August 25: Britain Saved by a Full Moon (Vol. 30, pp. 27-33)

On this day in 1882, Sir William Thomson Kelvin delivered a lecture on tidal patterns, which is our reading today.

We'll let Lord Kelvin, who had one of the most creative and wide-ranging scientific minds of the 19th century, take over from here as he discusses how little the Romans knew about tides and how that knowledge foiled Julius Caesar's attempt to conquer Britain.

It is curious to look back on the knowledge of the tides possessed in ancient times, and to find as early as two hundred years before the Christian era a very clear account given of the tides at Cadiz. But the Romans generally, knowing only the Mediterranean, had not much clear knowledge of the tides. At a much later time than that, we hear from the ancient Greek writers and explorers—Posidonius, Strabo, and others—that in certain remote parts of the world, in Thule, in Britain, in Gaul, and on the distant coasts of Spain, there were motions of the sea—a rising and falling of the water—which depended in some way on the moon. Julius Cæsar came to know something about it; but it is certain the Roman Admiralty did not supply Julius Cæsar’s captains with tide tables when he sailed from the Mediterranean with his expeditionary force, destined to put down anarchy in Britain. He says, referring to the fourth day after his first landing in Britain—”That night it happened to be full moon, which time is accustomed to give the greatest risings of water in the ocean, though our people did not know it.” It has been supposed however that some of his people did know it—some of his quartermasters had been in England before and did know—but that the discipline in the Roman navy was so good that they had no right to obtrude their knowledge; and so, although a storm was raging at the time, he was not told that the water would rise in the night higher than usual, and nothing was done to make his transports secure higher up on the shore while he was fighting the Britons. After the accident Cæsar was no doubt told—”Oh, we knew that before, but it might have been ill taken if we had said so.”

If the Roman navy was a little more on the ball, the whole course of history might have changed.