But inasmuch as the prevailing wind along the English Channel is from the south-west, the reader will observe on consulting a chart that the position of Boulogne for the Gallic traders bound for Dover or the Thames was singularly well placed, inasmuch as it gave the mariner a fair wind outward-bound on most occasions. That fact was doubtless appreciated by Cæsar when he elected to use this port as his starting-place for Britain. He therefore gave orders that his fleet was here to be got in readiness, and then sent forth Volusenus in a galley to reconnoitre the British coast. The ship was a Roman galley manned by oarsmen who had been trained by years of work for the task, and with such a craft as this Volusenus could be independent of wind and accomplish his task with the utmost dispatch. He was away cruising about the English Channel for a period of three days, during which time he had doubtless been able to locate a suitable place where his master’s troops could be disembarked. He had had the opportunity of taking soundings, and--perhaps most important of all to one accustomed almost exclusively to the Mediterranean--of noticing both the range of tide and the force and direction of the strong tidal streams. Similarly, he was able to make a note of the cliffs of Dover and other landmarks. With this knowledge he returned to place himself at Cæsar’s disposal. On August 25, then, the transports came out from Boulogne. The time was midnight, it wanted five days to full moon, and high water that evening was at 6 p.m., so that the tides were neaps, or at their weakest. We can be quite sure that, acting on the experience of Volusenus in the Channel, it was deliberately intended to avoid spring tides. (It is high water at Boulogne at new and full moon at 11.28.) The transports thus came out of the haven with the last drain of the ebb. But in the offing the tide that night did not make to the eastward till 4 a.m., so there would be the Channel ebb to contend against for some time.