Page:Popular Science Monthly Volume 43.djvu/510

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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 resemblance 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 obtain 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 HYDROGRAPHIC OFFICE.

NEXT to the oil which is used in the beacons of the world to give light to save life, that which is most effective in forestalling 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 extensive 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 seaport 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 hurricane, 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, keeping men stationed by them to replenish the supply. I also filled a