The Paris Pneumatic Clock Network>
In 1879 the Austrian engineer Viktor Antoine Popp and his co-worker Resch demonstrated a system of pneumatic clock synchronisation in the Austro-Hungarian section of the Universal Exposition. It appears the actual inventor was Carl Albert Mayrhofer. The Paris city council granted Popp's Compagnie des Horloges Pneumatiques (CGHP) authorization to install a compressed air network to drive both public and private clocks. It was not a power distribution network but a time distribution network, which synchronised a large number public clocks, in particular those of railway stations, by sending a pulse of air every minute. The pipes ran through the sewers of the city, and the tunnels of the Metro and the RER. (The RER is a commuter rail network serving Paris and its suburbs) Each Popp clock contained a metal bellows which advanced a 60-tooth wheel by one tooth per minute. Operation began in 1880; it is interesting to note that this was a long time after the Paris pneumatic post had opened in 1866.
At the time compressed air was in the air, so to speak, following the extensive use of compressed air for rock drilling in the construction of the Mont Cenis tunnel, which was initially expected to take 25 years but was opened in 1871 after only 14 years following the introduction of pneumatic equipment. (Dynamite was also a help in the later years of construction)
Popp obtained French nationality in 1881. He was later involved in the compressed air power network installed in Paris, and then the Branly-Popp radio system.
Left: A Popp clock in a Paris street
Left: Two more Popp clocks at unknown locations in Paris
I am however sure about the time- 11 o'clock in the morning.
Left: Multiple-faced Popp clock in the Place de la Madeleine, Paris
The following information in quotes is taken from Popular Science Monthly, Vol 20, January 1882 "Time-Keeping in Paris."
"At a central point a steam-engine drives pumps which compress air to five atmospheres (73 psi) 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, (10.3 psi)or a little less—a pressure determined empirically to be sufficient to move the dials."
Left: A Distributing clock
"The "distributing reservoir" is opened to transmit an impulse into the pipes each minute, for about twenty seconds, by a distributing clock. This consists of two distinct clock 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 open; the other two are normally closed. The automatic escape of the lever G, at the end of each minute, moves the slide-valve, opens the second orifice, and sends an impulse into the pipes; at the end of a number of seconds, determined by experience and dependent on the length of the pipes (a number which varies from ten to fifteen seconds), the slide-valve is brought back to its original position by the clock-work, closes the two orifices, and allows the extra pressure which has been introduced to escape into the air. This operation is repeated every minute.
"The motive-power for the clock-work of both movements is furnished by the compressed air, which automatically lifts the pistons in the cylinders, C, at the end of each minute. The pistons move the levers B and A; the first of these, B, winds up the counter-weights as much as they have fallen during the preceding minute; the second, A, imparts motion to the slide-valve."
I find this description defective. The clock movement on the right is just a timer to control the 10 - 15 second pulse of air. The duration does not have to be determined very accurately, and so instead of an escapement its speed is regulated by the fly or vane labelled L. The lever A does not impart motion to the slide-valve directly, but winds up the weights of the timer by means of the gear teeth on its right extremity.
Left: The Distributing room
Going back to Popular Science Monthly:
"The impulse given by the clock-work is distributed through the city by means of pipes laid like ordinary gas-pipes. In the streets the pipes are of iron, and have a diameter of twenty-seven millimetres (about one inch); but in the houses the pipes are of lead, and of different sizes—the diameters being fifteen, six, or three millimetres (practically one half, one quarter, or one eighth of an inch), depending on the number and size of the dials to be operated. These pipes are entirely hidden from view, and in no way interfere with the appearance of the dials."
Left: The mechanism of a Popp pneumatic clock
Back to Popular Science Monthly again:
"All the machinery of the system is in duplicate, for use when repairs are needed. Delicate manometers indicate the pressure at all times, and the most approved electric apparatus is used to indicate the particular point at which a defect has occurred. A skilled engineer is on the watch at all times. Provision is also made so that, in case of any interruption in the regulator, the dials may be run by hand. Accuracy of time is secured by daily comparison with the observatory clock."
All very sensible, except that reference to the observatory clock, which must mean it was in the Paris Observatoire (the equivalent of Greenwich Observatory in London) and so some six kilometres away on the other side of the Seine. Investigation was called for...
Left: Electrical time distribution from the Paris Observatoire