The levelling also shows, that by adopting 6 _met._, 50, for the bed of the Canal, there will be a length of 18 _kil._ in the Bitter Lakes, where there will not be a shovelful to remove, and for another 18 _kil._ there will be very little to do; and as these lakes are dry at a depth of 8 _met._, 39, below low water, all the earth-works for the whole length of them could be performed in the dry, if found advantageous to do so. The numerous transverse sections taken with the levelling of 1847, enable us to ascertain approximately the superfice of the Bitter Lakes at the water line. This superfice is about 330,000,000 _square metres_. If, then, the action of the tide, which brings two _metres_ of moving water, be admitted into these lakes, a disposable volume of 660,000,000 _cubic metres_ of water would be accumulated, and which might be raised to 800,000,000 by adding Lake Timsah and the retaining basins at Suez and Pelusium to these immense reservoirs. Before pointing out the various directions of the adopted track, it appears necessary to arrive at a fixed opinion as to the formation of the Isthmus and of the downs by which it is partly covered, and also as to the accumulations of sand which exist both on the coast of Pelusium and at the bottom of the Gulf of Suez; for it is from the explanation of these phenomena that we shall start in our justification of the arrangements of the direct track in general and in detail. By attentively observing what is passing before our eyes at the present time, in respect of the destruction and recomposition of continuity, we may come to an exact conclusion regarding the laws which operated towards the first ages of the world in the formation of alluvial lands. Let us first examine what is going on in the English Channel; for this narrow sea having a large number of ports both on the French and English coasts, has on that account been the object of numerous observations by engineers. The first well recognised fact is the destruction of the coast from the point of Barfleur as far as the Somme, a distance of 338 _kilometres_; and on the other side of the channel, from the Isle of Wight to Dover, a distance of 250 _kilometres_. This action is produced by the alternation of frost and thaw, by dry and moist winds, and by the saline evaporation of the sea. The abrasion observed on the coast of Calvados is an average of 0 _met._, 25, _per ann._ and on the coasts of Normandy and England 0 _met._, 30. The mean height of the cliffs on either side being sixty _metres_, it follows that the channel swallows up an amount of 10,000,000 _cubic metres_ of earth and stones every year, which must find a place somewhere.