THE BIRDS OF HELIGOLAND 573 



this manner, it now has recourse to stratagem. Instead of continu- 

 ing to swim along rapidly in a horizontal direction, it bends off 

 towards the side, at a right angle below the water, or dives right 

 down to the bottom, allowing the boat to glide over and past it. 

 Sometimes this trick succeeds, and while the shooter is spying 

 eagerly forward, ready to tire, the oarsmen suddenly see the bird 

 diving up once more to the surface far in their rear. An experienced 

 sportsman, however, generally knows what the bird is up to by 

 the manner in which it dives, and usually rows no farther than 

 the spot where it disappeared. Then, however, is the moment 

 when attention combined with rapidity and surety of aim are 

 required, for in most cases the bird rises quite close to the boat, 

 often only a few paces off, but dives again at the same instant ; if 

 one misses it, it is all over with the sport ; for even if the bird 

 should be sighted again at all, it is sure to be at a distance of from 

 one hundred to one hundred and fifty paces, and all farther pursuit 

 will be useless. 



The actions of this Diver, as described above, present us with 

 a problem as interesting and as difficult to explain as is the soaring 

 upward of many birds without beatings of their wings or the help of 

 air-currents, which we have already fully discussed in treating of 

 the Buzzards and Gulls. This latter phenomenon is of course 

 opposed to the established laws of gravity, according to which it is 

 impossible that the heavier body should rise in the lighter atmos- 

 phere; consequently doubt has been thrown on the reliability of 

 my observation, to which, nevertheless, I adhere with the firmest 

 conviction. Now, the case of this Diver presents us with an act of 

 kindred nature, though opposite in the manner of its execution, 

 and too well known to be questioned by either sportsman or 

 naturalist. This act consists in the immersion at any desirable 

 time, and for any desirable period, of the lighter avian body in a 

 much heavier and denser element i.e. the water. The volume of 

 the body of the Great Northern Diver amounts to about a cubic 

 foot, and its weight to fifteen pounds ; the weight of a cubic foot of 

 water, on the other hand, amounts to more than sixty pounds. 

 Nevertheless, the Diver is able, without any apparent effort, not 

 only to immerse its much lighter body in the heavy mass of water, 

 but also to remain for a protracted period below the surface, in 

 order to escape pursuit by moving forwards in a horizontal direction. 

 This kind of immersion and horizontal forward movement below 

 the surface of the water must not, however, be confounded with 

 the more or less vertical diving movements towards the bottom 

 executed by the bird in its search for food ; this latter movement 

 is accomplished by mechanical action, the bird assuming an almost 



