24- Mr. Blackwall's Remarks on the Diving of Aquatic Birds. 



who are commencing the study of natural history. Scrupulous 

 accuracy must always be regarded as an object of primary 

 importance in a publication of this description, and I feel con- 

 fident that Dr. Drummond has too much candour and good 

 sense to be offended with the correction of an error into which 

 he has been inadvertently led by the hasty adoption of a doc- 

 trine which is directly opposed to the established principles of 

 dynamics. 



It is asserted by the advocates of this doctrine, that the 

 action of the legs in diving not only gives to birds a progressive 

 motion, but also a tendency to rise, which tendency being 

 overcome by the pressure of the water above them, the entire 

 moving force is directed in the line of the body, accelerating 

 thereby the velocity with which they pursue their subaqueous 

 course. 



Now it is a law of hydrostatics, that the pressure of fluids 

 in a state of equilibrium is equal in all directions at the same 

 depth ; whatever obstacle, therefore, the circumstance of pres- 

 sure may present to the ascent of a bird when diving, it must 

 also present, cccleris 'paribus, to its progressive motion. More- 

 over, it is manifest from the exceeding facility with which the 

 particles of water move among one another, that if any ten- 

 dency upward did result from the action of the limbs of watei- 

 fowl in diving, 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 fallacy of the argument, that the pi'opelling power 

 is increased on such occasions by the pressure of the super- 

 incumbent 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 upward arising from 

 their small specific gravity ; and as Mr. White has illustrated 

 this subject in his usual felicitous manner, in treating upon 

 the Northern Diver, {Coly7nhus glacialis,) Linn,, in the second 

 volume of the octavo edition of his Works in Natural History, 

 p. 184-186, I cannot do better than avail myself of his obser- 

 vations. 



" 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 wisdom 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 



