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THE POPULAR SCIENCE MONTHLY.
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),
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/PSM_V20_D324_Distributing_clock_of_the_pneumatic_system.jpg/430px-PSM_V20_D324_Distributing_clock_of_the_pneumatic_system.jpg)
Fig. 4.—Distributing Clock of the Pneumatic System.
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 move-