494 THE POPULAR SCIENCE MONTHLY. 



Probably the most important advantage derived by the frogs 

 from their power of color change is that of concealment from 

 birds and other enemies. Many of the larger waders devour these 

 animals whenever opportunity offers, and a protective resem- 

 blance would help greatly in escaping detection. In the case of 

 the wood frog, I suspect that the resemblance to the carpet of 

 pine needles helps to preserve them from birds of other kinds 

 the hawks and owls. Last summer I placed a wood frog in a 

 cage containing a red-tailed hawk (Buteo borealis), and it was 

 immediately gobbled up by the bird. 



It also seems likely that the resemblance to the environment 

 may be of benefit to the species in enabling it more readily to ob- 

 tain its insect food, but in the present state of our knowledge of 

 the vision of insects one can not place very much stress upon this 

 phase of the subject. 



WHY A FILM OF OIL CAN CALM THE SEA. 



BY G. W. LITTLEHALES, 



CHIEF OF DIVISION OF CHART CONSTRUCTION, UNITED STATES HYDROORAPHIC OFFICE. 



"VTEXT to the oil which is used in the beacons of the world to 

 JJN give light to save life, that which is most effective in fore- 

 stalling the loss of life and the destruction of property is the 

 quantity that is expended by mariners in forming a film around 

 their vessels to subdue the violence of breaking waves. The ex- 

 tensive practice of using oil for this purpose is the outgrowth of 

 an age of quick ocean passages which has impelled seamen to 

 crowd on every foot of canvas and every pound of steam in the 

 attempt to run through storm and calm alike. In any large sea- 

 port a visit to the docks where mariners tell the experiences of 

 their voyages will afford evidence of the extent and efficacy of 

 this practice ; but, before proceeding to point out the principles 

 involved, it will be of interest to give extracts from the log-books 

 of a few vessels, to show the manner and effect of the use of oil. 

 From the official notes of Captain Tregarthen, of the British 

 steamship Marmanhense, the following extract has been made : 

 " On March 3d, off Cape Hatteras, in a very strong northwest hur- 

 ricane, finding the ship could make no headway, I hove to. The 

 wind was blowing in hurricane force from the northwest, and the 

 tremendous sea which was running broke on board and did great 

 damage. The vessel was very unsteady, coming up and falling 

 off several points, so that I could not steer her nor keep her head 

 to the sea, although the engines were working well. I filled the 

 water-closet bowls with oakum and poured fish oil over it, keep- 

 ing men stationed by them to replenish the supply. I also filled a 



