[Footnote 1: Vortex-rings for illustration may be made by having a wooden box about a foot on a side, with a round orifice in the middle of one side, and the side opposite covered with stout cloth stretched tight over a framework. A saucer containing strong ammonia water, and another containing strong hydrochloric acid, will cause dense fumes in the box, and a tap with the hand upon the cloth back will force out a ring from the orifice. These may be made to follow and strike each other, rebounding and vibrating, apparently attracting each other and being attracted by neighbouring bodies. By filling the mouth with smoke, and pursing the lips as if to make the sound _o_, one may make fifteen or twenty small rings by snapping the cheek with the finger.] A vortex-ring produced in the air behaves in the most surprising manner. [Illustration: FIG. 4.--Method of making vortex-rings and their behaviour.] 1. It retains its ring form and the same material rotating as it starts with. 2. It can travel through the air easily twenty or thirty feet in a second without disruption. 3. Its line of motion when free is always at right angles to the plane of the ring. 4. It will not stand still unless compelled by some object. If stopped in the air it will start up itself to travel on without external help. 5. It possesses momentum and energy like a solid body. 6. It is capable of vibrating like an elastic body, making a definite number of such vibrations per second, the degree of elasticity depending upon the rate of vibration. The swifter the rotation, the more rigid and elastic it is. 7. It is capable of spinning on its own axis, and thus having rotary energy as well as translatory and vibratory. 8. It repels light bodies in front of it, and attracts into itself light bodies in its rear. 9. If projected along parallel with the top of a long table, it will fall upon it every time, just as a stone thrown horizontally will fall to the ground. 10. If two rings of the same size be travelling in the same line, and the rear one overtakes the other, the front one will enlarge its diameter, while the rear one will contract its own till it can go through the forward one, when each will recover its original diameter, and continue on in the same direction, but vibrating, expanding and contracting their diameters with regularity. 11. If two rings be moving in the same line, but in opposite directions, they will repel each other when near, and thus retard their speed. If one goes through the other, as in the former case, it may quite lose its velocity, and come to a standstill in the air till the other has moved on to a distance, when it will start up in its former direction. 12. If two rings be formed side by side, they will instantly collide at their edges, showing strong attraction.