There are two valves to an air-brake, one for ordinary stops and the other for sudden stops in case of emergency.
In the first only partial pressure of the confined air is used, in the latter the full pressure is employed and the brakes brought against the wheels with all force at once.
One of the most important duties of an engineer is to be well assured that the air-brakes are in proper working order.
After the call for hand brakes has been given, the air brakes must not be applied until the hand brakes are released. Air and hand brakes should never be used at the same time on a car.
When cars having different air pressures are coupled together the brakes will work first on that having the highest pressure.
Special instruction is needed to fully comprehend the working of air brakes.
Here is a speed table which may be useful. We have taken the liberty of extracting it from Alexander's "Ready-reference for Locomotive Engineers," an excellent hand-book with which all candidates for the foot-plate should provide themselves. Published by the author, S. A. Alexander, York, Pa.
Key: M = Minutes. S = Seconds. T = 10th of a Second.
M S T 10 miles per hour is 6.00 to 1 mile 11 " " " " 5.27 " 1 " 12 " " " " 5.90 " 1 " 13 " " " " 4.37 " 1 " 14 " " " " 4.17 " 1 " 15 " " " " 4.00 " 1 " 16 " " " " 3.45 " 1 " 17 " " " " 3.32 " 1 " 18 " " " " 3.20 " 1 " 19 " " " " 3.09.5 " 1 " 20 " " " " 3.00 " 1 " 21 " " " " 2.51.5 " 1 " 22 " " " " 2.43.5 " 1 " 23 " " " " 2.36.5 " 1 " 24 " " " " 2.30 " 1 " 25 " " " " 2.24 " 1 " 26 " " " " 2.18.6 " 1 " 27 " " " " 2.13.3 " 1 " 28 " " " " 2.08.5 " 1 " 29 " " " " 2.04 " 1 " 30 " " " " 2.00 " 1 " 31 " " " " 1.56 " 1 " 32 " " " " 1.52.5 " 1 " 33 " " " " 1.49 " 1 " 34 " " " " 1.45.6 " 1 " 35 " " " " 1.42.6 " 1 " 36 " " " " 1.40 " 1 " 37 " " " " 1.37.3 " 1 " 38 " " " " 1.34.7 " 1 " 39 " " " " 1.32.3 " 1 " 40 " " " " 1.30.0 " 1 " 41 " " " " 1.27.7 " 1 " 42 " " " " 1.25.7 " 1 " 43 " " " " 1.23.5 " 1 " 44 " " " " 1.21.7 " 1 " 45 " " " " 1.20.0 " 1 " 46 " " " " 1.18.2 " 1 " 47 " " " " 1.16.6 " 1 " 48 " " " " 1.15.0 " 1 " 49 " " " " 1.13.5 " 1 " 50 " " " " 1.12.0 " 1 " 51 " " " " 1.10.6 " 1 " 52 " " " " 1.09.4 " 1 " 53 " " " " 1.07.9 " 1 " 54 " " " " 1.06.6 " 1 " 55 " " " " 1.05.4 " 1 " 56 " " " " 1.04.3 " 1 " 57 " " " " 1.03.2 " 1 " 58 " " " " 1.02.2 " 1 " 60 " " " " 1.00.0 " 1 " 65 " " " " 0.55.3 " 1 " 70 " " " " 0.51.4 " 1 " 75 " " " " 0.48.0 " 1 " 80 " " " " 0.45.0 " 1 " 85 " " " " 0.42.3 " 1 " 90 " " " " 0.40.0 " 1 " 95 " " " " 0.37.9 " 1 " 100" " " " 0.36.0 " 1 "