568 POPULAR SCIENCE MONTHLY. 



ductor d is located. On tlie top of tube / a flexible rail & is secured, 

 and this connects with studs c, wbicli are within the tube, as clearly 

 shown in the drawing, and so situated that they may be forced 

 down into contact with d. Normally these studs are separated 

 from d, but when the car comes along, the wheel a, mounted upon 

 the end of plow P, flattens the tube f and thus forces one or more 

 of the studs c into contact with d. The distance between the studs 

 c is such that at least two will always be in contact with d, thus 

 insuring a continuous electrical connection with the motors so long 

 as the plow is depressed. 



The first impression upon looking at this design would be that 

 it is entirely free from objections; for if we assume that the tube 

 / is made of rubber, we can see it in our mind's eye springing up 

 after the plow passes by and thus separating the contacts c from d, 

 and at the same time yielding freely to the pressure of the wheel a. 

 All of this is true, but rubber is not very durable when under such 

 exposed conditions, and to maintain a length of several miles of it 

 in a perfect state for even two or three years could not reasonably 

 be expected; and if it became necessary to renew the tube oftener 

 than this the cost of maintenance would be entirely too great. 

 There is another objection, however, which is more serious, and 

 that is that the conduit will gradually fill up with dirt, and this 

 pressing against the rubber tube would force it out of shape, and 

 thus cause the contacts c to bear permanently upon d, or else to 

 become so far displaced that they would not touch it when depressed 

 by the plow. 



As the rubber tube can not be depended upon, inventors have 

 sought to improve the construction by using sheet steel and mak- 

 ing the tube flatter and much wider, so that a section of it would 

 present an outline much resembling an elliptic carriage spring. 

 Such a construction will meet the requirements as to strength and 

 the retention of the contacts c in their proper position; but steel 

 expands when warm and contracts when cooled, therefore a long 

 tube would be stretched so much in winter that it might pull apart, 

 while in summer it would be compressed and tend to buckle up 

 and thus be forced out of place. These difficulties can be over- 

 come by providing expansion joints at suitable intervals, so that 

 they are not necessarily proof of the impracticability of devices 

 based upon the principles involved in this design; they simply serve 

 to forcibly bring to mind the fact that the path of the inventor of 

 underground systems is not strewn with roses, no matter in what 

 direction he may turn to find a solution of the problem. 



The object in the designs Figs. 25 and 26 is to shield the con- 

 ductor so that it will remain dry should the conduit be filled or par- 



