48 TECHNOLOGY. 
covered with laths. In order to have the means of regulating the height of 
water, frames with flood-gates, a a! a” (figs. 17 and 18), are built on top, 
and between these frames and the sides bulkheads are built to the height 
which the water is to assume when the gates are closed. 
Stone overfall-dams are built massively of heavy dressed stone, on a 
foundation of piles and grillage, unless the bed of the river is rocky. The 
stones are clamped together by brass clamps, to prevent their being dis- 
placed separately. A stone dam, built by Smeaton, is represented on pi. 
10, jig. 15 being a top view, jig. 16 one half of a longitudinal section, and 
jig. 17 a cross-section. 
Between the two slopes of stone dams an open space is sometimes left, 
which is lined with two rows of closely fitted planks, and then filled up with 
rammed clay, in order to oppose an impermeable barrier to the water which 
may pass through the joints of the walls. Stone dams are protected against 
the undermining action of the water in the same manner as described in 
speaking of wooden dams. The form of an arch, with the convexity up 
stream, is often given to stone dams (pl. 10, jig. 15), by which they are 
enabled better to resist the action of the current. 
B. Canals. 
Canals are open trenches filled with water from lakes, streams, or springs, 
to a sufficient depth to bear loaded vessels, thus affording a means of inland 
navigation. They are formed either by excavations in the solid earth, or by 
embankments upon it. In some cases aqueducts are built, of which we shall 
treat separately. 
For the invention of canals we scarcely know to whom or to what age we 
are indebted, such is their antiquity. The most ancient vestiges seem to 
exist in Egypt, where a canal was once undertaken to connect the Red Sea 
with the Mediterranean. Other canals of antiquity still remain ; for instance 
the Yussuff Canal, and others in Persia and Afghanistan, where they had 
reached great perfection, and where canals had been constructed under 
ground for miles in length. We also find ancient canals on the Tigris and 
Euphrates. The Greeks and Romans did very little in the construction of 
canals. Charlemagne was the first to plan the connexion of the Danube 
with the Rhine, which work was commenced under him, but completed 
only in modern times. Within the last three hundred years canals have 
been constructed in all civilized countries, the Dutch, English, and French 
leading the way in improvements in the system of construction. 
Excavation is the simplest and cheapest method of forming canals, and 
is resorted to wherever existing conditions make it possible; but few cases 
occur where any great length is obtained without embankments, which 
become necessary when a shallow stream is formed into a canal, and when 
a canal is carried along the side of a hill or across low or marshy land. 
Embankments are mostly formed of rammed clay, and when they attain 
a considerable height the outside slope at least is protected from washing 
and caying by a stone wall of dry masonry. In cases of great height both 
slopes of the embankment are formed of stone walls, while the space between 
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