It follows, then, that, under the circumstances in question, the discs cannot be made to move asunder with a velocity greater than one-sixty-fourth of that of the blast. Of course, all the force of the current of air through the tube will be expended on the moveable disc, and the thin ring of air which exists around the orifice between the discs: and, since the moveable disc can only move with one-sixty-fourth of the velocity of the blast, the ring of air in the interstice must experience nearly all the force of the jet, and must be driven outwards, the blast following it in various currents, radiating from the common centre of the tube and discs. PRINCE RUPERT’S DROPS. Let fall melted glass into cold water, and it will become suddenly cooled and solidified on the outside before the internal part is changed; then, as this part hardens, it is kept extended by the arch of the outside crust: and, if the finely drawn-out point of the drop be broken off, the cohesion of the atoms of the glass is destroyed, and the whole crumbles to dust with a smart explosion. VEGETABLE HYGROMETER. The dampness of the air, and the consequent approach of rain, is denoted by several simple means, which are termed hygrometers. Thus, if an ear of the wild oat be hung up, its awn or bristly points will be contracted by a rotatory motion in damp air, and relaxed by a contrary motion when the air is dry. Similar effects are observable on all cordage, string, and every description of twisted material; as the moisture swells the threads, and increases their diameter, but reduces their length; hence, catgut is used in the construction of a weather-house, in which the man and woman foretel wet or dry weather, moving as the catgut stretches or contracts, according as the air is moist or dry. To prove the moving power of the awn, separate one from the ear, and, holding the base between the finger and thumb, moisten the awn with the lips, when it will be seen to turn round for some time. THE PNEUMATIC DANCER. [Illustration]