54 TECHNOLOGY. 
We have thus far confined our remarks to locks of a single water-way, 
which pass only one boat at a time. Where navigation is frequent, double | 
locks are built, consisting of two separate chambers side by side, by which 
arrangements two boats can be locked through at the same time, either in 
the same or in opposite directions. Double locks of the best and most 
durable construction have been built on the Erie Canal in the State of New 
York. 
D. Aqueducts. 
When a canal meets in its course with a river or ravine, it must be car- . 
ried across on a bridge, which differs from ordinary bridges only in the 
superstructure, which embraces the canal and tow-path. As a specimen of 
a stone aqueduct, we give on pl. 11, jigs. 6, 7, and 8, a side-view, cross- 
section, and top-view of the Cesse Aqueduct, designed by Vauban for the 
Languedoc Canal. The water-way is frequently carried across the bridges 
in wooden trunks; of this there are many examples in the United States, 
where wooden aqueducts have been more extensively constructed than else- 
where. 
The first aqueduct of cast-iron was the Chirk Aqueduct on the Ellesmere 
Canal, built in 1795, by Thomas Telford, who, encouraged by its success, con- 
structed immediately afterwards, on the same canal, the Dee Aqueduct, in 
the valley of Llangollyn, 127 feet above the bed of the Dee, and 1000 feet 
in length. It consists of 19 arches of cast-iron, abutting on stone piers (i. 
10, jig. 10). Each arch consists of four ribs, as shown in the cross-section 
(jig. 11), secured against lateral motion by connecting-plates (jig. 18). 
An abutting plate or skew-back is shown in jig. 12. The bottom plates 
(jig. 14), as well as the side-plates, are firmly connected by flanges, and are 
made water-tight by iron cement. The position of the tow-path is seen in 
jig. 11. The canal is 12 feet wide, and passes boats of 7 feet in width. 
The carrying of canals across rivers is not the only object of aqueducts. 
They have been built since the remotest times for the purpose of conveying 
water into cities. The Egyptians, Greeks, and Romans had large structures 
of that kind, and they continue to be built in modern times. The Croton 
Aqueduct, by which the city of New York is supplied with water, com- 
pleted in 1842, under the direction of J. B. Jervis, is the most gigantic 
modern work of the kind, of which we will here give a description. 
It was constructed at the expense of the city of New York, and cost 
about twelve millions of dollars. The conduit commences at the Croton 
river, in Westchester county, where a dam has been constructed which 
raises the water of that stream 40 feet above its natural level, and 116 feet 
above mean tide, setting back the water of the river about 5 miles, and 
forming a reservoir of about 400 acres surface. The aqueduct runs down 
the valley of the Croton to the shore of the Hudson, which it leaves again 
at the village of Yonkers, and, crossing the valley of the Sawmill river and 
Tibbitt’s Brook, gains the summit between the Hudson and East Rivers, 
and continues on it to the Harlem River, a distance of 83 miles of con- 
tinuous masonry. Iron pipes are then laid 1450 feet on an arched bridge 
634 
