554 i»HILOSOPHlCAL TRANSACTIONS. [aNNO 1722. 



pipes are full, if the mercury be equally high in both shells, on raising the 

 barrel one inch, the mercury will rise 13.28 inches in the outer shell; which I 

 call charging for forcing; because in continuing to raise the barrel, the forcing 

 valve immediately rises, and the water comes out at top during the rest of the 

 stroke, which is 12 inches, and delivers 1.6 gallon of water, wine measure. 



Fig. 10 represents the forcing stroke halfway up; with the mercury 17 

 inches in the outer shell, 4 inches in the inner, and the whole space at bottom 

 under the middle cylinder 7 inches. 



From this it appears, that in the whole stroke of 13 inches in length, there 

 is only i inch lost, to charge for suction, and in the next stroke, which is 

 likewise of 13 inches, there is only one inch lost, to charge for forcing; so 

 that in a motion of 0.6 inches, there is but 1-^ inch, or about ^'^ P^''^ ineffec- 

 tual. But this is owing to the too large space of the outer shell, which contains 

 4 times more than the inner one, because the cylinders were only hammered, 

 and not turned ; for if the outer space had been no larger than the inner, 

 then i inch of the stroke would have charged for forcing; so that only 4 

 inch in 26, or -^v part of the whole stroke would have been ineffectual; and 

 in that case, * of the quantity of mercury, or a little more than 12 pounds, 

 would have been sufficient. 



There may still less mercury be used, if the middle cylinder be made of plate 

 iron turned on the outside, and bored within, the outer cylinder bored, and 

 the inner one turned; so that if the work be well performed, 8 or 10 pounds 

 of mercury will be sufficient in this engine, though the bore of the middle 

 cylinder, or diameter of 'the column of water which is raised, be of 6.35 inches. 

 If the bore of the said cylinder were only 3 inches, less than 3 pounds of mer- 

 cury would suffice, and less than 6 if there were two barrels, in order to keep 

 a constant stream through a pipe of almost the same diameter. This will very 

 much lessen the expence of mercury, which would otherwise be an objection 

 against this engine; and by making the inner and outer cylinder of hard wood, 

 as box, or lignum vitae, the cost of the engine may still be reduced. But if 

 the engine be very large, cast iron bored will be proper for the outer cylinder, 

 and cast iron turned on the outside for the inner cylinder or plug, and ham 

 mered iron bored and turned for the middle cylinder. 



There is an objection, which seems at first to take off the intended advan- 

 tage of this engine, which is this, viz. that instead of the friction of the leather 

 of a piston, when we lift up our barrel to force, the resistance, that the 

 mercury finds to rise in the outer shell, is at least as great as the friction 

 that we avoid. Now that resistance is never greater than the weight of a 

 concave cylinder of mercury, whose height is the greatest to which the mercury 



