168 ON THE DIVING OF AQUATIC BIRDS. 
it could not be wholly counteracted by the pressure 
of the mass of fluid above them: indeed, the specific 
gravity of such birds being less than that of water, 
it would not be possible for them to continue beneath 
its surface, even for a much shorter period than they 
are known to do, without the employment of physical 
force to effect their purpose; hence the fallacious- 
ness of the argument that the propelling power 
is increased on such occasions by the pressure of 
the superincumbent water, is rendered sufficiently 
obvious. 
It remains to consider what means are actually 
made use of by birds in diving to overcome the 
resistance of the medium in which they move, and 
the tendency upwards arising from their small spe- 
cific gravity; and as Mr. White has illustrated this 
subject in his usual felicitous manner, in treating 
upon the Northern Diver (Colymbus glacialis, 
Linn.), in the second volume of the octavo edi- 
tion of his ‘Works in Natural History,’ pp. 184- 
186, I cannot do better than avail myself of his 
observations. 
“Every part and proportion of this bird (the 
Northern Diver) is so incomparably adapted to its 
mode of life, that in no instance do we see the wis- 
dom of God in the creation to more advantage. The 
head is sharp and smaller than the part of the neck 
adjoining, in order that it may pierce the water ; 
the wings are placed forward and out of the centre 
