“Just how much power can be thus utilized cannot well be told in advance of experiment; but we have calculated that under certain supposed conditions it may be as much as some six-horse power for an aeroplane with one thousand square feet of sustaining surface; and we have also seen that while but few experimenters have resorted to the wind as a motor, those few have accomplished remarkable results.” The indications seem to be that we must try to construct a machine analogous to the sailing-yacht rather than to the steamship, though perhaps the aerial machine of the future will be, so far as power is concerned, analogous to the yacht Sunbeam with its auxiliary screw. Before continuing further with this subject, I wish to call attention to certain facts concerning the storage of power and the flight of soaring birds. First, in regard to the storage of power. It is well known that the construction of a useful electric storage-battery presents a most difficult problem. Such a storage device is needed for use upon the surface of the earth; yet, for purposes of aerial navigation, there is a much simpler accumulator which can be used. Take, for example, one hundred pounds of lead and let energy be stored in it by giving it altitude, just as energy is stored in the weight of a clock when it is wound. What is known as one-horse power is the amount of energy which must be exerted in lifting thirty-three thousand pounds at the rate of one foot per minute, or five hundred and fifty pounds at the rate of one foot per second, or fifty-five pounds at the rate of ten feet per second. To give an illustration, it may be stated that if a man weighing one hundred and sixty-five pounds ascends a flight of steps ten feet high in three seconds, he exerts for the time being just one standard horsepower. A small balloon which can lift one hundred pounds of lead three hundred and thirty feet high in one minute exerts one-horse power. The lead when lifted to this height has stored within itself thirty-three thousand foot-pounds of energy. Now, if weights can be made to slide downhill upon aeroplanes at very gentle grades, then the balloon becomes a valuable motor which stores energy in its load by giving it altitude, and the weight lifted becomes a reservoir of the very power needed for its own transportation, and the name of Montgolfier, the inventor of the under-estimated balloon, takes its place as that of the real founder of the useful art of aerial transportation. Whether or not it is possible to transport freight by sliding it down long and gentle inclines by means of aeroplanes will be considered further on; just here we must consider the soaring power of birds.