FLY 



FLY 



lass or common winch, where the motion 

 is quick; for in pulling 1 upwards from 

 the lower part, a person can exercise more 

 power than in thrusting forward in the 

 upper quarter, where, of course, part of 

 his force would be lost, were it not accu- 

 mulated and conserved in the equable 

 motion of the fly. Hence, by this means, 

 a man may work all clay in drawing up a 

 weight of 40/6. whereas 30/6. would create 

 him more labour in a day without the fly. 



In order to calculate the force of the 

 fly, joined to the screw for stamping the 

 image upon coins, let us suppose the two 

 arms of the fly to be each fifteen inches 

 long, measuring from the centre of the 

 weight to the axis of motion, the weights 

 to be 50 pounds each, and the diameter 

 of the axis pressing upon the dye to be 

 one inch. If every stroke be made in 

 half a second, and the weights describe 

 an half circumference, which, in this case, 

 will be four feet, the velocity will at the 

 instant of the stroke be at the rate of eight 

 feet in a second, so that the momentum 

 of it will be 800 ; but the arms of the fly 

 being as levers, each fifteen inches long, 

 whilst the semi-axis is only half an inch, 

 we must increase this force 30 times, 

 which will give 24,000; an immense force, 

 equal to 100/6. falling 120 feet, or near 

 two seconds in time; or to a body of 750/6. 

 falfing 16 JL feet, or one second in time. 

 Some engines, for coining crown-pieces, 

 used to have the arms of the fly five times 

 as long, and the weights twice as heavy, 

 so that the effect is ten times greater. 

 See COINING. 



FLY, in the sea language, that part of 

 the mariner's compass on which the se- 

 veral winds or points are drawn. "Let 

 fly the sheet," is a word of command to 

 let loose the sheet, in case of a gust of 

 wind, lest the ship should overset, or 

 spend her top-sails and masts ; which is 

 prevented by letting the sheet go-amain, 

 that it may hold no wind. 



FLY boat, a large vessel with a double 

 prow, carrying from four to six hundred 

 tons. 



FLYERS, in architecture, such stairs as 

 go straight, and do not wind round; nor 

 have the steps made tapering, but the 

 fore and back part of each stair, and the 

 ends, respectively, parallel to one ano- 

 ther ; so that if one flight do not carry 

 you to your intended height, there is a 

 broad half space, from whence you begin 

 to fly again, with steps every where of 

 the same length and breadth, as before. 



FLYING, the progressive motion of a 

 bird, or other winged animal, in the li- 



quid air. The parts of birds chiefly con- 

 cerned in flying are the wings, by whicn 

 they are sustained or wafted along. The 

 tail, Messeurs Willoughby, Ray, and many 

 others, imagine to be principally employ- 

 ed in steering and turning the body in 

 the air, as a rudder : but Borelii has put 

 it beyond all doubt, that this is the least 

 use of it, which is chiefly to assist the bird 

 in its ascent and descent in the air; and 

 to obviate the vacillations of the body 

 and wings : for, as to turning to this or 

 that side, it is performed by the wings, 

 and inclinations of' the body, and but very 

 little by the help of the tail. The flying 

 of a bird, in effect, is quite a different 

 thing from the rowing of a vessel. Birds 

 do not vibrate their wings towards the 

 tail, as oars are struck towards the stern, 

 but waft them downwards : nor does the 

 tail of the bird cut the air at right angles, 

 as the rudder does the water; but is dis- 

 posed horizontally, and preserves the 

 same situation what way so ever the bird 

 turns. 



In effect, as a vessel is turned about on 

 its centre of gravity to *the right, by a 

 brisk application of the oars to the left, so 

 a bird, in beating the air with its right 

 wing alone, towards the tail, will turn its 

 fore part to the left. Thus pigeons, 

 changing their course to the left, would 

 labour it with their right wing, keeping 

 the other almost at rest. Birds of a long 

 neck alter their course by the inclinations 

 of their head and neck, which altering 

 the course of gravity, the bird will pro- 

 ceed in a new direction. 



The manner of flying is thus : the bird 

 first bends his legs, and springs with a 

 violent leap from the ground ; then opens 

 and expands the joints of his wings, so as 

 to make a right line perpendicular to the 

 sides of his body ; thus the wings, with all 

 the feathers therein, constitute one con- 

 tinued lamina. Being now raised a little 

 above the horizon, and vibrating the 

 wings with great force and velocity per- 

 pendicularly against the subject air, that 

 fluid resists those succussions, both from 

 its natural inactivity and elasticity, by 

 means of which the whole body of the 

 bird is protruded. The resistance the air 

 makes to the withdrawing of the wings, 

 and consequently the progress of the 

 bird, will be so much the greater, as the 

 waft or stroke of the fan of the wing is 

 longer : but as the force of the wing is 

 continually diminished by this resistance, 

 when the two forces come to be in equi- 

 librio, the bird will remain suspended in. 

 the same place ; for the bird only ascendt 



