Weights.—­The provision of weights which can be slipped up and down a rod may present some difficulty.  Of iron and lead, lead is the more convenient material, as occupying less space, weight for weight, and being more easily cast or shaped.  I have found thin sheet roofing lead, running 2 lbs. to the square foot, very suitable for making weights, by rolling a carefully squared strip of the material round the rod on which it will have to move, or round a piece of brass tubing which fits the rod.  When the weight has been rolled, drill four holes in it, on opposite sides near the ends, to take nails, shortened so that they just penetrate all the laps but do not enter the central circular space.  These will prevent the laps sliding over one another endways.  A few turns of wire round the weight over the heads makes everything snug.

Just one caution here.  The outside lap of lead should finish at the point on the circumference where the first lap began, for the weight to be approximately symmetrical about the centre.

An alternative method is to melt up scrap lead and cast weights in tins or flowerpots sunk in sand, using an accurately centred stick as the core.  This stick should be very slightly larger than the pendulum rod, to allow for the charring away of the outside by the molten metal. (Caution.—­The mould must be quite dry.)

Failing lead, tin canisters filled with metal scrap may be made to serve.  It will in this case be necessary to bore the lid and bottom centrally and solder in a tube fitting the rod, and to make an opening through which the weighting material can be inserted.

Adjustment of Weights.—­As lead is too soft a metal to give a satisfactory purchase to a screw—­a thread cut in it soon wears out—­it is better to support a leaden weight from underneath by means of a brass collar and screw.  A collar is easily made out of a bit of tubing thickened at the point where the screw will pass by soldering on a suitably shaped piece of metal.  Drill through the reinforcement and tubing and tap to suit the screw used, which may well be a camera tail screw, with 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.