The Druids taught the immortality of the soul, and its transmigration from one body to another, till, by some extraordinary act of virtue or courage, it merited to be received into the assembly of the gods. Cæsar, in his "Commentaries," also informs us that they instructed their pupils in the movements of the heavenly bodies, and the grandeur of the universe. Their knowledge of mathematics must have been considerable, since we find it applied to the measurement of the earth and stars. In mechanics they were equally advanced, judging from the monuments which remain to us. Of these, the most remarkable in England are Stonehenge, consisting of 139 enormous stones, ranged in a circle; and that of Avebury, in Wiltshire, which covers a space of twenty-eight acres of land. But the largest of all the Druid temples is situated at Carnac, in the department of Morbihan, in France. It is formed of 400 stones, varying from five to twenty-seven feet in height, and ranged in eleven concentric lines. It should be mentioned, however, that some authorities consider these erections to belong to a period anterior to the arrival of the Celts in Europe, though they were probably utilised by them. [Illustration: STONEHENGE FROM THE NORTH-WEST. (_From a Photograph by Frith & Co, Reigate._)] [Illustration: STONEHENGE (RESTORED). (_From the Model in the Blackmore Museum, Salisbury, after the Restoration by Dr. Stukeley._)] Such was the country and such the condition of its inhabitants when in 55 B.C. Cæsar undertook its invasion, to which he was led not so much by the thirst of dominion as by the necessity he found himself under of doing something to acquire a great name at Rome. He had already partially subdued the Gauls, and determined on striking a blow at Britain. Having decided on the expedition, the victorious general commenced his preparations with his accustomed energy. His first care was to obtain hostages from the Gauls: he questioned the merchants and others who had visited Britain as to its resources and extent, the natives which inhabited it, their manners, customs, and religion, and sent Commius, whom he had created King of the Atrabates in Gaul, to demand the submission of the islanders. [Illustration: DRUIDS INCITING THE BRITONS TO OPPOSE THE LANDING OF THE ROMANS. (_See p._ 6.)]