Note 2813 — PORTUS ITIUS

Caesar started ‘about sunset’ (ad solis occasum): the wind dropped ‘about midnight’ (media circiter nocte); and the drift ceased at daybreak (orta luce). The sun set at 8.16 p.m.; and day broke about 3.20 a.m. It would be absurd to suppose that the voyage must have begun at the moment when the sun dipped under the horizon: we may fairly assume that it began at any time between 7 and 8. Similarly the drift may be assumed to have begun at any time between 11 p.m. and 1 a.m. Let us suppose that the wind lasted 5 hours, and the drift 3 hours. For some time before the wind dropped it must have been gradually dying down; but, as the vessels kept steerage way, it may be assumed, so Captain Iron, the harbour-master at Dover assures me, that, even during the drift, there was not a dead calm. Major Rennell, indeed, affirms (Archaeologia, xxi, 1827, p. 503) that when the wind dropped the ships were ‘left to the resources of their oars’: but Caesar does not confirm this; and if the oars had been used, why should the ships have drifted out of their course? Captain Iron says that with a light south-westerly wind the ships could easily have sailed 6 knots an hour. The voyage took place about the time of new moon (see pp. 728-30, infra), that is to say, a day or two before spring tide. For the first two hours the ships had to encounter the ebb tide, the rate of which, however, was not more than one knot an hour; and the flood, the rate of which increased from about three-quarters of a knot to nearly 3 knots, helped them from about 9 or 9.30 p.m. We may estimate, then, that in the 5 hours they sailed not less than 25 knots; while in 3 hours, aided by a faint breeze, they would have drifted about 6 knots (see p. 656, infra). I think, then, that the entire distance which Caesar sailed up to the moment when he ‘saw Britain lying behind on the port quarter’, may be estimated at not less than 31 knots, or about 57½ kilometres. After making this calculation, which must be taken for what it is worth, I find that, according to Napoleon’s map (Hist. de Jules César, Atlas, pl. 16), the distance was 57 kilometres. The reader may check my estimate by referring to the Admiralty Tide Tables, pp. 112-9, and Tidal Streams, English and Irish Channels.