420 NATATORES. 



loaded their feathers, and then they pause upon the wave and 

 remove the oil with their bills. 



The oil which they thus collect, is not, however, to be con- 

 sidered as a burden which either impedes their progress, or 

 makes them sink deeper in the water. The effects of it are 

 exactly the reverse : it is less specifically heavy than water, 

 and thus it renders the bird more buoyant ; and it also 

 greatly diminishes the friction, and thereby renders the mo- 

 tion of the bird more free than it would be if performed with 

 the naked feathers. The repulsion which there is between 

 oil and water also increases the buoyancy of the birds. It 

 acts as a power lifting them up, so that the immersed portion 

 descends very little below the film of oil ; and the water 

 glides under them, so they do not produce any ripple or 

 splash in the water, except* when they occasionally " tip " 

 the surface with their wings. The effect of oil in diminishing 

 the friction of the air against the surface of the water is 

 well known ; and so is the great extent to which a very 

 small quantity of oil will soon extend, and consequently the 

 very thin pellicle of it which can calm the turbulence of the 

 waves. 



Whether these birds can discharge the oil from their 

 stomach on the sea, and thereby procure a calm for them- 

 selves, in order that they may repose, has not been ascer- 

 tained ; and very little, indeed, is known of their position in 

 repose, although there is no doubt that they and also the 

 other petrels at some seasons, and during some states of the 

 weather, repose upon the high seas. But there is, at least, 

 some probability that all the different species do sometimes 

 discharge oil in this manner, though not, of course, for this 

 purpose. The least alarm when the birds are off the water, 

 causes those of all the genera to disgorge from their stomachs 

 a considerable quantity of oil, and they do so indiscri- 



