[ 32 ] 106 
ait* as well as water, being drawn together by as majiy trenailings or 
iiailings as there are]a}ers, except one, and as many thicknesses may 
be given to the bottom or to the sides, it is necessarily strong and du- 
rable. 
There is still another method, however, which was resorted to at 
Venice in the construction of naval docks. That of building a cais- 
son, and then, by the erection of the walls witiiin it cause it to settle, 
and finally to sink, thus loaded, into its place. This method would, 
i;! our case, be attended with more expense, but is not impracticable. 
The caisson might, on that occasion, be built of lamina, as described 
for the lock chamber. 
In a conimsinication of this importance, one lock or entrance of the 
large dimensions necessary would scarcely be a sufficient accommo- 
dation, and two vrould be a guard against interruption from any tem- 
porary accident. 
The adjacent shores furnish an abundance of rock for the construc- 
tion of piers. The sliore is bold, and at the distance of 250 fathoms, 
is deep enough to float any ship that could pass through. 
The engineers lieictoforc employed on the survey of this route, 
traced a line of canal to Back River Harbor, half a mile west of the 
mouth of Monument River. The harbor is found to have 11 to 17" 
feet at low water, the inner harbor eight feet; but there is a sand bar 
between the former and the bay, extending from Toby's Creek to 
jMarshner's Island, which has eight feet at low water. If a channel 
w ere to be cut through this bar, vessels might, of course, enter at all 
tinics; but vessels drawing fifteen feet would be the largest that could 
enter at high w ater safely, and then w ould grwmd at low water. 
Perhaps the depth of this harbor is the best rate for tlie depth of 
the canal, unless the locks should be carried beyond the inner shoal 
and th-e outer shoal removed, when there would be 17 to 20 feet at the 
enti'ance. 
The water from Herring Pond appears to have been relied on in 
some measure to supply the canal. It is supposed to have an area of 
four square miles; at this time the outlet afforded but 1 0^^^^^ cubic 
feet per second. This po)id, however, as a rcsei-voir, may be occa- 
sionally very valuable. If it could be raised, or if the outlet (of which 
tliere is less doubt) were deepened, foursquare miles four feet deep, re- 
duced by evaporation to three feet, would amount to 334,570.200 cubic 
leet. Should the canal be 80 feet wide and eight miles long, its area 
is 3 379,200 square feet; that quantity would raise the surfaca of the 
Canal four feet 24 four times, six feet 16 times. The section of such 
a canal would be 650 feet, the prism 22,256,000. The reservoii* 
would hold 15 times the prism. The upper section of the Languedoc 
caiml for the whole year uses eleven prisms of its contents. 
Applying tlie rule of estimate we have, in the early part of this re- 
port, shown to be according to experience, viz: 58.13 per mil*, per 
