July 27, 1SS3.] 



SCIENCE. 



99 



pulled to safety, it is directly controlled by 

 the signalman. 



Connections to switches are generally made 

 by means of rods or pipes jointed together, 



DSVICE 70R KEEPING SIGNAL- WIRE TIGHT. 



and running on rollers. A ' trunking ' or 

 wooden covering is then placed over them to 

 protect them from snow and the feet of any- 

 one walking about the yard. 

 As it is very important that 

 the movement of switches should 

 be absolute and exact under all 

 conditions, — that is to say, 

 that the switch be always either 

 tightly closed or wide open, and 

 never stand i)artly open 

 arrangement 



always been experienced in keeping trains 

 running in the same direction, on the same line 

 of rails, from running into one another, as 

 naturally, on a crowded line, an accidental stop- 

 page to even a fast train may enable a slow 

 train to overtake it and cause a rear collision. 

 The Pennsylvania railroad ado])ted, some years 

 ago. what is known as the block system, by 

 which a definite interval of space (the dis- 

 tance between two adjoining signal-cabins) 

 can always be maintained between two follow- 

 ing trains. The system is too well known to 

 need description here; but Mr. George West- 

 inghouse has invented a system in which the 

 same results are obtained, not by men signal- 

 ling from one cabin to another, but by the 

 trains themselves operating signals through 

 the medium of electricity. The principle of 

 the invention is easily understood, although the 

 details are complicated and the results mar- 

 vellous. A battery is connected to each signal 

 by means of the rails, the current flowing 

 to the signal by one rail, and returning by 

 the other. The presence of an axle and pair 

 of wheels on the track enables the current to 

 flow through them, instead of through the 

 signal a|)paratus. Directly the current is thus 

 short circuited, the signal tiics to danger. 



This simple principle is so ingeniously worked 

 out in detail, that a train approaching a road- 

 crossing rings a bell fixed on a post at the 



1 ■ - r »- 



^ 



s. 



compensating 

 introduced half way between 

 the switch and the signal, so that, whatever 

 the variation of length of connection from tem- 

 perature, the switch is unatfected. and its 

 movements can always be under exact control. 



TO sieau. MAM. 



COMPENSATIN 



SIGN OF RODS. 



The full lines show the position in cold 

 weather ; the dotted lines, in hot. It is evi- 

 dent, however much the rod expand, the 

 distance between switch and signalman is un- 

 altered, and therefore the movements of the 

 switch and lever are unaffected. 



In working railroads, some difficulty has 



BATTERY 



AUTOMATIC ELECTRIC BLOCK SYSTEM. 



crossing until the crossing is reached, when the 

 bell stops ringing ; and this is done by trains 

 travelling in either direction. In working on 

 an ordinary piece of road, two signals behind 

 tlic train are always kept at danger : and, on 

 a single line, two signals in advance of the 

 train are always kept at danger against a train 

 advancing in the opposite direction. In a few 

 words, the trains warn one another of their 

 [iroximity. 



We have dwelt on the subject of signals at 

 considerable length, as the question is novel, 

 and of great and growing importance ; and 

 we have no doubt that those who take an inter- 

 est in railroads have found much to be gained 

 by visiting the exposition, and studying this 

 question on the spot. The two exhibits we 

 have mentioned represent the best results 

 attained in England after forty years' patient 

 and careful study of signals, under such trying 



