dialogues, each twelve of which have a distinct title.
220 Frankf. 1713, 8vo.
221 I shall here lay before the reader the whole passage, taken from Histoire de l’Académie for 1717, p. 7:—“Here we have a small magnet. It is a stone found in a river of the island of Ceylon. It is of the size of a denier, flat, orbicular, about the tenth part of an inch in thickness, of a brown colour, smooth and shining, without smell and without taste, which attracts and afterwards repels small light bodies, such as ashes, filings of iron, and bits of paper. It was shown by M. Lemery. It is not common, and that which he had cost twenty-five livres (about twenty shillings sterling). When a needle has been touched with a loadstone, the south pole of the loadstone attracts the north pole of the needle, and repels its south pole: thus it attracts or repels different parts of the same body, according as they are presented to it, and it always attracts or repels the same. But the stone of Ceylon attracts, and then repels in the like manner, the same small body presented to it: in this it is very different from the loadstone. It would seem that it has a vortex. …”
222 “I must not omit to mention that the rivers contain the electric stone, which is of the size of a halfpenny, flat, orbicular, shining, smooth, of a brown colour, one-tenth of an inch in thickness, without smell and without taste, and which attracts light bodies, such as ashes, filings of iron, shavings of paper, &c., and afterwards repels them. A wonderful and singular property, discovered and observed in this stone alone, when neither heated by motion nor by friction.”
223 [Light is called polarized, which, having been once reflected or refracted, is incapable of being again reflected or refracted in certain positions of the second medium. Ordinarily, light which has been reflected from a pane of glass or any other substance, may be a second time reflected from another surface, and will also freely pass through transparent bodies. But if a ray of light be reflected from a pane of glass at an angle of 57°, it is rendered totally incapable of reflexion from the surface of another pane in some positions, whilst it will be completely reflected by it in others. If a plate of tourmaline, cut in the manner described above, or a Nichol’s prism be held between the eye and a candle, and turned slowly round in its own plane, no change will take place in the image of the candle; but if the plate or prism be fixed in a vertical position, on interposing another of the same kind between the former and the eye, parallel to the first, and turning it round slowly in its own plane, the image of the candle will be found to vanish and re-appear alternately at each quarter turn of the plate, varying through all degrees of brightness down to total or almost total evanescence, and then increasing again by the same degrees as it had before decreased. These changes depend upon the relative positions of the plates; when the longitudinal sections of the two plates are parallel, the brightness of the image is at its maximum; and when the axes of the sections cross at right angles, the image of the candle vanishes. Thus the light, in passing through the first plate of tourmaline, has acquired a property totally different from the ordinary light of the candle; the latter would penetrate the second plate equally well in all directions, whereas the altered light will only pass through it in particular positions, and is altogether incapable of penetrating it in others. The light is polarized by passing through the first plate or prism. Thus, one of the properties of polarized light is proved to be the incapability of passing through a plate of tourmaline perpendicular to it in certain positions, and its ready transmission in other positions at right angles to the former.]
SPEAKING-TRUMPET.
Instruments by which the voice could be so strengthened as to be heard at a much greater distance than would otherwise have been possible, were known in the earliest ages; for of all musical instruments, wind instruments were first invented, and their use in war to give the signal of battle, we find mentioned in Job224. It had been remarked, even in Pliny’s time, that the least touch of a beam of wood could be heard when the ear was applied to the other end225. It was known likewise that the larger trumpets emitted a louder and stronger sound. The Grecians had a wind instrument with the bellowing noise of which the people who were placed to guard the vineyards frightened away the wild animals226. All these wind instruments however were little in comparison with the monstrous trumpets of the ancient Chinese, a kind of speaking-trumpets, or instruments by which words could not only be heard at the greatest distance possible, but could be also understood227. This invention belongs to the 17th century, though some think that traces of it are to be found among the ancient Grecians.
Kircher, as far as I have been able to learn, was the first person who made known, from a very ancient manuscript of Aristotle, De Secretis ad Alexandrum Magnum, preserved in the Vatican, that Alexander had a prodigious large horn with which he could assemble his army at the distance of a hundred stadia, or eight Italian miles. It was, according to the manuscript, five cubits in diameter; and Kircher, who gives a figure of it, which he says he found in the manuscript, thinks that, on account of its size, it must have been suspended from a beam by a ring. This horn has by many been considered as the oldest speaking-trumpet228, but in my opinion without reason. Aristotelis Secretum Secretorum ad Alexandrum Magnum I have never had an opportunity to see. It appears to have been printed only once, and is, like all the other works ascribed to that philosopher, extremely scarce; for they have all had the fate of being little regarded after it became the unanimous opinion of the learned that they were forged. These works, however, are old; some of them indeed very old: and, if some one would take the trouble to fix their antiquity, they might be used with advantage on many occasions. Morhof had in his possession the edition of that book published by Alexander Achillinus, a physician at Bologna, in 1516, which is a Latin translation from the Arabic229. If we compare what is said there and by Kircher, we may make the following conclusion:—
In the first place, it is certain that the book itself, as well as the whole account, is not the production of Aristotle, for in all the writers who relate the actions of Alexander we do not find the least mention of such a horn. Secondly, it is not expressly said in that work that Alexander spoke through this horn, but only that he assembled his soldiers by it, which in past times was done by the sound of a trumpet, and at present is done both by trumpets and drums. It appears also that the author of the book, perhaps an Arabian, intended to give the reader an idea of a horn that had an uncommonly strong and loud sound. Thirdly, Kircher’s account and figure of the horn do not agree with that which Morhof found in the edition of Achillinus230. Lastly, none of these descriptions are such that an instrument to serve as a speaking-trumpet could be constructed from them.
Wolf and other mathematicians are of opinion that the most advantageous form of a speaking-trumpet would be found with more certainty by experience than by theory. It may then be asked, whether any one ever caused such an instrument to be made from these descriptions. Kircher, who attempted things much more improbable, says he never tried it. Duhamel however relates that a Frenchman tried it, and discovered the real instrument231; but this information is of little weight, as it is much to be doubted that this Frenchman caused it to be made sufficiently exact according to the ancient description. I am as little acquainted with Bettini as Morhof; but I suspect that Duhamel meant Mar. Bettini, who, without making the smallest mention of Alexander’s