. The Artizan. tial differences between thepaddle and the screw; in the former case the motion is altogether backwards,but in the latter case it is altogether sideways, the motion of the blades being atright angles to the direction in which the ship travels. The disturbing of thocurrents immediately in front of the screw will modify any action that wouldoccur if it were alone ; but as it is important to decide general principles beforesecondary effects can be fairly estimated, it will be best to assume the ship any-where except directly in front. Each particular section of the blade describesa
. The Artizan. tial differences between thepaddle and the screw; in the former case the motion is altogether backwards,but in the latter case it is altogether sideways, the motion of the blades being atright angles to the direction in which the ship travels. The disturbing of thocurrents immediately in front of the screw will modify any action that wouldoccur if it were alone ; but as it is important to decide general principles beforesecondary effects can be fairly estimated, it will be best to assume the ship any-where except directly in front. Each particular section of the blade describesa path which, although circular, is really equal in length to the circumference,and whethei this length be measured circularly, or it be developed into nstraight line, the effect in propelling the ship will be precisely alike. P0r example, the end of the blade ol a screw 16ft. 6in. diameter travels 18ft, revolution, its course being exactly at right angles I ; the direction in which the vessel It 1the screw, but th lacious; sifarfi the sy may uo driven and . i. it. would be in , Ice a liquid Meld then nnmii Won.: ii. This idea U pi 33 258 THE ARTIZAN. [NOVEMBEB 1, 1868. more deeply rooted than many of the other curious theories concerning thescrew propeller. The chief influences upon currents of water are two ; friction is one_ of these,and wherever there is an impelling force, the line of least resistance is alwaystaken hy the current in preference to any other. Let A. B. (Pig 1) illustrate the end of a blade of a screw, 15ft. 6in. diameterand43ft. 4£in. pitch; P, E is a space through which the blade moves. Theforces acting upon this section are represented just as accurately by supposingthe water to move from B to P instead of the blade from P to E, and it is muchmore convenient to show this arrangement upon the diagram than the other. Aparticle of water moving from E strikes the surface A. B at P with a certainforce, and is at once reflected to
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