M. P. S. Girard on Mivigable Canals, 315 



evaporation and the filtrations absorb one fifth part of this 

 reserve, there will remain 4,864,000 tons of water for the 

 purposes of navigation alone. 



By the application of our principles to the lift of the 

 locks, and the draft of water of the boats, we found that the 

 expense of water necessary for that same navigation, may be 

 reduced to 200,000 tons; whence it appears that at least 

 II of the water specially reserved forthe use of this canal are 

 consumed in pure loss, and yet, the navigation on this canal 

 is often interrupted for several months in the year for the 

 want of water. 



This imperfection of the canal de Briare, inevitable 

 consequence of the excessive lift of its locks, compared 

 with the small draft of water of the boats which frequent 

 it, is also common to some other canals of more recent 

 construction. 



The relation between the lift of the locks of a navigable 

 canal, and the draft of water of the boats which navigate it 

 ascendiiig find descending through those locks, constitutes 

 as it were the regime of that canal ; and this regime is es- 

 sentially variable. It depends not only on the disposable 

 volume of water in different localities, but also on the terri- 

 torial or manufacturing resources of the countries through 

 which it passes, and on the nature of importations into 

 these countries. 



Thus, in passing a canal in a direction where it could re- 

 ceive directly a large mass of the productions of mines, 

 quarries, forests, vineyards &c. which would descend by it 

 into some valley where the canal should terminate, a 

 much smaller quantity of water would be necessary than 

 though this canal should traverse a country less produc- 

 tive of articles which would be advantageously exported; 

 and as the difficulty of procuring a sufficient quantity of 

 water on the summit level, has heretofore been the prin- 

 cipal obstacle to their construction in many places, we can 

 judge of the importance of our theory since it enables us to 

 elude this obstacle, and points out the way in which the 

 execution of navigable canals becomes easiest, precisely in 

 the places where they will be most useful. 



The better we are acquainted with the nature and ex- 

 tent of the exportations and importations of any section of 

 country, the better we shall be enabled to improve the 

 regime of an artificial canal which is to traverse it : thus we 



