It is also said that Archimedes set the Roman ships on fire by means of burning mirrors, composed of a combination of plane mirrors, adjusted so as to reflect all the incident rays of light to the same point. The possibility of this has several times been the subject of inquiry to modern philosophers. Kircher took so much interest in the subject, that he went to Syracuse expressly to inquire into the probable position of Marcellus’s fleet, and he arrived at the conclusion, that it might have been within thirty yards of the walls. Buffon’s experiments, made as well as those of Archimedes with a combination of plane mirrors, are conclusive as to the facility of setting tarred fir plank on fire at a distance of one hundred and fifty feet, and the possibility of doing it at considerably greater distances. Similar planks, and even more combustible materials, were precisely what Archimedes had to deal with. He is said to have operated in this way at the distance of a bow–shot, in which there may very probably be exaggeration. The sequel of the siege contains no matter of interest. Syracuse was taken by surprise through the negligence of the guard, and Archimedes is said to have been slain by a soldier, as he was deeply intent on the solution of a problem. Lines of circumvallation continued long to be the principal means employed by the Romans in the reduction of strong places. Even the inventive genius of Cæsar does not appear to have devised the means of dispensing with this tedious and most laborious process. In his Gallic wars he had frequent recourse to it, though the Gallic fortifications, it might be thought, could not be of the most formidable description; and the siege of Alesia furnishes one of the most remarkable instances of it on record. The town stood on an eminence, surrounded on three sides by hills of equal height, at a moderate distance: in front extended a plain, three miles in length. Round the foot of this eminence he dug a trench, twenty feet in width; and again, at an interval of 400 feet, two more, of which the inner one was filled with water: behind them he built a rampart twelve feet high, crowned with battlements, and strengthened with towers at intervals of eighty feet; and, more effectually to confine the besieged, and enable a smaller force to guard the works, the space between them and the inner ditch was filled with three distinct rows of obstacles. The first consisted of a sort of abattis, made with large branches of trees, with the ends squared and sharpened, set firmly in the earth (_cippi_). The next were called lilies (_lilia_), from their resemblance to the calix of that flower, with its upright pistil: these were circular cup–shaped cavities, three feet deep, with a sharpened stake in the centre, projecting about four inches above ground, and covered over with brushwood to deceive assailants. Still nearer to the town iron hooks (_stimuli_, like the Scottish _calthrop_, often used with effect