116 M. Girard on Nanigable Canals. 
damage. ‘The economy of water did not enter at all into 
the calculation which produced this change ; although this 
would have been the most powerful motive had the expense 
of water at the locks been submitted toa vigorous analysis. 
The most celebrated Engineers of France and England 
have continued to contribute to the maintaining of ancient 
practices in this species of construction. 
We read in a memoir of Mr. Perronet, on the canal of 
Burgundy, that the greater or less space occupied by a boat 
in the lock of a canal, has no influence on the quantity of 
water expended for the passage, either in ascending or de- 
scending, and that there is therefore no reason for diminish- 
ing the lift of the locks, which, he says, is generally eight, 
ten or twelve feet. 
This opinion, advanced by an engineer so justly renowned, 
has never been contradicted ; and if we may be permitted 
to judge from the canals which have been constructed since, 
it has been continually admitted without discussion or ex- 
amination. 
It is true that Mr. Gauthey, in the memoirs above cited, 
remarks that the locks of a canal which has a summit level, 
should not be of equal lift, that the smallest lifts should be 
nearest the summit level, and that their height may be aug- 
mented as the facility of supplying the expense by feeders 
increase. But Mr. Gauthey has not formally distinguished 
in what precise circumstances, nor under what restrictions 
this variation may take place; and although his idea indi- 
cates some notion of the existence of a certain relation be- 
tween the lift of the locks and the quantity of water required 
for their service, he did not turn his attention to the exact 
determination of that relation. 
He merely observes that the greatest lifts in common use 
are generally of the height of the 3" 90° (12 feet 9 inches, 
English or thereabouts,) and the smallest 1" 30° (4 feet 3 
inches). Whence he concludes that the most eligible 
height is 2" 60° (about 8 feet 6 inches) as being an arith- 
metical mean proportional between the two extremes in 
common use: this is the only rule on the subject which he 
nas deduced from his enlightened and extensive practice, 
and from the numerous observations with which his impor- 
tant work is filled. 
Let us now establish the principles which should serve us 
