Chap. VIII.
599
circular valve-box placed underneath the pipe; the valve is opened by the wheel of the firstcarriage depressing a lever on the outside of the pipe, which disengages a catch thatoccasions a weight to move a slide valve across two openings of the valve-box on eitherside. In one position of the valve the air in the chamber finds its way into the exhaustedtube; and on the other, the valve which is attached to the same fulcrum as the first isopened by the external pressure ; this is a very ingenious part of the contrivance. Thedisc in the inclined position is a little larger than that in the pipe, and thus keeps itclosed until the slide is moved. To the fulcrum of this valve is attached the weight, bywhich the action of the wheel is disengaged; there are two sets of these valves, to allowof a transit in either direction.
When the first steam-engine is worked, a vacuum is made in the first length of the pipe ;the piston is then introduced, and the valve opened ; when the train begins to move asignal is given by means of an electric telegraph, and the second engine is set to work, sothat when the first length of pipe is passed through, the second is exhausted, and afterpassing the branch connected with the first pump, the exit valve of the first division isopened, as well as that of the second and the train passes on without any interruptionwhatever; the third engine then docs its work in a similar manner, and so on. When | ofa vacuum are produced before the train enters the section of pipe it is to pass over, it ispropelled by nearly twice the power required to work the air-pump during its motion ;for the mean pressure on the piston of the air-pump, working at this degree of vacuum, isonly 5 \ pounds per inch, whilst that on the piston in motion is 10 pounds.
Experiments were made on the Dalkey railroad, upon the amount of leakage of themain there laid down, which was 15 inches in diameter ; the total contents of this main orpipe were estimated as containing 10,680 cubic feet, and in the space of 41 seconds itwas not found more than 356 cubic feet, an inch of mercury being all the reduction whichtook place in the vacuum ; this leakage has been estimated as equal to 10 horse-power permile, or 30 horse-power for each section of 3 miles.
The Electric Telegraph , adopted on some of the lines of railroads, is remarkable for thegreat ingenuity displayed in applying electricity to communicate intelligence from onepoint to another; telegraphs so set in motion were the invention of Messrs. Cooke andWheatstone.
When the required communication is to be made, the operator upon the machine com-mences by sounding a bell, which he does by a very simple apparatus. In connection withthe instrument are two coils of wire, the ends of which, passing through the bottom of theframe, are attached to the general wires of communication ; a piece of soft iron is fastenedto a lever, which is firmly held against a pin. The electric current attracts the iron bymagnetic influence, releases the pin, and, giving motion to the wheels of the machine,produces a loud and clear ringing, which is attended to at the station where the informationis to be conveyed. The communication is then made by letters, every word being spelt,each letter having the distinct motion of a pointer or index applied to it, which works ona dial divided into five circles, each containing a number of letters, all indicated by needles.The left-hand needle moved twice gives A, three times B ; once to the right and once to theleft, C ; once to the left and once to the right, D ; once to the right E, twice F, three times G.The order is then taken up by the right-hand needle moving once to the left for II, twicefor I, three times for K, once to the right and once to the left for L, once to the left andonce to the right for M, once to the right for N, twice for O, and three times for P. Theremaining signs are made by the two needles conjointly, so that the simultaneous movementof the two once to the left indicates It, twice for S, three times for T, once to the right andonce to the left for U, once to the right for W, twice for X, and three times for Y. Atthe end of each word the left hand needle moving once to the right to the cross, indicatesthe word is complete, and when the receiver understands it, he moves the same pointertwice to the left and twice to the right, which indicates yes. When not understood, theneedle E is pointed twice to the right and twice to the left, which indicates 710, and thenthe original word is repeated. Doubling the motions for each letter, figures are obtained.
The signals are given by two magnetic needles or pointers, each suspended vertically onits axis, passed through the dial; behind the dial is fixed another pointer on each cor-responding axis. A portion of the conducting wire, many yards in length, is coiled roundthe galvanometer frame, in which the magnet moves, so as to subject the magnet to themultiplied deflecting force of the electric current; the motion of the pointers is limited bya fixed stop or pin at either side.
When a communication is to be made, the conductor turns the handle to the right orleft, so as to break the electric circuit, and press the w T ire against pins connected with thebattery poles ; the coils of wire thus receiving the full deflective force attract the magneticneedles to either side, according to the course of the current; thus, if the stream ofelectricity first passes into the coil, the upper part of the needle will be attracted to theleft, thus giving the whole motion necessary to the pointers. The movement of the handles,and consequent deflection of the pointers at either end of the line, are simultaneous, and
QQ 4