In the front of the apparatus is seen a rank of small handles, which can be turned from side to side with as much ease as the keys of a piano can be depressed. Turning one of these handles admits compressed air to the end of a pipe containing liquid. Instantly the pressure is transmitted 500 or 1,000 feet to the valve at the switch to be moved. The small levers are interlocked perfectly, and in that particular perform the duties of the ordinary machine. A model of the tracks controlled is placed before the operator, showing the switches and signals, and when a movement is made on the ground it is at once repeated back by electricity and duplicated on the model. This beautiful system is due to the same genius that gave us the perfected air-brake and the triple valve, and is the greatest improvement that has been made in interlocking in the last dozen years. [Illustration: Interlocking Apparatus for Operating Switches and Signals by Compressed Air, Pittsburg Yards, Pennsylvania Railroad. (A model of the track is shown above the levers, on which the movements of the switches and signals are electrically indicated after they are completed.)] [Illustration: Torpedo Placer. (The torpedo is carried forward by the plunger and exploded by the depression of the hammer shown near the rail.)] If the reader has grasped the full significance of interlocking, he understands that it makes it impossible to give a signal that would lead to a collision or to a derailment at a misplaced switch. The worst that a stupid, or drunken, or malicious signalman could do would be to delay traffic, if the signals were obeyed. Here comes in the failing case. The brake-power may be insufficient to stop a train after a danger signal is given. That is a rare occurrence, but may happen. The engineer may not see the danger signal because of fog, or he may carelessly run past it. Provision against a failure to see and to obey a signal may be made by placing on the track a torpedo, which will explode with a loud report when struck by a wheel. The use of hand-torpedoes in fogs, and for emergencies in places unprovided with fixed signals, is very common. These are little disks filled with a detonating powder, and provided with tin straps that are bent down to clasp over the top of the rail. A simple and very efficient torpedo machine, which has been used for some years on the Manhattan Elevated and elsewhere, is here shown. This machine has a magazine holding five torpedoes. It is connected to a signal-lever in such a way that, when the signal is put to danger, one torpedo is placed in a position to be exploded by the first passing wheel. When the signal returns to the clear position the torpedo, if unexploded, is withdrawn to the magazine. If the torpedo is exploded another one takes its place at the next movement of the signal-lever. One of these machines on the Elevated Road moves about five thousand times every day. In such a case a