[Illustration: FIG. 4.--Porta's Apparatus, A. D. 1601.] The use of steam in Hero's fountain was as necessary a step as, although less striking than, any of the subsequent modifications of the machine. In Porta's contrivance, too, we should note particularly the separation of the boiler from the "forcing vessel"--a plan often claimed as original with later inventors, and as constituting a fair ground for special distinction. The rude engraving (Fig. 4) above is copied from the book of Porta, and shows plainly the boiler mounted above a furnace, from the door of which the flame is seen issuing, and above is the tank containing water. The opening in the top is closed by the plug, as shown, and the steam issuing from the boiler into the tank near the top, the water is driven out through the pipe at the left, leading up from the bottom of the tank. [Illustration: FIG. 5.--De Caus's Apparatus, A. D. 1605.] Florence Rivault, a Gentleman of the Bedchamber to Henry IV., and a teacher of Louis XIII., is stated by M. Arago, the French philosopher, to have discovered, as early as 1605, that water confined in a bomb-shell and there heated would explode the shell, however thick its walls might be made. The fact was published in Rivault's treatise on artillery in 1608. He says: "The water is converted into air, and its vaporization is followed by violent explosion." In 1615, Salomon de Caus, who had been an engineer and architect under Louis XIII. of France, and later in the employ of the English Prince of Wales, published a work at Frankfort, entitled "Les Raisons des Forces Mouvantes, avec diverses machines tant utile que plaisante," in which he illustrated his proposition, "Water will, by the aid of fire, mount higher than its source," by describing a machine designed to raise water by the expanding power of steam. In the sketch here given (Fig. 5), and which is copied from the original in "Les Raisons des Forces Mouvantes," etc., _A_ is the copper ball containing water; _B_, the cock at the extremity of the pipe, taking water from the bottom, _C_, of the vessel; _D_, the cock through which the vessel is filled. The sketch was probably made by De Caus's own hand. The machine of De Caus, like that of Porta, thus consisted of a metal vessel partly filled with water, and in which a pipe was fitted, leading nearly to the bottom, and open at the top. Fire being applied, the steam formed by its elastic force drove the water out through the vertical pipe, raising it to a height limited only by either the desire of the builder or the strength of the vessel. In 1629, Giovanni Branca, of the Italian town of Loretto, described, in a work[13] published at Rome, a number of ingenious mechanical contrivances, among which was a steam-engine (Fig. 6), in which the steam, issuing from a boiler, impinged upon the vanes of a horizontal wheel. This it was proposed to apply to many useful purposes.