so far devised can compare with the overhead trolley; but if any one should devise 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 New York and Washington, where the overhead 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 conductors. 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 electrical 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 insulation 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 conductor, while the last is a nearly perfect one. It is evident, therefore, that the insulation proper in an underground or surface system 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 that would withstand the strain it is subjected to; but rain in summer and snow in winter will at times cover the tracks and fill the conduits, hence the securing of perfect insulation presents great difficulties. The manner in which inventors have sought to surmount the obstacles can be made clear by the aid of a few illustrations of typical designs. [Illustration: FIG. 25.--UNDERGROUND CONDUIT WITH PROTECTING SHIELD FOR THE CONDUCTOR.]