120 AM. Girard on Navigable Canals. 
whatever be the expense of rari and the draft of the as- 
cending and the descending boat 
But we have already conctited from the equation which 
expresses the relation between these quantities, that if the 
lift of the lock be made equal to the difference in the 
drafts of water of the two boats, the expense of water from 
the r level is nu 
In this particular case, the expense of active force neces- 
sary for ee passage of the two boats is therefore as it were 
repaid the descending boat, in the same manner as if 
this boat in descending on an inclined plane, drew the oth- 
er boat up along the plane, at the same time, by means of 
a chain passing over a pulley. In the same manner, when 
shia lift of the lock is less than the difference of draft of wa- 
ter of the two boats, we have seen that a certain volume of 
water was raised from the lower to the upper lavel ; in this 
case the active force of the descending boat is not only em- 
Leta in ree the icwagee boat but in raising at the same 
clined plane, and a- certain volume of water was Maded to 
the weight of the lightest boat. 
_ We must also observe that the quantity of active force 
expended by the descending boat to raise a mass of water 
into the higher level, is not deducted from the ‘uselub mn 
movement, offer this singular advantage, to the exclusion 
of all other machines, that the expenditure of active force 
necessary to the production of movement is in itself a por- 
tion of the useful effect which the machine i is intended to 
Po ob obtain this advasieng e, it istrue, the following; condi 
$ are necessary ; Ist. the of water of the boats 
which descend should be greater than that of the boats which 
ascend; 2ndly, That the rise and fall of the locks should 
in no case exceed that difference of draft of water. =~ 
Tis evident that the last condition can alwayebe be fulfilled 
first shall exist : and, | ‘in determin- 
ing the quantity of water necessary to supply a pater it has 
