118 M. Girard on Navigable Canals. 
wholly applied to the use of the following lock. Hence it 
becomes necessary to diminish either the lift of the second 
lock, or the depth of the water it contains. 
The preservation of a given depth of water in each stage 
of a canal is an indispensable condition of the existence of 
such canal as a means of communication. The lift of the 
second lock from the summit level must therefore be less 
thon that of the first 
_ For the same reasons the lift of the third lock must be 
less than that of the second, and go on as in like 
manner till the last and lowest. 
Therefore when the locks of a canal can only be suppli- 
ed with water from the summit level, the lift of the locks 
respectively, should diminish as they recede from that level; 
and if the ground be homogenous, these diminutions should 
be in exact proportion to the length of the level which ae 
cede the locks respectively. 
When, on ee fresh supplies of water — be 
replace th and fi 
baat taataont” f 
e canal descends in the | Plains itis siden that 
ny ot wraxst ivinea the Hit $e der the loc 
esta A below it to have a higher lift thas the one which - 
immediately precedes the introduction of such supply; from 
this point to where another feeder can furnish a fresh sup- 
ply of water, the lift of the locks must diminish as before, 
from the first feeder to the second; from the second to the 
third, and so on to the lowest levels whence we see that, 
taking into account the losses sioned by oe 
and filtration, a navigable ocho obaas should be consi 
ed as asystem of partial canals, each extending” from one 
Raaivess another, sania in Sheb of secant the lifts msn: ae 
evaporation and “ion tei eer at the origin “boul 
the contrary, sry hae a smaller Tift when these losses are not 
nsated supplies. 
In or. if we suppose all te tevals. obi a navigable ¢a- 
al once filled with a sufficient depth of water to float the 
heaviest loaded boats, in order to maintain that depth con- 
