M. P.S. Girard on Navigable Canals. °89 
D would express the draft of water of a single ascending 
boat ; thus we have 
+D, 
for the generalexpression of the expenditure of water from 
an upper level of a lock, when a boat whose draft of wa- 
ter is expressed by D passes that lock in ascending or in 
descending. 
It is often difficult and expensive to accumulate on the 
summit level of a canal, a volume ofwater sufficient tosup- 
ply the deficiency occasioned by evaporation, filtration and 
the passage of the locks ; whilst its lower level, almost al- 
ways confounded with a river, naturally contains a greater 
or less quantity of superabundant water. The essential ob- 
ject of the theory should therefore be to seek the means, if 
rimitive height of water contained in the upper level will 
augmented by the quantity 
g Lf $ (D— 
\—a) , 
ot. VII.—No. 2. 
