Telescope bell machine (or 'zone clock') made by Alexander Herschel and used by William Herschel with his 40-inch telescope.

Bell machines were designed to solve a problem William Herschel encountered as he used his 20-foot telescope for ‘sweeping’ for nebulae, directing the instrument to the south and examining the heavens as they drifted past. The method of sweeping for nebulae that William adopted in December 1783 required the workman to raise the tube to the required altitude, and then to raise and lower it in a succession of up-and-down motions that traversed the ‘zone’ under examination.

Experience showed that 2° or so was the most suitable angle between the upper and lower limits of the zone, and it was important for the workman to know when he had reached the limit in one direction and should reverse the sense in which he was turning his handle. The length of a cord leading from a narrow part of the same winding barrel to some graduated scale would of course reflect the number of turns the workman had made. However, it was sometimes necessary to have an arc larger or smaller than 2º, and an additional complication was that any given arc represented slightly different numbers of turns depending on the altitude at which the telescope was directed. A bell machine was therefore designed to ring bells at the upper (low bell) and lower (high bell) limits of the zone, alerting the workman to the need to reverse direction, the mechanism being adjustable for the number of turns currently required to effect the desired angular separation.

This bell machine was the one fitted to William’s 40-foot reflector as it is identical with that shown in his published description of the telescope. There seems little doubt that William's brother Alexander was responsible for the concept, if not for the actual making of this machine.

The basic construction is relatively crude, though somewhat better made than the other (possibly earlier) mechanism, and this suggests an evolution of thought in its layout. The frame consists of a pair of solid iron plates held apart by nicely turned brass pillars. As with the other bell mechanism, it has no train of wheels at all, only one large central ‘drum’ wheel within the plates, with the rope wrapped around its outer periphery and three fixed ‘primary’ pins operating one of the two bell hammers. The use of three pins provides two extra, ‘reminder’ blows on the bell.

Unlike the other bell mechanism, the arbor of the drum wheel projects through the plates of the mechanism and a pin wheel is arranged on the front of the mechanism. Then, for the adjustable part, a single spring-blade on the wheel, like those on the other mechanism, with three pins at its end performs the same function as those on the other mechanism, the end of the spring-blade being pulled out when a new timing was required for the up/down bells, a peg on the end of the blade being inserted into one of the holes further round the wheel, as necessary. Instead of the bells being stacked one above the other, as in the earlier machine, this mechanism has them on separate standards, side by side (only one of the two bells now survives).