himself at the same time. Some time ago I bought a play thing, very much in vogue at that time, called a Stropheor. This toy was composed of a small rotating screw propeller, which revolved on its own support when the piece of string wound round it was pulled sharply. The screw was rather heavy, weighing nearly a quarter of a pound, and the wings were of tin, very broad and thick. This machine, however, was rather too eccentric for parlour use, for its flight was so violent that it was continually breaking the pier glass, if there was one in the room; and, failing this, it next attacked the windows. The ascending force of this machine is so great that I have seen one of them fly over Antwerp Cathedral, which is one of the highest edifices in the world. The air from underneath the machine is exhausted by the action of the screw, which, passing under the wings, causes a vacuum, while the air above it replenishes and fills this void, and under the influence of these two causes the apparatus mounts from the earth. But the problem is not solved by means of this plaything, whose motive power is exterior to it. Messrs. Nadar, Ponton, D'Amécourt, and De la Landelle teach us better than this, although the wings of their different models are entirely unworthy of men who desire to demonstrate a truth to short-lived mortals. We have only arrived as yet at the infancy of the process, but we have made a good beginning, for, having once proved that a machine capable of raising itself in the air, wholly unaided from without, can be made, we have overcome with this apparently small result the whole difficulty. The principle of propulsion by means of a screw is by no means a novelty. It was first utilised in windmills, whose sails are nothing more nor less than an immense screw which is turned by the action of the wind on its surface. In the case of
