The configuration of the coasts is perhaps the chief cause of the peculiarities of tides in the Channel. From the entrance as far as Portland Bill the time of high water is found to be progressively later in passing from west to east, being influenced by the oceanic tidal stream from the west under conditions which are on the whole normal. But eastward of a line between Portland Bill and the Gulf of St Malo these conditions are changed and great irregularities are observed. On the English coast between Portland Bill and Selsey a double tide is found. At Portland this double tide corresponds approximately with the time of low water in the regular tidal progression, and the result is the occurrence of two periods of low water, separated by a slight rise known locally as "gulder." But farther east the double tide corresponds more nearly with the time of high water, and in consequence either the effect is produced of a prolonged period of high water, or there are actually two periods of high water, as at Southampton. Various causes apparently contribute to this phenomenon. The configuration of the coast line is such as to present at intervals barriers to the regular movement of the tidal wave (west to east), so that reflex waves (east to west) are set up. In the extreme case at Southampton the tidal effect is carried from the outer Channel first by way of the Solent, the strait west of the Isle of Wight, and later by way of Spithead, the eastern strait. Finally the effect of the tidal stream entering the Channel through the Strait of Dover from the North Sea must be considered. The set of this stream towards the Strait of Dover from the east corresponds in time with that of the Channel stream (i.e. the stream within an area defined by Start Point, the Casquets, Beachy Head and the mouth of the Somme) towards the strait from the west; the set of the two streams away from the strait also corresponds, and consequently they alternately meet and separate. The area in which the meeting and separation take place lies between Beachy Head and the North Foreland, the mouth of the Somme and Dunkirk. Within this area, therefore, a stream is formed, known as the intermediate stream, which, running at first with the Channel stream and then with the North Sea stream, changes its direction throughout its length almost simultaneously, and is never slack. Under these conditions, the time of high water eastward of Selsey Bill as far as Dover is almost the same at all points, though somewhat earlier at the east than at the west of this stretch of coast. The configuration of the French coast causes a very strong tidal flow in the Gulf of St Malo, with an extreme range at spring tides of 42 ft. at St Germain, compared with a range of 12 ft. at Exmouth and 7 ft. at Portland. In the neighbourhood of Beer Head and Portland and Weymouth Roads the streams are found to form vortices with only a slight movement. On the eastern