The progressive motion of fish beneath the water is produced principally by the undulation of their tails. One oblique plain of a part of the tail on the right side of the fish strikes the water at the same time that another oblique plain strikes it on the left side, hence in respect to moving to the right or left these percussions of the water counteract each other, but they coincide in respect to the progression of the fish; this power seems to be better applied to push forwards a body in water, than the oars of boats, as the particles of water recede from the stroke of the oar, whence the comparative power acquired is but as the difference of velocity between the striking oar and the receding water. So a ship moves swifter with an oblique wind, than with a wind of the same velocity exactly behind it; and the common windmill sail placed obliquely to the wind is more powerful than one which directly recedes from it. Might not some machinery resembling the tails of fish be placed behind a boat, so as to be moved with greater effect than common oars, by the force of wind or steam, or perhaps by hand?] [Footnote: _On pinions broad display'd_, l. 375. The progressive motion of birds in the air is principally performed by the movement of their wings, and not by that of their tails as in fish. The bird is supported in an element so much lighter than itself by the resistance of the air as it moves horizontally against the oblique plain made by its breast, expanded tail and wings, when they are at rest; the change of this obliquity also assists it to rise, and even directs its descent, though this is owing principally to its specific gravity, but it is in all situations kept upright or balanced by its wings. As the support of the bird in the air, as well as its progression, is performed by the motion of the wings; these require strong muscles as are seen on the breasts of partridges. Whence all attempts of men to fly by wings applied to the weak muscles of their arms have been ineffectual; but it is not certain whether light machinery so contrived as to be moved by their feet, might not enable them to fly a little way, though not so as to answer any useful purpose.]