This telescope bell machine (or 'zone clock') is traditionally said to have been used in John Herschel's 20-foot telescope during the 1820s and 1830s. This may well be so, as the very brief description of his bell-machine, as far as it goes, fits this device. However, it is possible this mechanism is in fact the remains of Alexander Herschel's machine for the 20-foot telescope of the 1780s, perhaps later adapted for use with John’s telescope.

The 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 two 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 2°, 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.

The basic design and function of this machine is the same as that described by Alexander in the 1780s, with a frame held apart by pillars and two hammers on two bells mounted on a single standard. By contemporary clockmaking standards, the mechanism is crudely made In the ratio of rope movement to pin motions, this mechanism has no reduction through a train of wheels at all, merely one large central ‘drum’ wheel (two iron discs held apart by a ring of little iron rods) within the frame, with the rope wrapped around the rods forming its outer periphery. It could be that Alexander realized (or was told by William?) that the complication of a wheel train could be avoided by simply reducing the motion of the rope using a smaller diameter drum feeding off the telescope, thus paying out less overall for altitude change.

There are two pins fixed at the back of the drum wheel, these being the fixed ‘primary’ pins. Two pins are fitted instead of one as it was evidently decided that a ‘reminder’ blow on the bell would be useful in case the first was ignored. Then, for the adjustable part, a single blade is mounted directly on this wheel with two pins at its end (this also has a permanently fixed ‘reminder’ pin). The end of the blade is clamp-able to the periphery using a threaded wing-nut once the correct separation is chosen.

It is possible that the machine originally had a separate pin wheel mounted in front of the main wheel, and that the machine was later simplified to its present form. The device has certainly been altered at least once and is now mounted up on two legs standing on a small platform. It also now has a wooden baton jammed through the ring of iron rods, which projects down to form a lever that can be moved from side to side between the legs, permitting a 30º to-and-fro action of the drum wheel. The lower part of this wooden lever (replaced in recent years) may originally have had a peg projecting out sideways for attachment to a cord for sounding the bells where a 30º action of the mechanism was all that is required, perhaps for use with John’s telescope.

During the 18th century, the role of the Royal Observatory was to produce tables to aid navigation. The work was routine and practical. Astronomical discoveries were left to amateurs such as the Herschels. William Herschel was originally a musician from Hanover but became interested in astronomy in 1773. In 1781 he spotted what he thought might be a comet using a seven-foot telescope. He immediately wrote to Nevil Maskelyne, then Astronomer Royal. The comet turned out to be the planet Uranus.