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HISTORY OF MARINE ARCHITECTURE.

and after yielding laterally for the same space that occasioned its division, falls inbehind, creating what is defiued to be minus pressure, that is to say, the tempo-rary and superficial vacuum, which for a period certainly existing, though soshort as to be nearly incomprehensible to those who are not accustomed toconsider even a second of time divided into an almost infinite number of parts.This vacuum varies in its extent and consequence, in proportion to the powerwhich creates it; as for instance, a round sterued vessel, built after the Dutchfashion, and passing through the water with the same rapidity, as a ship having aclear run abaft, will create a vacuum far more extensive than a cutter will. Inshort, the vacuum made by the cube in question will be very great; that byfig. b , in the same plate, so trivial as to be scarcely perceptible. In the experi-ment made on the plane, the fluid is divided with the same violence as in theinstance of the cube, but owing to its being as instantaneously obliged to uniteby its own collapsing weight, without being kept asunder during the smallestparticle of time, as is the case in the preceding experiment, the vacuum isrendered far greater, and all, or at ieast the greater part of the fluid pressure onthe stern face, (to use a new term) being taken off, the total natural resistanceof the water is left almost at perfect liberty to act on the front or bow face,without being coutiter-ballanced by the stern impulsive pressure of the fluidwhich is destroyed by the vacuum alluded to. The four succeeding experimentsafford a further corroboration of this principle. Fig. e is a round iron plane, ofthe same thickness with the last, which was square, its velocity differs veryimmaterially from the former, being 4. 968. two decimal parts only less than theformer; its motive weight 205. 15. The superficies are equal, the diameter being13 in 54. which is exactly equal to a squareof one foot. Fig. /is a cylinder present-ing the same face, and one foot in length. The difference in velocity, when actedupon by the same weight, which was uniformly used throughout, is as irregular asany result that has hitherto appeared. The variation between the square andround plane is, as just observed, in velocity no more than two decimal parts persecond, and that of the motive weight required to impel it through the fluid at therate of eight nautical miles per hour, is only 1. 34. in addition to 203. 79. made useof in the first instance; but the cylindrical body marked/, not only acquires a velo-city of 5. 131. per second, which is 131. more than the cube, but requires for itsallotted impulse of eight miles each hour only 190. 78, which is 13.01. lessthan the former, in respect to motive weight. By the addition of a semi-globeto the stern end, as represented in fig. g, and the consequent augmentation of the

stern