( 13 ) ^ 
be fufficienc in this Engine, tho’ the Bore of the 
middle Cylinder, or Diameter of the Pillar of Wa- 
ter which is rais’d, be of 6,35 Inches. If the Bore 
of the laid Cylinder was but 3 Inches^ lefs than 3 
Pounds of Mercury would fuflice, and lefs than fix 
if there were two Barrels, in order to keep a condanc 
dream thro’ a Pipe of almoft the fame Diameter. 
This will very much leflen the Expcnce of Mercu- 
ry, which would otherwife be an Objedion againft 
this Engine; and by making the inner and outer Cylinder 
of hard Wood, as Box^ or Lignum the Cod of 
the Engine may dill be reduc’d. But if the Engine 
be very large, Cad Iron bor’d will be proper for 
the outer Cylinder, and Cad Iron turn’d on the out- 
fide for the inner Cylinder or Plug, and hammer’d 
Iron bor’d and turn’d for the middle Cylinder. 
There is an Objedion, which leems at fird to take 
off the intended Advantage of this Engine, which is 
this, njtz.. That indead of the Fridion of the Leather 
of a Pidon, when we lift up our Barrel to force, the 
Reddance, that the Mercury finds to rife in the outer 
Shellj is at lead as great as the Fridion that we avoid. 
Now that Refidance is never greater than the Weight 
of a concave Cylinder of Mercury^ whofe Height is 
the greated to which the Mercury rifes in the Paid 
Shell, and the Bafe is the Area of the Shell it felf. 
This Weight in our Engine is equal to 57,5 Pounds, 
and therefore one would think it greater than the Re- 
fidance made by the Fridion of a Pidon. But if it 
be confider’d, that in the Defcent of the Barrel for 
fucking, the Mercury fhifts immediately into the inner 
Shell, rifing to the fame Height, and dill keeping the 
fame Bafe; the aforelaid Weight of 57,5 Pounds 
helps down the Barrel, and facilitates the overcoming 
of the Force of the Atmofphere, confequently the 
C Weight 
