This inner angle, as located by Mr. Grossmann, is at the intersection of the short arc i with the line g n, which limits the ten-degree angular motion of the pallets. If we carefully study the drawing, we will see the pallet has only to move through eight degrees of angular motion of the pallet staff for the tooth to escape, because the tooth certainly must be disengaged when the inner angle of the pallet reaches the peripheral line a. The true way to locate the position of the inner angle of the pallet, is to measure down on the arc i ten degrees from its intersection with the peripheral line a and locate a point to which a line is drawn from the intersection of the line g m with the radial line a c, thus defining the inner angle of the entrance pallet. We will name this point the point x.
It may not be amiss to say the arc i is swept from the center g through the point u, said point being located ten degrees from the intersection of the radial a c with the peripheral line a. It will be noticed that the inner angle of the entrance pallet A seems to extend inward, beyond the radial line a j, that is, toward the pallet center g, and gives the appearance of being much thicker than the exit pallet A'; but we will see on examination that the extreme angle x of the entrance pallet must move on the arc i and, consequently, cross the peripheral line a at the point u. If we measure the impulse faces of the two pallets A A', we will find them nearly alike in linear extent.
Mr. Grossmann, in delineating his exit pallet, brings the extreme angle (shown at 4) down to the periphery of the escape, as shown in the drawing, where it extends beyond the intersection of the line g f with the radial line a 3. The correct form for the entrance pallet should be to the dotted line z x y.
We have spoken of engaging and disengaging frictions; we do not know how we can better explain this term than by illustrating the idea with a grindstone. Suppose two men are grinding on the same stone; each has, say, a cold chisel to grind, as shown at Fig. 17, where G represents the grindstone and N N' the cold chisels. The grindstone is supposed to be revolving in the direction of the arrow. The chisels N and N' are both being ground, but the chisel N' is being cut much the more rapidly, as each particle of grit of the stone as it catches on the steel causes the chisel to hug the stone and bite in deeper and deeper; while the chisel shown at N is thrust away by the action of the grit. Now, friction of any kind is only a sort of grinding operation, and the same principles hold good.
THE NECESSITY FOR GOOD INSTRUMENTS.
It is to be hoped the reader who intends to profit by this treatise has fitted up such a pair of dividers as those we have described, because it is only with accurate instruments he can hope to produce drawings on which any reliance can be placed. The drawing of a ratchet-tooth lever escapement of eight and one-half degrees pallet action will now be resumed. In the drawing at Fig. 18 is shown a complete delineation of such an escapement with eight and one-half degrees of pallet action and equidistant locking faces. It is, of course, understood the escape wheel is to be drawn ten inches in diameter, and that the degree arcs shown in Fig. 1 will be used.
We commence by carefully placing on the drawing-board a sheet of paper about fifteen inches square, and then vertically through the center draw the line a' a''. At some convenient position on this line is established the point a, which represents the center of the escape wheel. In this drawing it is not important that the entire escape wheel be shown, inasmuch as we have really to do with but a little over sixty degrees of the periphery of the escape wheel. With the dividers carefully set at five inches, from a, as a center, we sweep the arc n n, and from the intersection of the perpendicular line a' a'' with the arc n we lay off on each side thirty degrees from the brass degree arc, and through the points thus established are drawn the radial lines a b' and a d'.
