306 ■ ON AEKIAL LOCOMOTION. 



Again the boat is under weigh, though the wiud is but just sutficient 

 for us to stem the current. An immense kite is soaring overhead, 

 scarcely higher than the top of our lateen yard, affording a tine oppor- 

 tunity for contemplating his easy and unlabored movements. The cook 

 has now thrown overboard some oft'al. With a solemn swoop the bird 

 descends and seizes it with his talons. How easily he rises again with 

 motionless expanded wings, the mere force and momentum of his de- 

 scent serving to raise him again to more than half-mast high. Observe 

 him next, with lazy tlai)])ing wings, and head turned under his body; 

 he is placidly devouring the pendent morsel from his foot, and calmly 

 gliding onwards. 



The Nile abounds with large aquatic birds of almost every variety. 

 During a residence upon its surface for nine months out of the year, 

 immense numbers have been seen to come and go, for the majority of 

 them are migratory. Egypt being merely a narrow strip of territory, 

 passing through one of the most desert parts of the earth and rendered 

 fertile only by the periodical rise of the waters of the river, it is proba- 

 ble that these birds make it their grand thoroughfare into the rich dis- 

 trict of Central Africa. 



On nearing our own shores, steaming against a moderate headwind, 

 from a station abatt the wheel the movements of some half-dozen gulls 

 are observed, following in the wake of the ship in patient expectation 

 of any edibles that may be thrown overboard. One that is more famil- 

 iar than the rest comes so near at times that the winnowing of his wings 

 can be heard; he has just dropped astern, and now comes on again. 

 With the axis of his body exactly at the level of the eyesight, his every 

 movement can be distinctly marked. He approaches to within 10 

 yards, and utters his wild, plaintive note, as he turns his head from side 

 to side, and regards us with his jet black eye. But where is the angle 

 or upward rise of his wings, that should compensate for his descending 

 tendency, in a yielding me<lium like air? The incline can not be de- 

 tected, for, to all appearance, his wings are edgewise, or parallel to his 

 line of motion, and he appears to skim along a solid supi)ort. No 

 smooth-edged rails, or steel- tired wheels, with polished axles revolving 

 in well-oiled brasses, are needed here for the purpose of diminishing 

 friction, for nature's machinery has surpassed them all. Tbe retarding 

 effects of gravity in the creature under notice, are almost annulled, for 

 he is gliding forward upon a frictionless plane. There are various rea- 

 sons for concluding that the direct flight of njany birds is maintained 

 with a much less expenditure of power for a high speed, than by any 

 mode of progression. 



The first subject for consideration is the proportion of surface weight, 

 and their combined effect in descending perpendicularly through the 

 atnu^sphere. The datum is here based upon the consideration of safety, 

 for it may sometimes be needful for a living being to drop passively, 

 without muscular effort. One square foot of sustaining surface for 

 every pound of the tot;d weight, will be sufficient for security. 



