Marvelous it surely seems. Yet the cause of our astonishment is to be sought in the relative newness of the phenomena rather than in the nature of the phenomena themselves. At first glance it may seem that the intangible character of the electrical power gives it a unique claim on our wonderment. But a moment's reflection dispels this illusion. After all, electricity is no more intangible than heat. Neither the one nor the other can be seen or heard, but each alike may be felt. Yet we observe without astonishment a locomotive propelled by the power of heat--simply because the locomotive has become an old story. Again, electricity is far less intangible than gravitation. Not merely may electricity be felt, but it may be generated through transformation of other forms of energy; it may be stored away and measured; may be conducted at will through tortuous channels, or obstructed in its flight by the intervention of non-conductors. But gravitation submits to no such restrictions. It eludes all of our senses, and it absolutely disregards all barriers. To its catholic taste all substances are alike. It holds in bondage every particle of matter in the universe, and can enforce its influence over every kind of atom with an impartiality that is as astounding as it is inexorable. Moreover, this weird force, gravitation, has thus far evaded all man's efforts to classify or label it. No man has the slightest inkling as to what gravitation really is. If, as you glance at these lines, you should chance to release your hold and allow the volume to drop to the floor, you will have performed a miracle which no scientist in the world can even vaguely explain. As regards our electric train, then, the fact that it stands there firmly, held fast to the rails by gravitation, is in reality as great and as inexplicable a marvel as the fact that the electric current gives it propulsion. Not only so, but the fact that the train goes forward of its own inertia, as we say, for a time after the current is shut off, presents to us yet another inexplicable marvel. It is a fundamental property of matter, we say, when once in motion to continue in motion until stopped by some counter-force; but that phrasing, expressive though it be of a fact upon which so many physical phenomena depend, is in no proper sense of the word an explanation. Once for all, then, there is nothing unique, nothing preternaturally marvelous, about the phenomena of electricity. And indeed, it is interesting to note how quickly we become accustomed to these phenomena, and how little wonder they excite so soon as they cease to be novel. Even imaginative people have long since ceased to give thought to the trolley car; and within a week of the opening of New York's subway the average man came to regard it as much as a matter of course as if he had been accustomed to it from boyhood.