Mr. Blackmail's Remarks onthe Diving of /Iqiiatic Birds. 23 



dences they have the singular forms shown in Figs. 2. and 3, 

 the line where there is no interval of retardation being the 

 boundary of the four diffei'ent systems of fringes shown in 

 these figures. 



As the paths of the interfering pencils are performed in three 

 media, crown-glass, flint-glass, and air, and as their lengths 

 vary very quickly and irregularly, as the angle of incidence 

 varies, and as the point of incidence changes its position, the 

 analytical expression of the interval of retardation will be very 

 complex. 



VII. RemarJcs on the Diving of Aquatic Birds. By John 

 Blackwall, Esq. F.L.S. Sfc.* 



T"JR. DRUMMOND of Belfast, in his interesting " Letters 



-*-^ to a Young Naturalist," p. 201-202, has the following 

 passage on the diving of water-fowl. "Does a cormorant, or a 

 duck, or a grebe, move more rapidly under the surface of 

 water than on it? In several parts of Montagu's Ornitholo- 

 gical Dictionary, and the still more valuable Supplement to 

 it, you will find illustrations on this point, showing that the 

 same power will cause a much more rapid motion in diving 

 than in swimming; and the cause is this: — When a bird 

 moves in water, or upon it, there is a movement upwards as 

 well as forward ; but in swimming, the momentum upwards is 

 lost, and the bird derives benefit only from the forward im- 

 pulse. But in diving, the pressure of the water above pre- 

 vents the ascending movement, and consequently the impetus 

 is not lost, as if the bird were on the surface, and therefore 

 the propelling power is greater ; and the bird moves faster, 

 because, in diving, the whole moving power is effective; 

 whereas in swimming, a part of it is lost, and the progress is 

 proportionally lessened." 



Many years since, when perusing for the first time the ob- 

 servations on the diving of aquatic birds contained in the In- 

 troduction to the Ornithological Dictionary, p. xxxix.-xl., the 

 insufficiency of Montagu's attempt to solve this problem was 

 perceived, and I was induced to make a {evf comments on the 

 subject in my zoological note-book : it is probable, however, 

 that they never would have filled a more conspicuous situa- 

 tion than that which they have so long occupied in its pages, 

 had not my attention been again directed to them by Dr. 

 Drummond's recent introduction of Montagu's hypothesis in 

 a work professedly written for the instruction of young persons 



• Communicated by the Author. 



