ÆOLIPILE, in Hydraulics, is a hollow ball of metal, generally used in courses of experimental philosophy, in order to demonstrate the possibility of converting water into an elastic steam or vapour by heat. The instrument, therefore, consists of a slender neck or pipe, having a narrow orifice inserted into the ball by means of a shoulder-screw. This pipe being taken out, the ball is filled almost full of water, and the pipe being again screwed in, the ball is placed on a pan of kindled charcoal, where it is well heated, and there issues from the orifice a vapour, with prodigious violence and great noise, which continues till all the included water is discharged. The stronger the fire is, the more elastic and violent will be the steam; but care must be taken that the small orifice of the pipe be not by any accident stopped up, because the instrument would in that case infallibly burst in pieces, with such violence as might greatly endanger the lives of the persons near it. Another way of introducing the water is to heat the ball red-hot when empty, which will drive out almost all the air; and then by suddenly immersing it in water, the pressure of the atmosphere will force in the fluid, till it is nearly full. Descartes and others have used this instrument to account for the natural cause and generation of the wind; and hence it was called Æolipila: q. d. pila Æoli, the ball of Æolus, or of the god of the winds.