Page:Popular Science Monthly Volume 20.djvu/323

Rh with the electro-magnet of the clock at sixty-five seconds before the end of each hour, and reverses the operation at five seconds after the end of the hour. Ten seconds after the end of the hour the first current from the regulator at the Hotel de Ville automatically stops, and the wires are restored to the telegraph. The clocks at the mairies, being thus corrected every hour, run with very small error; but, should for any reason the error become large, or the clock stop, this is indicated automatically by the fact that the current from the "horary center," instead of stopping precisely at twelve o'clock, continues for thirty seconds. By this, the operator at once knows that his clock is wrong, and can have it set right. From the other "horary centers" the number of lines is in no case larger than six, the lines are shorter, and the apparatus accordingly simpler.

But there is another novel and ingenious method for the distribution of time in use in Paris, which, though lacking in accuracy sufficient for scientific purposes, has both convenience and economy to recommend it for general uses, and for that reason has become quite extensively employed in a short time. Abandoning electricity as an uncertain means for moving clock-work at a distance, the inventors of this system, Messrs. Popp and Resch, have accomplished the same object by the use of compressed air and for this reason have called their clocks "pneumatic clocks." They were exhibited at the Exposition at Vienna in 1878, and are now widely distributed in that city.

The essential parts of the system are three: 1. Machinery whose function it is to compress the air, and to propel impulses of the same every minute; 2. Pipes led through the streets and into the houses; 3. Dials provided with mechanism for receiving the pneumatic impulses.

1. At a central point a steam-engine drives pumps which compress air to five atmospheres in a reservoir holding eight cubic metres. This compressed air is sent, by means of a special regulator, into a second receiver called the "distributing reservoir," where the pressure is kept constant at seventh tenths of an atmosphere, or a little less—a pressure determined empirically to be sufficient to move the dials. The "distributing reservoir" is opened to transmit an impulse into the pipes each minute, for about twenty seconds, by a distributing clock (Fig. 4). This consists of two distinct movements. The one to the left, provided with balance-wheel, counter-weights, etc., is simply an ordinary clock, and indicates the hour, minute, and second, as shown in the figure. The movement to the right is contrived especially for moving the distributing valve, R. This valve, ingeniously arranged in such a way that the pressure acts only on a minimum of its surface, is inclosed in a valve-box and has three orifices. The first of these puts the valve in communication with the "distributing reservoir"; the second puts it in communication with the street-pipes; and the third puts the pipes in communication with the atmosphere. The first orifice is always