WHAT MAKES THE TROLLEY CAR GO. 565 



just as great, if not greater, liability of obtaining shocks from 

 underground systems as from the trolley, therefore the only real 

 gain that can bie made by their use is in the artistic sense. From 

 a financial point of \'iew no underground system so far devised 

 can compare with the overhead trolley; but if any one should de- 

 vise anything hereafter that can be constructed at the same expense 

 and will not cost more for maintenance it will undoubtedly find 

 an extensive application. Until such a perfect solution of the 

 problem makes its appearance the field for these devices will be 

 confined to cities like Xew York and Washington, where the over- 

 head trolley is not permitted. 



Every system of conductors that dispenses with the overhead 

 wire is called by the layman an underground trolley, but, properly 

 speaking, these systems may be divided into surface and subsurface 

 conductors. Both of these may again be divided into exposed and 

 inclosed conductors, and also into continuous and sectional con- 

 ductors. Finally, we may designate the various modifications as 

 mechanical, electrical, and magnetic, the mechanical being those 

 that accomplish the result by purely mechanical means, the elec- 

 trical being those that employ electrical devices, and the magnetic 

 those that depend for their action upon the attraction of magnets. 

 The principal difficulties that the inventors in this field have to 

 contend with are the cost of construction and the effective insula- 

 tion of conductors. With the overhead trolley the current flows 

 out from the power house to the cars through wires carried on poles, 

 and the poles are themselves good insulators; but to make the 

 work doubly sure the conductors are secured to glass insulators, 

 which are practically perfect. The current returns to the power 

 house through the ground and the track rails. As it is easier for 

 the current to circulate in a short path than in a long one, there is 

 a continual tendency for it to jump from the overhead wire through 

 the insulation to the ground, but this is effectually prevented by 

 the very perfect character of the insulation. When the outgoing 

 and incoming wires are both placed upon or underground the strain 

 upon the insulation is very much increased, for then instead of the 

 two lines being separated by fifteen or twenty feet of pole, which 

 is a very fair insulator, they are separated by only a few inches of 

 earth or perhaps metal, the first of which is a fairly good con- 

 ductor, while the last is a nearly perfect one. It is evident, there- 

 fore, that the insulation proper in an underground or surface sys- 

 tem must be of the highest order. If the conduits in which the 

 wires are located could be kept perfectly dry, there would be no 

 difficulty in obtaining insulation tjiat would withstand the strain 

 it is subjected to; but rain. in summer and snow in winter will at 



