i18 M. Girard on Navigable Canais. 
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 asa means of communication. The lift of the 
second lock from the summit level must therefore be less 
than 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 diminishing in like 
manner til 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 pre- 
cede the locks respectively. 
When, on the contrary, fresh supplies of water can be 
provided to replace the water lost by evaporation and filtra- 
tion, as the canal descends in the plains, it is evident that 
the supply of water from the first feeder wil! allow the lock 
immediately below it to have a higher lift than 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 hft 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 level; whence we see that, 
taking into account the losses occasioned by evaporation 
and filtration, a navigable locked canal should be consider- 
ed as asystem of partial canals, each extending from one 
feeder to another, and in each of which the lifis should di- 
minish from its highest to its lowest level. 
The locks situated at the origin of these partial canals 
may have a greater lift in proportion as they are farther from 
the summit level, in all cases where the quantity of water 
they receive from their feeder is greater than their loss by 
evaporation and filtration ; the locks at the origin should, on 
the contrary, have a smaller lift when these losses are not 
compensated by fresh supplies. 
In general, if we suppose all the levels of a navigable ca- 
nal once filled with a sufficient depth of water to float the 
heaviest loaded boats, in order to maintain that depth con- 
