrance lip has passed the point indicated by the radial line e x the point of the escape-wheel tooth will commence to act on the cylinder lip and continue to do so through an arc of forty degrees, or from the lines x to l.

MAKING A WORKING MODEL.

To practically study the action of the cylinder escapement it is well to make a working model.  It is not necessary that such a model should contain an entire escape wheel; all that is really required is two teeth cut out of brass of the proper forms and proportions and attached to the end of an arm 4-7/8” long with studs riveted to the U arms to support the teeth.  This U arm is attached to the long arm we have just mentioned.  A flat ring of heavy sheet brass is shaped to represent a short transverse section of a cylinder.  This segment is mounted on a yoke which turns on pivots.  In making such a model we can employ all the proportions and exact forms of the larger drawings made on a ten-inch radius.  Such a model becomes of great service in learning the importance of properly shaping the lips of the cylinder.  And right here we beg to call attention to the fact that in the ordinary repair shop the proper shape of cylinder lips is entirely neglected.

PROPER SHAPE OF CYLINDER LIPS.