Take two heads of plaster of Paris, and place them on pedestals on the opposite sides of a room. Then take a tin tube, of an inch in diameter, and let it pass from the ear of one head through the pedestal, and under the floor, to the mouth of the other, observing, that the end of the tube which is next the ear of one head, should be considerably larger than that which comes to the mouth of the other.
The whole being so disposed that there may be no suspicion of a communication, let any person speak with a low voice into the ear of one bust, and the sound will be distinctly heard by anyone who shall place his ear to the mouth of the other; and if there be two tubes, one going to the ear, and the other to the mouth of each head, two persons may converse together, by applying their mouth and ear reciprocally to the mouth and ear of the busts, without being heard by any other persons in the room.
Another Recreation of the same kind, called the Oracular Head.
Place a bust on a pedestal in the corner of a room, and let there be two tubes, one of which goes from the mouth, and the other from the ear of the bust, through the pedestal and floor, to an under apartment.
Then if a person be placed in the under room, by applying his ear to one of the tubes as soon as a proper signal is given, he will hear any question that is asked, and can immediately return an answer; and if wires be contrived to go from the under jaw and eyes of the bust, they may be made to move at the same time, and by these means appear to deliver the answer.
It was by a contrivance of this kind, that Don Antonio de Moreno so much astonished the celebrated Knight of the Woeful Countenance, and his facetious squire Sancho Panza, by resolving certain doubts proposed by the former concerning his adventures in the cave of Montesinos, and the disenchantment of my lady Dulcinea.
How to make a Piece of Metal, or any other heavy Body, swim upon the Surface of Water, like a Cork.
The specific gravity of water is inferior to that of metals, and consequently water, absolutely speaking, cannot support a ball of iron or lead; but if this ball be flattened, and beat out to a very thin plate, it will, if put softly upon still water, be prevented from sinking, and will swim upon its surface like any light substance. In like manner, if a fine steel needle, which is perfectly dry, be placed gently upon some still water in a vessel, it will float upon the surface without sinking.
But if you would have a metallic body of large dimensions to swim upon water, you must reduce it into a thin concave plate, like a kettle; in which case, as the air it contains, together with the body itself, weighs less than the same bulk of water, it cannot possibly sink; as is evident from large copper boats, or pontoons, by which whole armies have frequently passed over rivers without danger.
If this concave metallic vessel be placed upon the water with its mouth downwards, it will swim as before, and the contained air will keep the bottom of it from being wet; for that the water will not rise into any hollow vessel which is immersed into it, may be made evident thus:—Take a glass tumbler, and plunge it into water with its mouth downwards, and you will find, when you take it out, that the inside of the vessel is perfectly dry, so that if a live coal were put there, it would not be extinguished.