256 APPLIED SCIENCE. 



parts or switches. Let the successive sides of the polygon 

 be called the odd and even sides ; the odd outer sides are con- 

 nected with the even inner sides ; and the even outer sides with 

 the odd inner sides. We thus have two continuous conductors 

 each going right round the model, but not joined to each other ; 

 these are connected to the two poles of a battery. So long as 

 no train bridges a gap no current flows, but whenever the train 

 bridges the gap a current flows from the positive to the 

 negative pole round the motor. This plan is called the ' cross- 

 over system ; ' all the trains are joined by it in parallel arc, and 

 the current is reversed each time a train passes a gap. This 

 reversal does not affect the working of the motor. This is the 

 plan which has been carried out on a large scale at Weston. 

 Its simplicity leads me to believe that it will be the plan most 

 usually adopted, but several other methods of driving have been 

 devised. A spark passes between the wheels and the line each 

 time the current is stopped, but this spark occurs between large 

 masses of metal, where it appears to be harmless ; it has given 

 no trouble whatever at Weston. Moreover, it has been found 

 very easy to make connection between the line and the train. 

 The ordinary truck wheels answer admirably, so that no com- 

 plicated brushes are required. There are some absolute advan- 

 tages in having interruptions at regular intervals, but the 

 discussion of these would lead me too far for my present purpose. 

 Only one of the two continuous conductors requires to be 

 insulated; this results in alternate insulated and uninsulated 

 sections all along each line. Fig. 1 shows a saddle, as we call 

 it, with an insulated attachment, B, at the one end, and an un- 

 insulated attachment, A, at the other, as used for a short sample 

 line which has just been sent to Peru for the Nitrates Railway 

 Company. The line itself is a |-inch steel rod with forged ends ; 

 and Fig. 2 sufficiently shows the mode of attachment. The in- 

 sulation is given by a vulcanite bell insulator, D, carrying a cast- 

 iron cap, 0. All the parts are designed to stand 2'2 tons strain ; 

 the vulcanite is secured between two layers of Siemens' cement. 

 The experiments at Weston have shown that vulcanite answers 

 perfectly, but the material is rather expensive I have here a 

 gmall porcelain insulator, which has been subjected to 2'2 tons 

 strain. I believe porcelain will answer well in all respects, but 



