CANTO ir. REPRODUCTION OF LIFE. 73 



High o'er their heads on pinions broad display'd 

 The feather'd nations shed a floating shade; 



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 progres- 

 sion 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 re- 

 sembling 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 ? 



On pinions broad display' 'd. 1. 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, ex- 

 panded 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 situa- 

 tions 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 con- 

 trived 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. 



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