50 THE VICTORIAN NATURALIST. 



of the sea. They are, in fact, the scavengers which clear the 

 surface of the water of those impurities which would otherwise 

 arise from floating dead bodies. This obliges them to scour 

 hundreds of miles of sea surface every day, and the muscular 

 power required to enable a frail creature to perform this task 

 would overbalance the other functional developments of its body 

 unless that labour were reduced to a minimum. As we see, these 

 birds do perform their flight in a marvellously easy and graceful 

 style, sailing about for hours at a stretch without any flapping 

 motion of their wings, and clearly with a minimum of fatigue to 

 themselves. This enables them to follow a ship for thousands of 

 miles. 



The question, how they manage to propel themselves without 

 striking the air with their wings, whilst, as often as not, following 

 a steamer in the face of a heavy gale, has puzzled many. 

 Their real means of so doing seems to have led to misapprehen- 

 sion among many of our keenest observers and most thoughtful 

 minds. How, then, is this performed? The subject naturally 

 separates itself into three divisions — the bird's means of suspen- 

 sion in the air, its power of rising, and its method of propulsion. 

 Although the other points present many interesting problems, 

 I shall confine myself at present more particularly to their 

 act of piopulsion. I will, however, call your attention to just 

 two points in regard to their means of suspension in the air : — 

 (i.) It is clear that a bird's power of hovering at all must result 

 from its weight being insufficient to compress the air underlying 

 its outstretched wings and tail — i.e., the area of the wing and 

 tail surface must be proportionate to the weight of the bird and 

 the density of the air. This evening we will not go into 

 calculation of what area would be required under different 

 conditions, as we have the experimental fact before us that 

 the area provided for the bird by nature is sufficient for the 

 purpose. This requisite expanse of wing in sea birds is 

 provided by their third pinion joint, which allows so much 

 narrower a wing and more command over the action of 

 turning the feathers than is the case with eagles, hawks, 

 and vultures, which have the same power of hovering and sailing 

 with motionless wings, but can rest more often, and, therefore, 

 do not require to husband their resources to the same extent. 

 Birds with a relatively smaller area of wing and tail have con- 

 tinuously to strike and compress the air in order to sustain the 

 weight of their bodies. (2.) Again, a piece of the heaviest metal 

 may be beaten out into so thin a sheet that if the air does not 

 pass through its substance it will float in a horizontal position, 

 because its weight is not sufficiently great to immediately com- 

 press the volume of air underlying it. After the lapse, however, 

 of a definitely long instant of time, the air is slightly compressed, 



