MEANS OF COMMUNICATION. 49 
them is filled with clay, which is wetted and worked into a mass of the 
consistency of potter’s clay, and well rammed down, an operation termed 
puddling. 
The two sides or banks of the water-way are sloped; in hard and clay 
soils the slope need not exceed 1} base to 1 rise ; but in softer soils a greater 
slope must be given, and in loose soil the banks must be well protected by 
fascines, piles, or stone walls, from the washing caused by the fluctuation of 
the water attending the passage of the boats. The depth and width of the 
canal depend of course on the size and capacity of the boats intended to be 
put upon it. The width at bottom should be sufficient to permit the passage 
of two boats abreast without their rubbing against the side slopes. 
If a canal is to be navigated by steam, both the slope of the banks and 
the depth should be increased, and the protection of the banks made more 
permanent, as the washing caused by the wheels and the rapid passage of 
the boats is very great. Most canals, however, are navigated by means of 
horses or mules, for which purpose the top of one bank is formed into a 
road called the tow-path, which continues uninterruptedly, occasionally 
changing sides by means of. bridges. The surface of the tow-path is formed 
either of hard sandy clay or of small broken stone; it should be from eight 
to twelve feet wide, to admit readily of the passage of two horses and 
riders abreast, and should have a lateral slope from the canal to shed off 
the rain-water. At suitable intervals drains are constructed to carry off 
the water either into the canal or away from it, as opportunity may 
serve. 
It is seldom practicable to continue a line of canal on the same level for 
a great distance. The points to be joined by the canal are frequently on 
different levels, and hills or valleys intervene on the route, which it is 
impossible or too expensive to cross on the same level by means of excava- 
tion or embankment. In such cases successive portions of the canal are 
built on different levels, the boats passing from one level or veach into the 
other by means of locks, of which we shall treat fully below. 
A longitudinal slope should be given to the bottom of a canal sufficient 
to cause a moderate current of water, not exceeding three feet per second. 
If the current is too slow the water is liable to become stagnant in places, 
and to accumulate mire and rubbish to a great degree; if too fast, the 
expenditure of water and the resistance to the boat is too great. In long 
reaches and on hill sides, openings are left at intervals in one of the banks, in 
which dams are built even with the water-line of the canal, for the purpose 
of discharging the surplus water occasioned by rains and lockage. These 
dams are called waste weirs, and are constructed either of wood or stone; 
they retain the water at the required height, but allow it to flow over in 
case of arise. They are also provided with draw-gates to act as drains for 
the purpose of carrying off mire and rubbish, and of emptying a reach in 
the canal when necessary for repairs. 
Among the most important canals in the world is that of Languedoc, in 
France, which connects the Mediterranean Sea with the Atlantic Ocean, 
and thus avoids the dangerous passage of the Straits of Gibraltar. It was 
629 
