Every small boy knows that by focusing the sun's rays through a lens he is able to kindle a fire. The action is caused by concentration of the rays, due to refraction. But a similar concentration, even to a point of greatest intensity, may be secured by employing a multiple system of mirrors. In this case, the arrangement of the numerous surfaces must be such that the reflected beams will converge at a focal point.
The principle, of both the ordinary burning glass and of what we may term the fire mirror, was known to the ancients, and through the ages such glasses have served as playthings for men of science. Yet, the practical result of innumerable experiments has been almost nothing. In recent years a solar motor has been placed in operation in California, and the device has proved itself to be efficient. It depends for power entirely on the rays of the sun, which are reflected to a focus from grouped mirrors. But this exception serves only to emphasize the fact that the solar energy has never been really exploited. The measureless force thus available remains unutilized, as does that of the tidal movement. Neither sun nor moon has yet been fully harnessed for the driving of man's engines.
One difficulty that stands in the way of success for the solar motor is its dependence on a cloudless sky. As a matter of fact, the brilliant sunlight required is not to be found in regions where are located the great industries of the world. Just those regions where the best effects might be secured are those too remote from the centers of civilization for our practical use of them.
That both burning glasses and fire mirrors were familiar to the learned long ago is shown by the testimony of Empe-docles. Euclid, too, gave the subject consideration in his works on optics and catoptrics. Evidently, knowledge was sufficiently widespread to render literary references intelligible to the general public; for Aristophanes did not hesitate to make use of the principle in one of his comedies, where a debtor contrives the cancellation of a bond against him by melting the seal with the sun's rays, concentrated through a globe of water. Plutarch describes disks of polished metal, which were used for the purpose of setting on fire combustible substances properly placed at the focal points. It appears that, in this instance, the mirrors used were concave. Indeed, long after the principle involved in such a concentration of the rays by a concave mirror was familiar to learned men, no extension was made to secure a more intense effect by concentrating the various rays from a number of plane surfaces.
The originator of the multiple-mirror contrivance was Father Kircher. It was obvious to him that the effect of the concave mirror might be obtained, and greatly heightened, by employing a group system of looking glasses, so disposed that the reflected rays of the sun from each should converge at a common focal point. He therefore arranged five mirrors in such a manner that the focal point of reflection from them should be at a distance of about 100 feet. He found the heat thus obtained so great that he became enthusiastic over the possibilities suggested by the device.
"If five mirrors," he wrote, "produced so considerable an effect, what would one hundred or one thousand do, arranged in the same manner? They would excite so violent a heat that it would set fire to everything and reduce all to ashes."
Nevertheless, there is little historical record of a reliable sort as to the practical use of either the burning glass or the fire mirror. It has been claimed that burning glasses were used by the Romans, on occasion, for the lighting of sacred fires at the altar. But the evidence in this regard is inconclusive. There is better justification for the belief that Archimedes made practical use of fire mirrors. We may at least credit the claim in his behalf because of what we know concerning his various ability. When Syracuse was besieged by the Romans, in the third century before our era, Archimedes resorted to the invention of numerous devices, to be employed against the enemy's fleet, in defense of the city. These included engines from which were hurled showers of rock, and also curious machines that could seize the hostile ships, raise them and capsize them. But it is declared, in addition, that his greatest triumph was obtained by the use of an elaborate grouping of mirrors, with which he was able to focus on vessels at a distance heat so intense as to set them afire, and to consume them. There is no doubt as to the success of the great man's efforts, for we know that the Roman leader speedily ordered the withdrawal of the fleet.
Accounts tell of a similar destruction by means of fire mirrors when Proclus defended Byzantium against the navy of Vitellian.
In later times, a notable effect was secured by Magius of Septala, who constructed a system of mirrors 3 1/2 feet in diameter. The concentration of heat from this machine was sufficient to kindle fire at a distance of 3 rods. Similar experiments were conducted by Vilette and Deschirinhausen. La Brocquiere, who visited Damascus in the fifteenth century, tells of having seen mirrors of polished steel with curved surfaces, which in the sunlight reflected the rays so strongly as to set on fire wood a rod distant.
Buffon made the most serious experiments in the use of the fire mirror. At the outset, he employed a group of 24 looking glasses. He found that the concentration of rays from these very easily kindled a combustible mass of mixed pitch and coal powder, at a distance of 66 feet. He next constructed a polyhedral arrangement, in which he assembled 168 separate pieces of flat looking glass, each 6 inches square. He tried the effect of this on sections of beechwood board, placed at a distance of 150 feet, where they were readily kindled to flame. In another experiment, a plate of silver was fused at a distance of 4 rods.
