[5] Owen’s “Lectures on Comparative Anatomy,” 2nd. ed. p. 605. Carpenter’s “Physiology,” 645. “Woodcroft on Marine Propulsion,” note, p. 1, and drawing of Nautilus in frontispiece to Woodcroft’s “Steam Navigation.” See also an interesting paper, read at the Society of Arts on the 14th of April, 1858, by John MacGregor, Esq., M.A., Barrister at Law.
[6] There is little difference between the action of an oar in sculling and that of the modern screw-propeller, which is fast superseding the paddle-wheel in all ocean-going steamers: the one has an alternate lateral motion, like the tail of a fish; the other is rotatory, but with the same effect. It may be added that fishes often have the power of “feathering” their tails, by puckering their lobes in their forward motion, and expanding them on their return, so as to displace as little water as possible, while they, at the same time, rely for their advancement on the reaction of the water in the direction of their body. These points have been carefully considered in the construction and arrangement of the blades of the screw, as well as the important fact that the tail of the fish or the sweep of an oar in their motions displace a quantity of water, great in proportion to the length of the instruments employed; and further, that it is by the resistance the water makes to this displacement by the oar or tail, in their continued oscillation, coming as these do from their extreme sweep to the axis of the boat or fish, that either is urged forward.
[7] An edition of Hero’s “Pneumatics” has been published by Mr. Woodcroft. Lond., 4th ed. 1851. His second experiment is referred to in Muirhead’s “Life of James Watt,” 2nd ed. p. 107.
[8] The principle of Hero’s steam-machine depends on the physical law that, when any fluid issues from a vessel in which it has been confined, the vessel is acted on by a force equal to that with which the fluid escapes, but in the opposite direction. Thus, if water issues from an orifice, a pressure is produced behind the orifice corresponding to the force with which the water escapes: hence, the recoil of a gun when fired. If the muzzle were turned at right angles to the length of the gun, the explosive gases would escape sideways, and the shooter, instead of being forced back, would spin round. The orifices in each case are exposed to the atmosphere, which tends to rush in with a force of a little less than 15 lbs. on the square inch: the force, therefore, with which the steam escapes represents the excess of its elasticity over that of the atmosphere, which furnishes, as it were, the fulcrum, and thus gives motion to the machine. Mr. Bourne states that the principle of the Æolipile is the same as that embodied in Avery and Ruthven’s engines for the production of rotatory power. “These engines,” he says, “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.”
[9] In another experiment (No. 37), Hero shows “how temple doors may be opened by fire on an altar.” He says, “Let the proposed temple stand on a pedestal, on which is also a small altar. Through the altar insert a tube, of which one mouth is within the altar, and the other nearly at the centre of a globe. The tube must be soldered to the globe in which a bent syphon is placed. Let the hinges of the doors be extended downwards, turning freely on pivots, and from the hinges let two chains running into one be attached by means of a pulley to a leaden weight, on the descent of which the doors will be shut, let the outer leg of the syphon bend into a suspended vessel and fill the globe half full of water. When the fire becomes hot, the heated air in the altar expands and, passing through the tube into the globe, will drive the liquid through the syphon into the suspended vessel which, descending by its weight, will tighten the chains and open the doors.”
[10] Although there may be a doubt how far the inventions recorded by Hero were used for superstitious practices, there is no question that, somewhat later, the agency of steam was employed for purposes anything but legitimate. Thus Gibbon (c. xl.) gives an amusing account of how Anthemius, the architect of Sta. Sophia at Constantinople, avenged, himself on Zeno, the orator. “In a lower room,” says he, “Anthemius arranged several vessels or cauldrons of water, each of them covered by the wide bottom of a leathern tube, which rose to a narrow top, and was artificially conveyed among the joints and rafters of the adjacent building. A fire was kindled beneath the cauldron; the steam of the boiling water ascended through the tubes; the house was shaken by the effects of imprisoned air, and its trembling inhabitants might wonder that the city was unconscious of the earthquake they had felt.” Still later, Arago, in his “Éloge de James Watt,” notices an ancient Teutonic god, called Bustarich, on the banks of the Weser, who was made by the priests to show his displeasure through the agency of steam. The head of the metal God was hollow and had within it a pot of water. Its mouth and another hole having been plugged, a charcoal fire was cleverly lighted under it, in such a way as not to be perceived by the expectant worshippers. After a while, the imprisoned steam forced out the plugs, with a loud report, followed by two jets of steam, which formed a dense cloud round the god and concealed him from his astonished worshippers.
[11] Paper read at the Society of Arts 14th of April, 1858, by John MacGregor, Esq., Barrister at Law.
[12] This work in Latin, printed at Verona, 1472, is the first book with woodcuts printed in Italy.
[13] This letter is written from Simancas, and bears the date 27th August, 1825. It was published 1826, in Lack’s “Astronomical Correspondence.”
[14] The interview with the Pope took place at Bupeto, 22nd of June, 1543, and the campaign against the duke of Cleves, the ally and general of Francis, followed.