M. Girard on Navigable Canals. 103 
2dly. Of that which is lost by filtration through the bot- 
tom and sides of the canal; and 
3dly. Of that which is necessarily expended at the locks 
ia raising or lowering the boats. 
_ Evaporation is a natural effect, and which no art can 
counteract : the loss arising from this cause Is. therefore in- 
evitable. 
Whatever be the nature of the ground through which a 
canal is intended to pass, the loss by filtration may always 
be greatly diminished, or even entirely prevented, by re- 
course to suitable means. 
There remains then the expense occasioned by the move- 
ments at the locks, and this portion is generally greater than 
the losses by evaporation and filtration together: therefore 
when the subject of making a canal is under consideration, 
it is necessary, in the first place, to make sure ofa sufficient 
quantity of water on the most elevated point of its direction, 
to supply the uses of that navigation to which it is destined. 
The impossibility of fulfilling this first condition has often 
prevented the execution of canals, which, could they have 
existed, would have contributed powerfully to the advance- 
ment of agriculture and the prosperity of commerce in cer- 
tain provinces.. We have seen other canals which answer 
but imperfectly the object for which they were intended. 
because the water collected for their use could suffice for 
their wants only during a few months of the year. For this 
reason many engineers and mechanicians, both in France 
and England, have endeavoured to discover some means of 
obviating the difficulty of a deficiency of water in navigable 
canals. ‘Thus the moveable locks of Soldages, the inclined 
planes of Fulton, the wheel boats of Chapman, the floating 
gates (ecluses a flotteur) of Bettancourt, and more recently 
the Pneumatic locks of Congreve, have been successively 
imagined ; but, however ingenious these means may ap- 
pear in theory, they require in practice, the application of a 
force with which we may always dispense, wherever the 
boats can be kept naturally afloat, and can circulate in the 
canal without any other obstacle than that of passing the 
locks as they were first invented. 
On the other hand, these inventions are practicable only 
on small canals ; and, where fuel is plenty, the least ex- 
pensive mode of supplying a deficiency of water, is to raise 
