METALLIC BURNING MIRRORS.
The classical student will remember that Archimedes
burned the fleet of Marcellus, by means of burning-glasses,
from the heights of the fortifications of his
native city of Syracuse. Unfortunately, any account of
the system of catoptrics, or the science of reflections,
employed by the ancient Syracusan in their construction
is lost to us, and many modern writers have
gone so far as to doubt the fact altogether. The
knowledge of the properties, however, of concave
mirrors which we have just been acquiring, will enable
us to form a pretty good guess as to the means adopted
by Archimedes for the destruction of the enemy's fleet.
The ancients, not having the means of either casting or
grinding such enormous mirrors, must have constructed
them of a large number of small ones, so arranged
that the images of the sun reflected by them would all
fall in the same place, or nearly so. In this case, the
larger the number of mirrors, the greater would be the
burning effect. In order to explain the reflection of
rays incident upon the surface of concave mirrors, we
supposed them to consist of an immense number of
plane mirrors placed in a curve, so that the reflected
rays might all meet in one point; but on examining
into the history of burning mirrors, we find that the
plan has been adopted in reality in a great number of instances. We have also said, that the reflection of the
heating rays was governed by similar laws to those
influencing the rays of light; consequently, by directing
a pencil of sunlight upon the surface of a concave
mirror, we obtain the maximum of light and heat at
the focal point.
Many modern writers give the ancients too little credit for their knowledge of optical principles, and late investigations seem to prove that the old school of philosophers were much more learned in these matters than has been generally supposed. The discovery of a rock crystal double convex lens in an Egyptian tomb of great antiquity is an instance of this. Descartes wrote a little treatise to prove that the stories related of the burning mirrors of Archimedes were pure fabrications, although many Latin authors have described them both as being used by that philosopher and in more modern times; Dion, for instance, who lived in the early part of the sixth century, states that at the siege of Constantinople, Proclus burnt the fleet of Vitalian with mirrors of brass; but the opinion of Descartes seemed to outweigh all other testimony. Buffon, who wished to sift the matter thoroughly, constructed for himself, after many previous experiments on the laws of reflection, a series of mirrors that closely imitated those ascribed to Archimedes. His first memoir, "On the Invention of Mirrors capable of burning at a great Distance," was published in the Transactions of the French Academy of Sciences for 1747. A few years later he combated both theoretically and practically the opinion of Descartes, in a memoir containing an account of an immense number of experiments. Before speaking of the extraordinary effects of burning mirrors, it will be as well to do justice to the predecessors of the learned naturalist we have just mentioned, by quoting a passage from the works of Father Kircher, who, 128 years pre-viously, experimented in this direction with great patience and perseverance, and tried to prove that the stories related of Archimedes were true. "The larger the surface of a mirror," says this philosopher (who, like Huyghens, was a practised astronomer), "the more light it reflects from the objects opposite to it. If it is only a foot square, it will throw a square foot of light upon any wall or screen placed before it. Experiment shows that this light is composed of an infinite number of rays reflected from different points on the surface of the mirror. Direct the rays from a second mirror upon the same place as those from the first, and the light and heat will clearly be doubled. They will become trebled if you direct the rays from a third mirror upon the same spot, and so on ad infinitum. In order to prove that the intensity of the light and heat is in direct proportion to the number of reflecting surfaces employed, I took five mirrors, and found that on exposing them to the sun I obtained with only one, less heat and light than if I used direct sunlight. With two the light and heat increased considerably; three gave as much heat as an ordinary fire, and four gave me a still greater effect. I therefore concluded that by multiplying these plane mirrors, I not only obtained greater effects than those got by using parabolic, hyperbolic or elliptical mirrors, but that I could use them upon objects at a much greater distance. With five mirrors I could obtain these effects at a distance of 100 feet, but what terrible phenomena would have taken place had I used one thousand instead of five?" He ends by begging mathematicians to experiment in this direction with greater care than they had hitherto done.
After Kircher we may cite as an experimentalist with these terrible instruments the French philosopher Villette, who constructed several mirrors, in direct imitation of those of Archimedes, for Louis XIV. and other sovereigns. The Journal des Savants for 1679 gives an account of his principal metallic burning mirror in the most eulogistic terms, adding an instance of ignorance which is singularly quaint and curious. It is of the fourth and most perfect of Villette's mirrors that the Journal des Savants speaks, the first having been bought by Tavernier, and presented to the Shah of Persia, who considered it as one of the rarest and most precious curiosities that he possessed: the second was sold to the King of Denmark, and the third was given by M. Villette to Louis XIV., from whom he received the praises and rewards that were due to his talent and perseverance. "It was thirty-four inches in diameter, and vitrified flints and bricks almost instantaneously, no matter how large they were. It consumed the greenest wood, burning it to ashes in an instant, and fused the most refractory metals with equal ease and quickness. Steel, no matter how hard, resisted its power no more than other metals, and melted so quickly that one part burnt away in inconceivably brilliant sparks, some of them forming stars as large as a franc piece, leaving a flowing mass of metal behind. The last made by Villette was still more powerful, being larger and more carefully made. It was forty-four inches in diameter, and three inches and a line deep. Its burning point, or focus, was situated at a distance of three feet seven inches from the surface, and was apparently as large as a five-sou piece; and it was at this spot, where the rays of light and heat were concentrated into so small a space, that the wonderful effects of its violent power became manifest, the spot of light being of such brilliancy that the eyes could no more withstand its brightness than that of the sun. Besides the property of burning which it possessed in so wonderful a degree, it was capable of exhibiting other effects just as curious as those already related. It had the power of sending the images of objects to a distance of fifteen feet or more, so that a man looking at himself in this mirror with a stick or sword in his hand, saw the image of them suspended in the air, apparently ready to strike the observer. On seeing such an effect for the first time, the observer could hardly fail to experience the greatest surprise, and even fear; and it is stated that the king having placed himself, sword in hand, before one of these mirrors, in order to observe the effect, was surprised to find himself face to face with an armed hand apparently directed against him. When he advanced, the hand seemed to spring forward to meet him. The king could not conceal his surprise and fright, and afterwards felt so ashamed at being terrified with a mere shadow that he ordered the mirror to be taken away, and could never be prevailed upon to look into it again." The Journal des Savants then goes on quaintly to remark on the various startling effects produced by these mirrors, winding up by stating that its powers of reflection were so great, that at night the light of a torch or flambeau was reflected so perfectly that an observer placed at four hundred feet distant could read the smallest print.
