be along the lines of possible development—that would be too commonplace and altogether out of keeping with the ideal of the greatness of the future achievements of mankind. They must necessarily assume that what is brought forth hereafter will be so far in advance of what we now know of as to be revolutionary in its character, and so much so, in fact, as to consign to the scrap heap the most perfect devices of the present time. Some of the means by which these results are to be attained are not capable of accomplishing such wonders; others, while of great theoretical possibilities, are surrounded by certain practical difficulties so well understood at the present time that we can almost with certainty say that they will never realize the dreams that are based upon them. The remainder are problems that can be solved to-day, and would be if it were not for the fact that it is by no means certain that their solution would be of any practical value. The improbability of ever realizing a substantial gain by the solution of many of the problems upon which prophecies as to the wonders of the future are based is fully appreciated by many of those who have given the subject careful consideration; but those who dream of the revolutionary character of future invention never take note of such things.
Nearly all those who succumb to the fascination of meditating upon the changes that may be wrought by inventive genius in days to come follow the same line of thought. The problems upon the solution of which their fancy paints its pictures are always the same, although some contemplate the whole category, while others only dwell upon a portion thereof. These problems are aerial navigation, the development of electric energy direct from coal or some other equally cheap substance, and the utilization of the various forces of Nature, such as solar heat, tide and wave motion, and wind currents. Of these, aerial navigation is supposed to be by far the most important, obtaining electricity direct from coal and the others following along in the order in which they are given above.
As to the utilization of solar heat, tides, wave motion, and wind currents, it can be truthfully said that they could be utilized at the present time if it were considered profitable to do so. The energy of wind currents, as every one knows, is made available on a very extensive scale, but always in small units, and this fact alone shows that it can not compete with the steam engine, which, according to the prophets, it is sure to supersede. The energy of tides and wave motion is also utilized to some extent, and solar engines have been made from time to time.
It can not be said that these unlimited sources of energy are not brought into the service of man because of our inability to devise apparatus with which to harness them successfully, for, as
