Titus Livius (Livy), The History of Rome, Book 1
Benjamin Oliver Foster, Ph.D., Ed.

36. He was also preparing to build a stone wall around the City, when a Sabine war interrupted his plans. And so sudden was the invasion, that they had crossed the Anio before the Roman army was able to march out and stop them, so that the City was thrown into a panic. [2] The first battle was indecisive, with heavy losses on both sides. The enemy then withdrew into their camp, affording the Romans an opportunity to renew their preparations for the war. Tarquinius believed that cavalry was what he chiefly lacked. To the Ramnes, Titienses, and Luceres, the centuries which Romulus had enrolled, he therefore determined to add others, and to give them his own name as a permanent distinction. [3] But since this was a matter in which Romulus had obtained the sanction of augury before acting, it was asserted by Attus Navius, a famous augur of those days, that no change or innovation could be introduced unless the birds had signified their approval. [4] The king's ire was aroused by this, and he is reported to have said, in derision of the science, “Come now, divine seer! Inquire of your augury if that of which I am now thinking can come to pass.” When Attus, having taken the auspices, replied that it would surely come to pass, the king said, “Nay, but this is [p. 133]what I was thinking of, that you should cleave a¹ whetstone with a razor. Take them, and accomplish what your birds declare is possible!” Whereupon, they say, the augur, without a sign of hesitation, cut the whetstone in two. [5] There was a statue of Attus standing, with his head covered, on the spot where the thing was done, in the comitium, even at the steps on the left of the senate-house; tradition adds that the whetstone also was deposited in the same place, to be a memorial of that miracle to posterity. [6] However this may be, auguries and the augural priesthood so increased in honour that nothing was afterwards done, in the field or at home, unless the auspices had first been taken: popular assemblies, musterings of the army, acts of supreme importance —all were put off when the birds refused their consent. [7] Neither did Tarquinius at that time make any change in the organization of the centuries of knights. Their numerical strength he doubled, so that there were now eighteen hundred knights, in three centuries. [8] But though enrolled under the old names, the new men were called the “secondary knights,” and the centuries are now, because doubled, known as the “six centuries.”