But perhaps the most powerful burning mirror ever constructed, was that of Mr. Parker, an eminent glass manufacturer of London; it was made in the begining of this century by one Penn, an ingenious artisan of Islington. He erected an outhouse at the bottom of his garden, for the purpose of carrying on his operations, and at length succeeded in producing, at a cost of £700, a burning lens of a diameter of three feet, whose powers were astonishing. The most hard and solid substances of the mineral world, such as platina, iron, steel, flint, &c., were melted in a few seconds, on being exposed to its immense focus. A diamond weighing ten grains, exposed to this lens for thirty minutes, was reduced to six grains, during which operation it opened and foliated like the leaves of a flower, and emitted whitish fumes; when closed again, it bore a polish, and retained its form. Ten cut garnets, taken from a bracelet, began to run into each other in a few seconds, and at last formed one globular garnet. The clay used by Wedgewood to make his pyrometric test ran in a few seconds into a white enamel; and several specimens of lavas, and other volcanic productions, on being exposed to the focus of the lens, yielded to its power.
A subscription was proposed in London to raise the sum of 700 guineas, in order to indemnify the inventor for the expense he had incurred in its construction, and retain it in England; but, through the failure of the subscription, and other concurring circumstances, Mr. Parker was induced to dispose of it to Captain Mackintosh, who accompanied Lord Macartney in his celebrated embassy to China; and the mirror, much to the loss and regret of European science, was left at Pekin.
MAGNETIC CORRESPONDENCE IN THE
SEVENTEENTH CENTURY.
In one of Addison's contributions to the Spectator (No. 241), we find the following curious instance of what may almost be considered as the foreshadowing of the electric telegraph. It is quoted from the writings of Strada, the celebrated Roman Jesuit, who died in 1649. In his "Prolusiones," a series of polished Latin essays upon rhetoric and literature, he gives an account of a chimerical correspondence between two friends, by the help of a certain loadstone, which had such virtue in it, that if touched by two several needles, when one of the needles so touched began to move, the other, though at ever so great a distance, moved at the same time and in the same manner. He tells us that two friends, being each of them possessed of these needles, made a kind of dial-plate, inscribing it with twenty-four letters—in the same manner as the hours of the day are marked upon the ordinary dial-plate. They then fixed one of the needles on each of these plates, in such a manner that it could move round without impediment so as to touch any of the twenty-four letters. Upon their separating from one another into distant countries, they agreed to withdraw themselves punctually into their closets at a certain hour of the day, and to converse with one another by means of this their invention. Accordingly, when they were some hundred miles asunder, each of them shut himself up in his closet at the time appointed, and immediately cast his eye upon his dial-plate. If he had a mind to write anything to his friend, he directed his needle to every letter that formed the words that he had occasion for—making a little pause at the end of every word or sentence, to avoid confusion. The friend, in the meanwhile, saw his own sympathetic needle moving of itself to every letter which that of his correspondent pointed at. By this means, they talked together across a whole continent, and conveyed their thoughts to one another, in an instant, over cities or mountains, seas or deserts.... In the meanwhile (adds the Essayist, playfully), if ever this invention should be revived, or put in practice, I would propose that upon the lovers' dial-plate there should be written, not only the twenty-four letters, but several entire words which have always a place in passionate epistles; as flames, darts, die, languish, absence, Cupid, heart, eyes, hang, drown—and the like. This would very much abridge the lover's pains in this way of writing a letter—as it would enable him to express the most useful and significant words with a single turn of the needle.
NAVIGATION BEFORE THE COMPASS.
Before the invention of the mariner's compass, the Phœnician, the Greek, and the early Italian navigators were compelled to creep from headland to headland, without venturing to quit the shore—except when an island, so near as to be distinctly seen from the continent, offered them an equally secure retreat from the violence of an accidental tempest. Yet, the bolder Norwegians, though exposed to far greater perils, from the habitual inclemency of a high northern latitude, and from the frequent cloudiness of their atmosphere, were in the habit of attempting, and often with success, a voyage of some length upon the ocean. It may be supposed that a patient observation of natural phenomena, attention to the flight of migratory birds and to the direction of currents, and some few simple devices which, being no longer necessary, are now forgotten, served as substitutes for the more valuable guides of modern navigation. Of one of the devices here enumerated, it is related that when Flok, a famous Norwegian navigator, was about to set out from Shetland for Iceland, then called Gardarsholm, he took on board some crows, "because the mariner's compass was not yet in use." When he thought he had made a considerable part of his way, he threw up one of his crows, which, seeing land astern, flew to it; whence Flok, concluding that he was nearer to Shetland (or perhaps Faroë) than any other land, kept on his course for some time, and then sent out another crow, which, seeing no land at all, returned to the vessel. At last, having run the greater part of his way, another crow was sent out by him, which, seeing land ahead, immediately flew for it; and Flok, following his guide, fell in with the east end of the island. Such was the simple mode of steering their course, practised by those bold navigators of the stormy northern ocean. This story at once and strikingly recalls the use made of birds by the first sea captain of whom we read—Noah; but such expedients evidently could not be supposed to have inspired the old northern navigators with the courage and confidence that enabled them, as there is reason to believe, to discover America before Columbus.