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HORNE & THORNTHWAITE'S ASTRONOMICAL CLOCK.

Fig. 31.

An accurate timekeeper is one of the most useful adjuncts to a telescope; in fact, the report of an observation, to be of any astronomical utility, must be always accompanied by the exact sidereal time at which it took place. An instrument of this perfection is generally very expensive, but Messrs. Horne & Thornthwaite have introduced the above in the hope that it will Supply a want long felt amongst amateur astronomers of a really reliable clock, fit for astronomical purposes, at a reasonable price.

The Escapement is dead beat, maintaining power to go whilst being wound, the pendulum is compensated so as to be uninfluenced by a rise or fall of temperature—the dial is enamelled, and the outer case is of polished mahogany.

Horne & Thornthwaite have a direct hourly signal from Greenwich Observatory to their Strand Establishment, for the purpose of thoroughly and readily testing the qualities of these clocks, and ascertaining their uniformity of rate, and they are thus enabled to speak in high terms of the regularity of their performance.

Clocks of a similar quality, showing Mean time, can be supplied at the same price.

The compensating Pendulum used is a slightly modified form of that so highly extolled by Captain Kater. It is easily adjusted, the centre of oscillation is low, and, unlike the mercurial form, cannot be put out of adjustment during transport.

The rod a, Fig. 32, is of straight white deal which, after two days’ baking in a japanner’s stove, was thoroughly impregnated with melted solid paraffin (this being found the best of all materials to prevent the action of damp). The deal rod passes freely through the pendulum bob b b, and is furnished at the lower end with a regulating screw, c, a divided nut, d., and a clamping nut, e; and at the upper part with the spring, / and suspension pin, g. A bar of zinc, /?, bent as shown in the cut, with a hole through which the regulating screw, c, passes ; and to the top of this bar of zinc the pendulum bob, b by is firmly fixed by screws at i.

The mode by which correction for variation of temperature is produced in this pendulum can be readily understood. Assuming that at a given temperature the Clock is keeping correct time—now, it is evident that if the temperature rises that the spring/, deal rod a, and screw r, will be lengthened by that increase, and this lengthening would (unless otherwise prevented) lower the pendulum bob, b by and thus lower the centre of oscillation, causing the Clock to lose. To prevent this, the bent zinc bar, h, is introduced, to which the pendulum bob is fixed at i. This on an increase of temperature can only expand upwards, resting as it does on the upper part of the

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