WHAT MAKES THE TROLLEY CAR GO. 



567 



The plow is also provided with the arms h h, upon the ends of which 

 are mounted small wheels a a, and these run upon tracks attached 

 to the covers c c. As is shown in the figure, the wheels a a, run- 

 ning upon the tracks attached to the covers c c, cause the latter to 

 spread out to the position in which they are shown. This spread- 

 ing, as can be readily understood, only takes place for a short dis- 

 tance ahead and behind the plow, but at all other parts of the con- 

 duit the sides assume the position i i, and thus close the conduit 

 and exclude the water. 



It can be easily seen that some difficulty would be encountered 

 in making a tight joint at h h, and also that the opening and closing 

 of the sides might not operate as perfectly in practice as upon 

 paper, but it does not follow from these facts that the design is not 

 practical; it simply illustrates that there are many minor difficul- 

 ties to be overcome in order that complete success may be attained. 

 Many designs operating upon this principle have been patented, 

 and in some of them a great amount of ingenuity is displayed. 



Fig. 26 illustrates another type of inclosed conductor which 

 at a first glance appears to be far superior to that just described, 

 but upon closer investigation it is found to be not wholly free from 

 objections that are difficult to overcome. The yoke F F, as in the 



Fig. 26. — Underground Conduit with Inclosed Conductor. 



design just described, is made wide enough to support upon its 

 outer ends the track rails R R, and is cut away in the middle to 

 an outline conforming with the shape of the conduit. The con- 

 ductor that carries the current is located at d, being supported by 

 the stands e. An elastic tube / is provided, which is water-tight 

 and thus excludes moisture from its interior, within which the con- 



