Page:Popular Science Monthly Volume 20.djvu/321

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circuit of the observatory clock. The operation is as follows: The secondary clocks are kept running with a very small gaining rate. At each vibration of the pendulum of the observatory clock the circuit is closed, and a current passes from a battery of six Daniell cells and magnetizes one of the electro-magnets at the foot of the pendulum of each secondary clock, which, attracting the piece of soft iron, retards its motion. The adjustment is delicately made, so that the retardation is just sufficient to keep the secondary clocks beating synchronously with the observatory clock.

This system, in Paris the device of M. Breguet, is a modification of the Jones system, which is considered by scientists the best ever invented for regulating clocks at a distance from the standard clock. Its main advantage lies in the fact that by no disaster to the wire of the circuit or to the regulator of the system can the secondary clocks be stopped. Should, by any accident, the wire be broken or the observatory clock stopped, the secondary clocks move right on, only slightly too fast; whereas, in any system of dials which are driven by a standard clock, any such mishap must of necessity stop the dials, whereby those depending upon them for time are misled, if not entirely deprived of time. In point of accuracy the results in this system are indeed all that could be desired, the error of the secondary clocks being kept less than one tenth of a second; but, because the secondary clocks must be fine time-keepers, the system is quite expensive. The estimated cost of each of these clocks is from 2,400 to 2,500 francs, or from 1480 to $500. On the two circuits, each terminating at both ends at the Observatory of Paris, there are distributed thirteen clocks, the farthest being at a distance of seven and a half kilometres, or nearly four and a half miles from the observatory. The clocks are furnished with second-hands, and are placed so that they can be easily seen from the street, and usually in prominent positions. The system is entirely under municipal management and has been in successful operation for about four years.

But the system thus far described is the basis of a much wider distribution of accurate time; for each of the secondary clocks is itself provided with apparatus, by means of which it sends a signal every hour to clocks placed on special circuits and to the public clocks of the city. For this reason the secondary clocks have come to be known as "horary centers." The methods employed for the distribution of the hourly signals from the "horary centers" are not uniform, nor are they of equal importance or extension; some of the principal watchmakers have invented methods of their own for special services, which are not of general interest, but the system which radiates from the "horary center" at the Hôtel de Ville (at present the Tuileries) to the twenty mairies of Paris is worthy of mention here both on account of its importance and ingenuity. There is a system of telegraph wires connecting all the mairies of the city with the Prefecture of