The Postal Dial Scale: The Weight and the Postage on One Dial

"Within the tube a, which may be mounted upon legs, or suspended from the ceiling, or attached to the wall, I confine the spring b, the upper end of which I attach to the cap c of the tube a, and the lower end of said spring I fasten to the rod d. To the lower end of the rod d, I suspend the platform e, while to the upper end of said rod is attached the arm f, holding the adjustable indicator o." "Near the periphery of the dial i are two circles, between which, in spaces formed by the radial lines and intersecting circles, is marked the rate of postage on pamphlets and newspapers."

**Tube and spring.** Cut 30 cm of PVC pipe. Screw a screw eye into a plywood disc glued over its top end and hang an extension spring from it inside the tube. Hook 30 cm of threaded rod to the spring's lower end so it runs down the tube's middle, and hang a plywood platform from its bottom with nuts. **Pointer.** Cut a slot 12 cm long down one side of the tube. Bolt an aluminium strip to the rod through the slot as the arm, and a second strip to the arm with an M5 bolt as the pointer, so it can be reset. **Dial.** Cut a card disc 15 cm across and pin it through its centre to a plywood backboard beside the tube on an M5 bolt with a nut, so it turns. **Mark.** Weigh objects on the kitchen scale, hang each on the platform and mark where the pointer sits; draw circles through the marks. Compare the spacing with the notebook. Look up the letter rates for three places you write to and write them on radial lines on the dial. Weigh five letters and read their postage.

**Marcus L. M. Hussey** of New York received US 73,976, dated 4 February 1868. His dial, of thick paper, metal or other material, turned on a centre pin and was held by a tightening nut; his pointer was a fork straddling the dial, set by a screw, and for ordinary weighing he suggested a segment of the dial marked in troy, apothecaries' and avoirdupois weight. **Honest limits.** A PVC tube, a bought extension spring and a card dial stand in for his instrument. No historical postage rates are given here; the dial carries the rates the maker looks up. The notebook assumes the spring obeys Hooke's law; its stiffness is an example. Your marks are the result.