of the syphon. The 11th proposition is the first application of heat to produce motion of fluids. An altar and its pedestal are hollow and air-tight. A liquid is poured into the pedestal, and a pipe inserted, of which the lower end passes beneath the surface of the liquid, and the upper extremity leads through a figure standing at the altar, and terminates in a vessel inverted above this altar. When a fire is made on the altar, the heat produced expands the confined air, and the liquid is driven up the tube, issuing from the vessel in the hand of the figure standing by the altar, which thus seems to be offering a libation. This toy embodies the essential principle of all modern heat-engines-the change of energy from the form known as heat-energy into mechanical energy, or work. It is not at all improbable that this prototype of the modern wonder-working machine may have been known centuries before the time of Hero. Many forms of hydraulic apparatus, including the hand fire-engine, which is familiar to us, and is still used in. many of our smaller cities, are described, the greater number of which are probably attributable to Ctesibus. They demand no description here. A hot-air engine, however, which is the subject of his 37th proposition, is of real interest. Hero sketches and describes a method of opening temple-doors by the action of fire on an altar, which is an ingenious device, and contains all the elements of the machine of the Marquis of Worcester, which is generally considered the first real steam-engine, with the single and vital defect that the expanding fluid is air instead of steam. The sketch, from Greenwood's translation, exhibits the device very plainly. Beneath the temple-doors, in the space A B C D, is placed a spherical vessel, H, containing water. A pipe, FG, connects the upper part of this sphere with the hollow and air-tight shell of the altar above, D E. Another pipe, KLM, leads from the bottom of the ves sel, H, over, in syphon-shape, to the bottom of a suspended bucket, NX. The suspending cord is carried over a pulley and led around two vertical barrels, OP, turning on pivots R FIG. 1.-Opening Temple-Doors by Steam, B. c. 200. at their feet, and carrying the doors above. Ropes led over a pulley, R, sustain a counterbalance, W. On building a fire on the altar, the heated air within expands, passes through the pipe, F G, and drives the water contained in the vessel, H, through the syphon, KLM, into the bucket, NX. The weight of the bucket, which then descends, turns the barrels, OP, raises the counterbalance, and opens the doors of the temple. On extinguishing the fire, the air is condensed, the water returns through the syphon from the bucket to the sphere, the counterbalance falls, and the doors are closed. Another contrivance is next described, in which the bucket is replaced by an air-tight bag, which, expanding as the heated air enters it, contracts vertically and actuates the mechanism, which in other respects is similar to that just described. In these devices the spherical vessel is a perfect antici pation of the vessels used many centuries later by several so-called inventors of the steam-engine. Proposition 45 describes the familiar experiment of a ball supported aloft by a jet of fluid. In this example steam is generated in a close cauldron, and issues from a pipe inserted in the top, the ball dancing on the issuing jet. No. 47 is a device subsequently reproduced-perhaps reinvented by the second Marquis of Worcester. FIG. 2.-Steam Fountain, B. c. 200. A strong, close vessel, AB CD, forms a pedestal, on which are mounted a spherical vessel, EF, and a basin. A pipe, HK, is led from the bottom of the larger vessel into the upper part of the sphere, and another pipe from the lower part of the latter, in the form of a syphon, over to the basin, M. A drain-pipe, NO, leads from the basin to the reservoir, AD. The whole contrivance is called "A fountain which is made to flow by the action of the sun's rays." It is operated thus: The vessel, EF, being filled nearly to the top with water, or other liquid, and exposed to the action of the sun's rays, the air above the water expands, and drives the liquid over, through the syphon, G, into the basin, M, and it will fall into the pedestal, A B C D. Hero goes on to state that, on the removal of the sun's rays, the air in the sphere will contract, and that the water |