INDEX. 



283 



W. 



Walking (man), 111 



— (horse), 142 

 Wing (bird's) 



— action downward and back- 



ward at each stroke, 235 



— active and passive parts of, 



276 



— ascent of, like action of boy's 



kite, 273 



— analogy to human arm, 211 



— at each revolution of, bird 



rises twice, 272 



— change of plane in, 244, 



257 



— compared to screw, 211 



— curves in, 210 



— depression of, elevates and 



carries forward the body, 

 269 



— descent of, gives all motive 



force, 273 



— duration of elevation and 



depression of, 228 



— frequency of strokes of, 227 



— Hartings on, 223 



— instrument to show change 



in plane of, 258 



— inclination of, changes gra- 



dually, 263 



— M. de Lucy on, 222 



— Louvrid, M. de, on angle of 



plane of bird's wing, 275 



— movements of, 226 



— ratio to weight, 222, 225 



Wing (hivds* )^contmued. 



— re-action of movements of, 



on body, 264 



— — — of wild duck, 



&c., 267 



— trajectory of pigeon's, 255 

 Wing (insects') 



— act as inclined planes, 200 



— artificial representation of, 



198 



— causes of movement of, 196 



— changes in plane of, 190-204 



— figure -of-8 movement of, 195 



— flexible membrane of, 208 



— flight instrument, illustrat- 



ing, 206 



— frequency of movement of, 



181-185 



— moves downward and for- 



ward, 197 



— movements of, determined 



optically, 187 



— propulsion of, from below 



upward and forward, 204 



— shape of, 276 



— structure of, 196 



— trajectory of (Dr. Pettigrew), 



201 



Work, mechanical, 60 



— unit of, 16 



Z. 



Zootrope, 137 



Zuckung, shock of muscles, 30 



THE END. 



