This question of wireless transmission lies within the province of discovery rather than that of invention. Before it can receive actuality we have to do more than utilise existing knowledge: we have to acquire new knowledge. In the meantime, portable energy will no doubt be achieved in ways other than electrical. Some very interesting compressed-air tools are already in limited use. Holes are drilled and rivets driven by little contrivances which have a store of force within themselves furnished by compressed air. One of the many uses of the cheap oxygen and hydrogen, and doubtless of cheaply liquefied gases of high-resisting power, [14] will no doubt be to work various kinds of machinery. This use of liquid airs has been much derided, and indeed a good deal of nonsense has been written as to its possibilities, drawing from a recent and accomplished writer the remark that "The statements which have sometimes appeared in the daily papers, announcing impending revolutions in the methods of obtaining cheap power by the application of liquid air, have originated from an imperfect comprehension of the problems involved." [15] In present conditions, and so far as we are able to see at present, liquefied gases are for a long time not likely to serve any greater mechanical purpose than that of furnishing a highly portable apparatus by which great power can be developed for a short time at any required place. It is easy to believe that it could not be otherwise employed with any economy, even when discovery has greatly simplified the now difficult process of liquefaction. But in regard to this matter, and to almost every other mechanical and engineering improvement suggested in the present work, it is of the first importance to remember that the conditions in which the work of the world a hundred years hence will be done are certain to differ very greatly from anything we know to-day; and that procedures at present not merely out of proportion, but in themselves actually chimerical, will become perfectly workable in the new circumstances of another century. No doubt the problems at present involved make many of the developments herein suggested almost laughable to those who examine the subject without imagination. But what could have been thought of a man who, when Oersted discovered the influence of a battery current on the compass needle, suggested that the discovery might, in much less than a hundred years, be practically developed in such unforeseen ways as to produce locomotive machines capable of carrying vast weight at a speed of perhaps a hundred miles an hour? He would have been told that such predictions "could only have originated from an imperfect comprehension of the problems involved." But we know that they would have been perfectly sound, though it would have been difficult to withhold assent from the derision which instructed hearers would have poured upon them. The