duced by sanding the rails from well-contrived sand-boxes, which should be so arranged as to be self-acting in cases. of emergency. Probably none of the breaks have yet reached the limit of rate of retardation possible or practicable. The practicable rate of retardation, subject to the considerations to which I have above referred as to the limitation of the possible force of retardation, depends on the amount of force which may be safely administered to a train without injury to the passengers. Up to this time the power available to arrest a train with hand breaks has been altogether disproportioned to the necessities of railway travelling and to the forces at work in a train at full speed. The amount of retarding force which may be applied to a train without injury to its inmates has up to this time been much under-estimated, and it may be with confidence predicted that in future breaks will exceed in power any of those yet introduced. LECTURE VI. SYSTEMS OF SIGNALLING-BLOCK SYSTEM-VISIBLE ELECTRICAL SYSTEMELECTRIC SLOT SIGNALS-ELECTRICAL INSTRUMENTS-THREE-WIRE SYSTEM-SURFACE RAILWAYS-TEMPORARY WORKS-DRAINAGE OF SCREW JACKS-LIFTING VEHICLES-BREAKDOWN TRAINS-NE IN my fourth lecture railway signalling was considered as it is carried out between the signalman and the enginedriver. We have now to notice the signalling which takes place between one signalman and another, through the agency of the electric telegraph. The time remaining at our disposal will not allow of my giving more than an outline of this somewhat complicated subject, but I shall hope to make the principles aimed at in such signalling clear, and at the same time to give a short description of a few of the various kinds of apparatus usually employed to carry out the system of electrical signalling, which is now well known as the Block System. Since the employment of the electric telegraph in working railways, the object aimed at in signalling is to preserve not an interval of time, but an interval of space, between trains. So long as the latter is preserved, no collision can take place, but the attempt to maintain an interval of time between trains is of necessity illusory. Between one signalling station and another an engine or carriage may break down, the rails may be slippery and the train may in various ways be prevented from running accurately to time. Thus time signalling is faulty in principle, for though all the time signals may have been correctly exhibited and a proper interval of time may have been observed at a signalling station between any train and the train following it, yet before the first train reaches the succeeding signalling station the second train may have caught up the first and have run into it. The introduction of the electric telegraph afforded means of discarding altogether the system of time signalling, and of substituting a system the object of which is, as I have said, to secure the preservation of an interval of space between the trains. The latter system received the name of the Block' system, either from the facility it afforded for blocking the line, and stopping the trains as required, or from the securing or blocking over of the handle of the signalling instrument in the requiring position, which was necessary in the instruments employed when the system was first introduced. The name is by no means a good one, but it has been used so long that it is not likely that it will now be discarded for a better. The mode in which the block system is carried into effect differs slightly on different lines; that is to say, the machinery by which the signals are transmitted varies, though the principle or object aimed at is the same (with one exception, to be referred to below) on all English railways. The exception referred to is what is called the 'permissive block' system, the operation of which will be explained after the ordinary or absolute block system has been treated of. To carry out the block system a railway must be divided into telegraphic districts by signal boxes, in each of which there are signalling instruments, enabling the signalman in it to communicate by electricity with the signal box on each side of him. Thus taking the case of one line of rails, and supposing, as shown in fig. 146, it is divided into four districts by five signal boxes, A, B, C, D, E, A will communicate with B, в will communicate with A and c, c with B and D, D with cand E, and E with D. The districts may be of equal or unequal lengths, as may be convenient for working the traffic. Supposing, then, that a train is ready to start from A, the signalman at A will warn в of the fact. B will acknowledge the signal, and say 'send the train,' which a will do by lowering his outdoor semaphore signal. A at the same time will notify to B that the train has left; B will acknowledge the signal and will at the same time give some sort of signal in the signal box at A, which will notify to a that no other train must be sent until further orders. This last operation is called 'blocking the line,' and when the signal of B is received, A will at once put up his outdoor semaphore to the position of danger.' Meantime B will have asked c permission to send the train between B and c, and if в receives this permission, he will lower his semaphore arm to let the train pass his box. As soon as the train has passed B, the signalman there will notify a of its arrival, and will take off' the block signal and give to A the signal of line clear.' Precisely the same series of signals passes between every signalman from A to E, excepting only that at the intermediate signalling stations between A and E the signalman may in some cases send on the warning signal which he receives to one, or perhaps two, stations in advance, so as to avoid any necessity of check ing the speed of such trains as have to pass the stations in question without stopping at them. Now suppose that a train breaks down or is delayed between C and D (fig. 147), and that a train is ready to start at A or B. Previously to the break down c will have asked permission from D to dispatch the train subsequently disabled, and as soon as this has been done, D will have blocked the line at c. This block, so far as the electric signal is concerned, cannot be taken off by c, or indeed by anyone but D, who will not do so until the disabled or delayed train reaches and passes the signal box at D. Meantime, supposing that the second train is travelling between B and c, its engine-driver when the train arrives at c will find the signal of c at 'danger,' in obedience to the orders of D, and will consequently pull up at c. c will previously have blocked the line at B, when warned that the second train has passed B, and will thus have protected the second train from being run into by a succeeding train starting from A. Thus, in the case of a break-down of a train between C and D, and if trains continued to be dispatched from A, the condition of the line would be that there would be one train on each block, and a train would be standing still at each of the signal boxes в and c (fig. 148); when this state of things has occurred, no more trains could be dispatched from A until the disabled train between C and D had been removed. A B D E FIG. 148. It follows, therefore, that however numerous the trains might be which were timed to be dispatched from the terminus of a line, still so long as the block signalling be |