182 



ANIMAL LOCOMOTION. 



strain on the feathers during flight increases in proportion to 

 their distance from the trunk. 



The manner in which the roots of the primary, secondary, 

 and tertiary feathers are geared to each other in order to 

 rotate in one direction in flexion, and in another and opposite 

 direction in extension, is shown at figs. 98, 99, 100, and 101, 

 p. ,181. In flexion the feathers open up and permit the air 

 to pass between them. In extension they flap together and 

 render the wing as air-tight as that of either the insect or bat. 

 The primary, secondary, and tertiary feathers have conse- 

 quently a valvular action. 



The Wing of the Bird not always opened up to the same extent 

 in the Up Stroke. — The elaborate arrangements and adaptations 

 for increasing the area of the wing, and making it impervious 

 to air during the down stroke, and for decreasing the area 

 and opening up the wing during the up stroke, although 

 necessary to the flight of the heavy-bodied, short-winged 

 birds, as the grouse, partridge, and pheasant, are by no means 

 indispensable to the flight of the long- winged oceanic birds, 

 unless when in the act of rising from a level surface ; neither 

 do the short-winged heavy birds require to fold and open up 

 the wing during the up stroke to the same extent in all cases, 

 less folding and opening up being required when the birds 

 fly against a breeze, and when they have got fairly under 

 weigh. All the oceanic birds, even the albatross, require to 

 fold and flap their wings vigorously when they rise from the 

 surface of the water. When, however, they have acquired a 

 certain degree of momentum, and are travelling at a tolerable 

 horizontal speed, they can in a great measure dispense with 

 the opening up of the wing during the up stroke — nay, more, 

 they can in many instances dispense even with flapping. 

 This is particularly the case with the albatross, which (if a 

 tolerably stifl" breeze be blowing) can sail about for an hour 

 at a time without once flapping its wings. In this case the 

 wing is wielded in one piece like the insect wing, the bird 

 simply screwing and unscrewing the pinion on and off the 

 wind, and exercising a restraining influence — the breeze doing 

 the principal part of the work. In the bat the wing is 

 jointed as in the bird, and folded durino; the up stroke. As, 



