[2] The principle of the Æolipile is the same as that embodied in Avery and Ruthven’s engines for the production of rotary motion. “These engines,” says Bourne, “are more expensive in steam than ordinary engines, and travel at an inconvenient speed; but in other respects they are quite as effectual, and their construction is extremely simple and inexpensive.” [Illustration: BRANCA’S MACHINE.] Hero’s book lay hidden in manuscript and buried in libraries, until the revival of learning in Italy in the sixteenth century, when a translation of it appeared at Bologna in 1547. By that time printing had been invented; and the multiplication of copies being thereby rendered easy, the book was soon brought under the notice of inquiring men throughout Europe. The work must, indeed, have excited an extraordinary degree of interest; in proof of which it may be mentioned that eight different editions, in different languages, were published within a century. The minds of the curious and the scientific were thus directed to the subject of steam as a motive power. But for a long time they never got beyond the idea of Hero’s Æolipile, though they endeavoured to apply the rotary motion produced by it in different ways. Thus, a German writer suggested that it should be used to turn spits, instead of turnspit dogs; and Branca, the Italian architect, used the steam jet projected from a brazen head to drive an apparatus contrived by him for pounding drugs. The jet forced round the vanes of a wheel, so as to produce a rotary motion, and this, being communicated to other wheels, set in motion a rod and stamper, after the manner shown in the preceding cut. [3] See Bennet Woodcroft’s ‘Pneumatics of Hero of Alexandria,’ from the original Greek. London, 1851. Solomon de Caus was another of the speculative inquirers whose attention was drawn to the subject of steam by the publication of Hero’s book. De Caus was a native of Normandy, and for some time studied the profession of an architect in Italy; from whence he returned to France early in the seventeenth century. Religious persecution was then raging, and, being a Protestant, he was glad to take refuge from it in England. He entered the service of the Prince of Wales, by whom he was for a time employed in designing grottoes, fountains, and hydraulic ornaments for the Palace Gardens at Richmond. While occupied in that capacity he gave lessons in design to the Princess Elizabeth; and on her marriage to the Elector Palatine he accompanied her to Heidelberg, to take charge of the Castle gardens there. It was while residing at Heidelberg that De Caus wrote his well-known book on hydraulics, which was published at Frankfort in 1615.[4] [4] Les Raisons des Forces Mouvantes, avec diverses machines tant utiles que plaisantes, &c., par Solomon de Caus, Ingénieur et Architecte du Roy. Frankfort, 1615.