O.F CONVEYING BOATS OR BARGES ON CANALS. 237 



be difplaeed out of the refer voir into the lock muft be equal 

 in cubical content to that of the fpace within the lock between 

 the furface of the lower line and that of the upper, it follows 

 that if the area of the refervoir be greater than that of the 

 lock the neceilary depth of the water to be difplaeed will be 

 fo much the lefs than in the cafe before dated ; and e contra if 

 the area of the refervoir be lefs than that of the lock the ne- 

 cefiary depth of the water to be, difplaeed muft be fo much 

 the greater : fo that whether this proportional quantity of 

 water is to be obtained by the greater depth or the greater 

 area of the refervoir, is a point for the conn* deration of the 

 artift. 



There muft be a communication fomewhere near the bottom and contrmum- 

 between the lock and refervoir, that the water in each may catxng Withlt > 

 be always on a level ; which level may by means of the lower / 

 fluice always correfpond with that of the water in the lower 

 canal, except during the actual operation of raifing or finking 

 a boat within the lock. 



To the inlide of the refervoir muft be fitted a * folid body in which a 

 Or plunger, in fpecific gravity fomewhat exceeding water, flikTordll 

 and of fuch a bulk and form as will exactly fill the whole of prefled. 

 the refervoir, allowing only fufficient room for the plunger to 

 move freely up and down therein. 



Let us now fuppofe the refervoir to be 16 feet deep, and Method of ope- 

 by means of the' communication with the lower canal to be r f' ,on : J s tj? e 

 precifely half full of water. Let us alfo fuppofe the plunger tne boat rifes and 

 to be fo fufpended by machinery as to be movable up and th e contrary, 

 down within the refervoir, and to be barely above the furface 

 of the water, when it, [the plunger] is at its greateft height. 

 And now let us fuppofe that a boat or barge is floated through 

 the lower fiuice into the lock. That fluice being now fliut, 

 as well as the upper one (which was all along fuppofed to be 

 fo) the plunger is let down to the bottom of the refervoir : by 

 this operation though (he plunger be not in actual contact with 

 the fides of the refervoir, much lefs what is called water '-tight , 

 (a neceflary circumftance in the cafe of forcing-pumps, pif- 

 tons, &c.) the water in the refervoir will be forced into the 

 lock, and thereby raife the boat or barge as much as the 

 plunger links, viz. 8 feet : in which fituation the water in the 



* At lead fo far folid as to outweigh its bulk in water. 



lock 



