In designing a barge canal the chief considerations generally are that it shall not be in such low ground or so near a river as to be liable to damage by floods, that it shall not traverse very permeable soil or gravel--this is often found near a river,--that the material excavated shall be as nearly as possible equal to that required, at the same place, for embankment, and that as far as possible high embankments, which are very expensive to construct and are more or less a source of danger, shall be avoided. The side slopes of the banks of a navigation canal depend on the nature of the soil. They are generally 1½ to 1, but the inner slope may be 2 to 1. The banks are generally 1½ or 2 feet above the water-level, the width of the bank on the towing-path side ranging from 8 to 16 feet, but being generally 12 feet and the width of the other bank 4 to 6 feet. The width of a canal is made sufficient for two boats to pass, and the depth is 1½ to 2 feet greater than the draught of the boats used. In some cases the banks are protected by pitching for short lengths, but generally they are merely turfed. The sides near the water surface wear away, so that the side slope becomes steeper in the upper part and flatter in the lower part. The resistance of a boat to traction in a canal is given by the formula R = _r_(8·46)/(2 + (A/_a_)), where _r_ is the resistance in a large body of water and A and _a_ are the areas of the cross-sections of the canal and of the immersed part of the boat. When A is six times _a_, R is only 6 per cent. more than _r_. In practice A is never less than six times _a_. Regarding methods of protecting banks, see CHAP. VI. A ship canal is a barge canal on a large scale. The speed of ships has to be strictly limited to avoid damage to the banks. The Manchester Ship Canal takes in the waters of the Irwell and the Mersey, and conveys them for several miles. It is thus a canalised river for part of its course. Below that it is a tidal stream, the tide being admitted at its lower end where it joins the estuary of the Mersey, and passing out higher up where it leaves the estuary after skirting it. This circulation of water is beneficial to the estuary. The Panama Canal might have been constructed at one level, but the cost of this, and the time occupied, would have been double that of making it a summit canal. The water of the river Chagres is to be impounded to form a lake of great extent that will not only supply water for lockage but will itself form part of the high-level reach of the canal, and ships will be able to traverse it at greater speed than in the rest of the canal. Some Indian irrigation canals have been constructed so as to be navigable. The increase in cost has usually been enormously in excess of any resulting benefits.