h a large flat
head.

I experienced some trouble from the crushing of wooden rods by a screw, but
got over it as follows. The tubing selected for the collar was large enough
to allow a piece of slightly smaller tubing to be introduced between it and
the rod. This inner piece was slit from one end almost to the other, on
opposite sides, and soldered at one end to the outer tube, a line joining
the slots being at right angles to the axis of the screw. The pressure of
the screw point was thus distributed over a sufficient area of the wood to
prevent indentation. (See Fig. 173.)

[Illustration: FIG. 173.]

[Illustration: FIG. 174.--Pivot for pen lever.]

Pen Levers.--The pen lever, of whatever kind it be, must work on its
pivots with very little friction, and be capable of fine adjustment as
regards balance. For the Rectilinear Harmonograph the form of lever pivot
shown in Fig. 174 is very suitable. The spindle is a wire nail or piece of
knitting needle sharpened at both ends; the bearings, two screws filed flat
at the ends and notched with a drill.

The brass standard should be drilled and tapped to fit the screws fairly
tight, so that when once adjusted they may not slacken off. If the lever is
made of wood, the tail may be provided with a number of metal pegs on which
to place the weights; if of wire, the tail should be threaded so that a
brass weight and lock screw may be moved along it to any desired position.
It is very important that the pressure of the pen on the card should be
reduced to a minimum by proper balancing, as the friction generated by a
"heavy" pen slows the pendulum very quickly; and that the centre of gravity
should be below the point of suspension, to put the pen in stable
equilibrium. The lever shown in Fig. 169 is suitable for the Twin Elliptic
Pendulum.

In this case the lever is not moved about as a whole. Mr. C. E. Benham
advocates the use of wood covered with velvet to rest the lever points on.

For keeping the pen, when not in use, off the platform, a small weight
attached to the lever by a thread is convenient. When the pen is working,
the weight is raised to slacken the thread.

[Illustration: FIG. 175.--End of pen lever.]

Attaching Pen to Lever.--In the case of wooden levers, it is sufficient
to slit the end centrally for a few inches after drilling a hole rather
smaller than the pen, at a point which lies over the centre of the card
platform, and quite squarely to the lever in a