72 



Professor W. E. Ayr ton 



[March 24, 



and is carried by the train, and by its motion presses forwards and 

 downwards a metallic fork on the contact box, thus making contact 

 between F and G. [Other diagrams were explained, illustrating 

 modifications of the contact boxes, in one case the well-insulated 

 cable is carried inside the flexible rail, which then takes the form of 

 a tube, shown in Fig. 3, in another case the cable is insulated with 



Fig. 3. 



paraffin oil instead of with guttapercha or indiarubber, shown in 

 Fig. 4, &c.] 



The existence of these contact boxes at every 20 to 50 feet also 

 enables the train to graphically record its position at any moment on 

 a map hanging up at the terminus, or in a signal-box or elsewhere, by a 

 shadow which creeps along the map of the line as the train advances ; 



Fig. 4. 



stops when the train stops ; and backs when the train backs. This is 

 effected thus : — as the train passes along, not only is the main contact 

 between F and G automatically made, as already described, but an 

 auxiliary contact is also completed by the depression of the lid of the 

 contact box, and which has the effect of putting, at each contact box 

 in succession, an earth fault on an insulated thin auxiliary wire run- 

 ning by the side of the line. And just as the position of an earth fault 

 can be accurately determined by electrical testing at the end of the 



