Page:The Zoologist, 1st series, vol 4 (1846).djvu/89

Rh land ; it is said to be common and resident on some of the lakes of the latter.

Little Grebe or Dabchick, Podiceps minor. This species might have been placed in my group a, as a certain number nestle with us, but the majority being migratory, I have preferred mentioning it here. The history of this species is better known than that of any other grebe, and most interesting accounts of its habits have appeared in 'The Zoologist' from the pens of Messrs. Atkinson and Parsons.

I do not believe that this bird has the power of stationary submer- sion, its flat and subcoriaceous feet being very ill-formed for taking hold of plants or other substances, which I consider the means employed by the moorhen (Gallinula chloropus). I do not think (and Mr. At- kinson seems to be of the same opinion) that birds possess the volun- tary power of suddenly expelling air from their air-cells, or of pre- venting it from introducing itself into them; this, however, even if it took place, would not much diminish the bird's bulk, as the cells have, I believe, no movements of contraction of their own, and the lungs could certainly not be deprived of air without causing almost instantaneous death by suffocation.

Although I do not like being at variance with any intelligent na- turalist, I am sorry not to be able to side with Mr. Slaney's state- ments (Zool. 667) ; and as the great aim of the studies of naturalists is the attainment of true principles and undeniable facts, I hope he will excuse the following lines, and that they will induce him to change his, perhaps, too hastily formed opinions respecting the submersion of water-birds.

It is well known to all who have opened a book on physics, that any substance floating on a liquid loses a quantity of its weight equal to the weight of the volume of liquid displaced. Now, a moorhen, or any other bird while swimming, displaces a volume of water which is equal, or more than equal, in weight to the weight of its body ; that is to say, in other words, that its specific gravity is less than that of water.

If, then, muscular contraction took place, sufficient to make the difference in bulk (which causes the displacement of the volume of water) equal to a sum which would be less than the weight of the bird, it would sink of itself ; but if no such contraction took place, the simple exclusion of air from the cells would not be sufficient in itself to occasion submersion. For instance, let us suppose a moorhen weighs 2 pounds, to remain above water it must displace a volume of water weighing as much or more than 2 pounds, let us say 2¼; for