esented by simple
line shading; thus in Figure 247 the two bodies A and C would readily be
understood to be a bevil gear and pinion. Similarly small spur wheels
may be represented by simple circles in a side view and by line shading
in an edge view; thus it would answer every practical purpose if such
small wheels as in Figures 246 and 247 at D, F, G, K, P, H, I and J,
were drawn as shown. The pitch circles, however, are usually drawn in
red ink to distinguish them.

[Illustration: Fig. 251. (Page 209.)]

[Illustration: Fig. 246.]

[Illustration: Fig. 247.]

In Figure 248 is an example in which part of the gear is shown with
teeth in, and the remainder is illustrated by circles.

In Figure 250 is a drawing of part of the feed motions of a Niles Tool
Works horizontal boring mill, Figure 251 being an end view of the same,
_f_ is a friction disk, and _g_ a friction pinion, _g'_ is a rack, F is
a feed-screw, _p_ is a bevil pinion, and _q_ a bevil wheel; _i_, _m_,
_o_, are gear wheels, and _J_ a worm operating a worm-pinion and the
gears shown.

Figure 249 represents three bevil gears, the upper of which is line
shaded, forming an excellent example for the student to copy.

[Illustration: Fig. 248.]

The construction of oval gearing is shown in Figures 252, 253, 254, 255,
and 256. The pitch-circle is drawn by the construction for drawing an
ellipse that was given with reference to Figure 81, but as that
construction is by means of arcs of circles, and therefore not strictly
correct, Professor McCord, in an article on elliptical gearing, says,
concerning it and the construction of oval gearing generally, as
follows:

[Illustration: Fig. 249. (Page 210.)]

[Illustration: Fig. 250.]

[Illustration: Fig. 252.]

"But these circular arcs may be rectified and subdivided with great
facility and accuracy by a very simple process, which we take from Prof.
Rankine's "Machinery and Mill Work," and is illustrated in Figure 252.
Let O B be tangent at O to the arc O D, of which C is the centre. Draw
the chord D O, bisect it in E, and produce it to A, making O A=O E; with
centre A and radius A D describe an arc cutting the tangent in B; then O
B will be very nearly equal in length to the arc O D, which, however,
should not exceed about 60 degrees; if it be 60 degrees, the error is
theoretically about 1/900 of the length of the arc, O B being so much
too short; but this error varies with the fourth power of the angle
subtend