Gratified by the result of his efforts thus far, Buffon now devised a machine containing 360 mirrors. These were each 8 inches in length and 6 in breadth. They were mounted on a frame, having a height of 8 feet and a breadth of 7. He made a trial of the instrument by using in succession different numbers of the mirrors for the concentration of their rays. When 120 of the glasses were focused on a combustible substance, at a distance of 20 feet, the material instantly burst into flames. At the same distance, a tin vessel was quickly melted when only 45 of the mirrors were set to reflect the sunlight at a common point. A flake of silver was fused by the combined heat of 117. Various other metals were melted at distances ranging from 25 to 40 feet. The extreme distance at which wood could be kindled, under the conditions of unobstructed sunlight and the full power of the machine, was 210 feet.
Buffon afterward built a machine that included 400 mirrors, but these were each only 6 inches square. This device was capable of melting lead and tin set at a distance of 140 feet.
A little more than a century ago, a series of interesting experiments was conducted by Parker, in London. The man had been a glass manufacturer, and, when he retired from active business, his interest in mirrors led him to undertake ambitious tests as to their power in concentrating the heat of the sun's rays. He was so enthusiastic over the matter that he erected an outbuilding, at the bottom of the garden which lay back of his town residence, and there installed special apparatus for testing his ideas.
He succeeded in constructing a very efficient burning lens, which had a diameter of 3 feet. Platinum, iron, steel, flint, and other hard substances were melted within a few seconds, when placed under the heat of the focal point. The records assert that Parker carried on some trials to determine the influence of this concentrated solar heat on diamonds, and that, in one instance, a stone weighing 30 grains was reduced to a weight of 6 grains, during a period of 30 minutes' exposure at the focus. It is alleged also that during this process the diamond opened and "foliated like the leaves of a flower"; at the same time, it gave forth white flames, perhaps from the combustion of carbonic-acid gas. Then, the gem again closed, and, at the conclusion of the experiment, the surface of the stone displayed its original polish, and, too, its former shape was exactly preserved, in spite of the fact that the bulk had been reduced to one-fifth its weight prior to the operation. In continued tests of this burning glass, Parker readily melted garnets, and other semiprecious stones. His work was so successful as to attract general attention, and the public appreciation was shown by a subscription of 700 guineas, to reimburse him for the expenses of his investigation. The lens used by him finally became the property of Lord Macaulay, who presented it to the Chinese Government.
In considering the above account, it must be borne in mind that, in Parker's day, experiments were not always watched and recorded with that scrupulous accuracy characteristic of investigators in our generation. We may, therefore, with all due respect to the zeal and honesty of those from whom we have received this narrative, regard some of the details with a degree of caution. We are permitted to suspect that there may have been unconscious exaggeration, or other errors, in the description of the effects shown by the diamond. Nevertheless, the account as it stands was given without comment by an American chemical authority, who was an instructor at West Point a century ago, and who was himself a devoted student of pyrotechny.
About the time of Parker's experiment, a French investigator contrived a fire mirror, which was formed out of numerous plane mirrors, so arranged as to concentrate the solar heat in a focus with the greatest precision. . This machine was differentiated from its predecessors in the method of adjusting the mirrors so that their position could be controlled with the utmost nicety, and the effect was correspondingly intense.
About the middle of the eighteenth century, following the experiments of Buffon, a polygonal mirror was erected in the Botanical Garden at Paris, by which an intense heat was generated at the focal point. This mirror was composed of 168 plates of silvered glass, each of which was capable of being moved in every direction and of being fixed at differ-ent degrees of inclination. The result was that the system of looking glasses could be so disposed as to form essentially one large concave mirror, of which the focus might be located wherever desired. This contrivance was so powerful as to set on fire wood placed at a distance of 200 feet. It fused metals under a focus 45 feet from the compound mirror.
Thus we find, after the passage of many centuries, something to justify a belief that the great brain of Archimedes did indeed evolve a method by which he could gather the sunlight and cast it forth to the destruction of his enemies. But, while we may credit him with such achievement, we are unable to discover any cause for pride over subsequent accomplishment in the matter of fire mirrors. There is nothing in our own day to rival the exploits of tradition.