The Paris Pneumatic Clock Network

Gallery opened 12 Dec 2017

Updated: 7 Jan 2018

Obervatory time distribution added
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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

Where is everybody?

At its maximum extent the system composed approximately 7800 pneumatic clocks in Paris. The compressed air was released from a central station in eastern Paris on the Rue St Fargeau. This was later enlarged and became the first usine on the xxx Paris compressed-air power network, which was another thing entirely, though also promoted by Viktor Popp.

Usine is a very handy French word which roughly translates as a place where something is made. It does not quite mean power-station; a room with a diesel generator in it counts as a usine.

Left: Two more Popp clocks at unknown locations in Paris

Once more a clock is combined with a lamppost.

The sign on the rightmost clock means 'Be kind to animals' which was directed at the drivers of horse-drawn vehicles, who in Paris did not have a reputation for consideration towards their animals. Also in this picture a gendarme with his distinctive kepi stands at the foot of the column.

Date unknown, though obviously in the horsedrawn era.
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 Madeleine is a large church built in classical style; only two of its columns are visible here.

Date of image currently unknown.

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

This the distribution room for the Popp network. In the glass case (vitrine) can be seen one of the distribution clocks, as illustrated above. The top of the case is labelled 'Horloge Centrale No 1', though lower down is a label saying 'Reserve' which seems contradictory; if it's the reserve clock, surely it should be called No 2? Presumably the case at the rear contains a second distributing clock. Here the valve is shown as outside the glass case.

The tank on the left is the distributing reservoir. The two dark things next to it that look a bit like coffee-urns are the pressure regulators between the high-pressure reservoir and the distributing reservoir. Note they are in duplicate for reliability, with isolating valves to disconnect a faulty regulator. In the original illustration it is possible to read 'regulateurs du pression' written on the wall just behind them.

I assume that stringent precautions were taken (presumably in the form of safety-valves) to prevent a regulator from failing and allowing the high pressure air to enter the network. This would burst the bellows in each clock and Paris would be treated to the alarming spectacle of 7,800 clocks exploding.

Date of image currently unknown.

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

The Popular Science Monthly tells us:

"A leather or rubber flap, seen in the cylinder, receives the impulse as it comes from the pipe and moves a piston, which acts upon a lever-arm arranged by simple connections to move the minute-hand one space forward. The ordinary clock-gearing (not shown in the figure) secures the proper motion for the hour-hand. This part of the apparatus can be inclosed in any case—as plain or as ornamental as desired. The cases are made in all the designs and sizes of ordinary clocks, and appear precisely like them, except that the minute-hands jump suddenly over one space at the end of each minute, and remain stationary during the minute, instead of moving gradually over the space."

I also find this description faulty. There is no 'flap'; what we have is a bellows in a can, made of soft rubber sections joined by brass rings, which pushes the 60-tooth wheel round by means of a pawl which is weighted to keep it in contact with the wheel. At the left a second weighted pawl prevents the wheel from slipping backwards. Probably the biggest design problem here is the danger of the inertia of the wheel causing it to advance by more than one tooth.

It is interesting that the mechanism uses weights to keep the pawls engaged rather than springs, which would have been more compact. The answer is probably that compactness is of no advantage in a big thing like a public clock, and that weights would be more reliable than springs which might break.

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

The Observatoire can be seen in the lower middle of the map. The Rue St Fargeau is coloured red.

At the Paris Observatoire a high-standard astronomical regulator clock was kept running on correct mean time by astronomical transit observations, being corrected daily. Pulses of electricity were sent every second to secondary clocks around the city, the wires being run through ducts in the sewers. Two loops starting and ending at the Observatoire carried thirteen clocks between them, the farthest being at a distance of seven and a half kilometres, or nearly four and a half miles from the observatory. The clocks were of a high standard, so they could keep good time even if the synchronising pulses failed. (The pulses synchronised the clocks but did not drive them, they were weight-driven in the conventional way) The secondary clocks were furnished with second-hands, and were placed so that they could be easily seen from the street, usually in prominent positions. They further distributed time by sending electric signals once an hour to synchronise various public clocks. The system came into operation in 1878.

The secondary regulator at the Hotel de Ville sent synchronising pulses once an hour to the twenty mairies of Paris, over the telegraph circuits connecting them. (A mairie is roughly equivalent to a borough town hall in London) The nearest to St Fargeau is the mairie of the 20th arrondisment, just to the east of the Pere Lachaise cemetery. This is still some 700 metres away, and I am left wondering how the time covered that last step. Given the short distance, carrying a good pocket watch between the mairie and the distribution centre would have been practical.

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