The methods by which time is determined in observatories belong to the details of practical astronomy. For the more ready transmission of correct time to the public, there is at Greenwich observatory, as well as some others, a ball which is dropped by means of electricity precisely at one o'clock. Within the last twenty yours, however, there was invented a most ingenious device by which public clocks in a town can he kept at every instant in perfect agreement with the mean-time clock in the observatory. This is effected by an electric connection, and a modification of Bain's electric pendulum, invented by Mr. R. L. Jones of Chester, on the suggestion of Mr. Hartnup, the astrono mer of the Liverpool observatory. For a description, see ELECTRIC CLOCK. The first public application of it was to the town-hall clock in Liverpool, when for the first time was seen the curious spectacle of a great clock with works nearly 100 years old keeping time with astronomical accuracy In the same way, a clock in the castle of Edinburgh, by whose mechanism a gun is fired precisely at one o clock every day, is controlled by the mean-time clock in the observatory on the Calton hill.
It is not known when the alarm or when the striking-mechanism of the clock was first applied. The alarm was adopted for the use of the priesthood, to arouse them to their morning devotions. Tlfe first striking-clock probably announced the hour by a single blow, as they still do, to avoid noise in churches. During the 17th c., there existed a great taste for striking-clocks, and hence a great variety of them. Several of Tompion's clocks not only struck the quarters on eight bells, but also the hour after each quarter; at twelve o'clock, 44 blows were struck; and between twelve and one, no less than 1131 Many struck the hour twice, like that of St. Clement Danes, in the
Strand, London.
The striking part of a clock is rather a peculiar and intricate piece of mechanism. In ordinary clocks, the impelling power is a weight similar to that moves the time-measuring mechanisM itself; but the pressure of this weight on the striking machinery is only permitted to come into play at stated periods in course of the work ings of the time-keeping apparatus—viz., at the completion of every hour; when the minute-wheel, which revolves once in an hour, and carries, the minute-hand of the clock along with it, brings it into action by the temporary release of a catch or detent, permitting the weight wound up on the cylinder of the striking-apparatus to run down for a little, in doing which, the hammer is forced into action, so as to strike the bell. Whether the strokes shall be one or many, is determined principally by two pieces of mechanism, one called a mail, from its form or outline, with twelve steps, and the other a rack, with twelve teeth; but the intricate action of the whole it would lie in vain hero to attempt to explain. Suffice it to say, that the time during which the striking-weight is allowed to descend, varies according to the turning of the twelve steps of the snail on its axis, and the position of the twelve teeth of the rack, at different hours of the day; being sometimes only long enough to permit one blow to be given by the hammer on the nell, and at another time long enough for twelve such blows.
The consideration of portable time-keepers (watches and chronometers) is reserved 'or the head WATCIL