M. Girard on Navigable Canals. 115 
ing the lift of the locks. The principles on which this con- 
clusion is founded are evident ; the calculations which jus- 
tify it are simple and easily verified. And yet it appears to 
have hitherto escaped the attention of engineers who have 
occupied themselves in projecting or in constructing canals. 
It is in the natural progress of our mind, and in the slowness 
with which certain branches of knowledge are propagated, 
that we must look for the cause of the seeming abandon- 
ment in which the questions which form the object of the 
present memoir have remained. 
The first inventors of canal locks, as well as those who 
first constructed them, captivated as they must have been 
by the effectof this ingenious contrivance, attributed to it so 
much the more merit as itserved to surmount a greater difli- 
culty at a single step. 
If, as we are assured, the first locks were constructed in 
the Venetian States, on a canal derived from the Brenta, 
‘no apprehensions need have been entertained of a want of 
water, since the canal was supplied by ariver; besides, to 
make any exact calculations of the expense of water at the 
passage of the locks, it was necessary that the physical sci- 
ences should have progressed farther than they had done 
before the days of Galileo, and that the imperfect notions then 
possessed of those sciences should have been more general- 
ly disseminated among the mechanics of those times. 
It is easier to imitate what has been already done in hy- 
draulic constructions, than to improve them, or even to ac- 
count for certain practices which usage seems to have ap- 
proved and consecrated. 
Every one knows that one of the principal obstacles to 
the execution of the canal of Languedoc, was the difficulty 
of collecting on the summit level a quantity of water suffi- 
cient for the service of the locks, and the navigation of the 
canal. It was therefore of the highest importance to econo- 
mise the water that could be procured. The means were 
easy, and consisted only in diminishing the lift of the 
locks. Nevertheless we learn from Mr. Gauthey that the 
first locks constructed were of so great a lift that it was 
necessary to demolish them, and substitute others of a less 
lift in their stead, even before the navigation of the canal was 
opened, and this because the great pressure of water which 
they had to support, subjected all their parts to too much 