It also mentions a curious piece of superstition on the authority of a scientific writer of the name of Robertson, who states that it happened at Liége. In reading the accounts of these experiments we can see how easily the minds of individuals were affected in those days by the wonderful. It happened while one of Villette's mirrors was at Liége, that the latter end of the summer was somewhat rainy, and great fears were entertained that a bad harvest and dear bread would be the result. Certain evil-minded people, who had taken a fancy to the mirror and wished to possess it by unfair means, spread the report that the continual rain was entirely caused by its action on the clouds and sun, and that the coming famine must be laid upon the shoulders of its owner and inventor. This absurd idea took such forcible possession of the minds of the populace of Liége, that great mobs collected together, uttering all kinds of maledictions against the mirror and its inventor, and at last became so violent that they attacked Villette's house with the intention of smashing his great work, and administering to the unfortunate philosopher the chastisement they supposed he deserved. Happily, however, for M. Villette and his mirror, Liége was governed in those days by the Prince Bishop of Cologne, who was a man of great enlightenment. He put the crowds round M. Villette's to flight by armed force, but he found that the conviction that all the coming mischief would result from the unlucky mirror was so strong, that he was obliged to issue a pastoral peremptorily declaring that the idea had originated with a number of malicious people, who spared no pains to propagate it for their own bad purposes, and that it was a mischievous and dangerous error to ascribe to a mirror a power which only belonged to the Almighty.
In 1747, Buffon performed many extraordinary experiments with burning mirrors, which were more surprising than any that had hitherto been described. They were mostly performed at the Jardin des Plantes, at Paris, of which institution Buffon was director; and many of them are worth describing.
On the 3rd of April, at about two o'clock in the afternoon, the great mirror was mounted on its stand, and was found to be capable of setting a plank of wood on fire at a distance of 138 feet, when 128 glasses were used, although the light was weak at the time, and the sun was covered with mist. In pursuing these experiments great care had to be taken to prevent the by-standers placing themselves within range of its terrible power, for several were nearly blinded by looking
Fig. 25.—Burning Mirror.
at the brilliant focal point of the instrument. The next day, at eleven in the forenoon, although the sun was still covered with mist and fleecy clouds they were able to produce such a heat at 150 feet distant, with 154 glasses, that a pitched plank began to smoulder and would have burnt into flame had not the sun disappeared at that particular moment. On the fifth of April, at three in the afternoon, with the light much in the same weak condition as it was on the other days, they succeeded in igniting at 150 feet distant, a heap of shavings of deal mixed with charcoal and sulphur, in less than a minute and a half, with 154 glasses. When, however, the sun shone with its natural power, a few seconds were sufficient to effect these results.
On the 10th, when the sun was shining pretty powerfully, a pitched pine plank was easily fired with 128 glasses, at 150 feet distant. In this case the ignition was very sudden, and extended over the whole of the radiant spot forming the focus, which at the distance named measured 16 inches in diameter. The same day at half-past two, a pitched elm plank covered in some places with chopped wood, was set fire to with extreme rapidity, and burnt with such violence that it had to be dipped in water before it could be put out. In this experiment 148 glasses were used, at a distance of 150 feet.
On the 11th of April, the burning point was fixed at 20 feet distant from the mirror, and combustible substances were easily burnt with only 12 glasses. With 21 glasses a half-burnt elm plank was set fire to, and with 45 a piece of tin weighing six pounds was almost immediately melted. Silver sheet was fused, and an iron plate was made red-hot with 117 glasses. In giving an account of these interesting experiments, Buffon expresses his conviction that at 50 feet it would have been easy to have melted metals if all the glasses of the mirrors had been used. When used at that distance, the burning spot was six to seven inches in diameter. He also noticed that when metals were melted, part of them were dissipated in brilliant vapour, which was so thick as to cast a shadow on the ground, although it seemed to be as bright as the sun itself. When the sun was at its full strength, and all the glasses were brought into requisition, wood was set on fire at a distance of over 200 feet, and metals and minerals were fused at 40 and 50 feet. Hence the possibility of making and using these mirrors as Archimedes was said to have done, was proved practically by the great naturalist. Fig. 25 represents a burning mirror in action.
Robertson, an English philosopher, residing in France during the days of the first Republic, reconstructed the mirrors described by historians as being used by Archimedes, and the results he obtained were thought sufficiently important by the Council of the Department of Ourthe to merit an attentive examination by two members of their body, who reported in favour of their being used as instruments of war.
It would be possible to pursue this subject still further, and give an account of numerous experiments made on burning mirrors by various philosophers, but we must not forget that it is light and heat that we have more especially to deal with in the present work. Already we have possibly strayed from our path a little too far, but the two influences are so closely connected with each other that it is almost impossible to speak of them separately when reflection is in question.