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Page:Popular Science Monthly Volume 54.djvu/387

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371
MODERN STUDIES OF EARTHQUAKES.

other just as water waves do. Thus are explained the earthquake bridges or spots which always remain unmoved through repeated earthquakes, either because they are firmer, or because the progress of the waves is arrested at them by interference.

The sounds, too, which so frequently accompany earthquakes are likewise simply results of this division of the waves and their escape into the air, for we perceive wave motions in the air as sound. The admirable delicacy of our sense of hearing is here manifested, for seismic movements are not rarely perceptible, or heard, as air waves, which we can not perceive as movements of the ground. Earthquake thunder is caused, like storm thunder, by shocks to the air, of which we hear the nearest and latest first, and the farthest and earliest last. The different tone shades of the earthquake sound depend upon their various sources, as from small, sharp fragments, clinking, rattling, and humming; from sand and earth, dull rumbling; from trees, whistling, etc. The echo in ravines not rarely operates to add strength to them. Earthquake sounds that seem to come out of the air from above are caused by earthquake waves reaching us by way of trees, houses, etc.; the different directions and degrees of force which they seem to indicate in different houses or in different rooms of the same house are explainable by the different elasticity conditions of the houses and rooms. But not the most insignificant conclusion can be drawn from these sounds concerning the nature and causes of earthquakes. It is important to emphasize this fact, for errors have often originated in conclusions drawn from such things.—Translated for the Popular Science Monthly from the Deutsche Rundschau.

 


 
Examples of a race of curiously protectively colored mice which inhabit the sandy island, the North Bull, in the Bay of Dublin, were exhibited by Dr. H. Lyster Jameson in the Zoological Section of the British Association. A considerable percentage of them were distinctly lighter hued than the ancestral type of house mouse, though every possible gradation occurred between the typical house mouse and the palest examples. The speaker regarded the marked predominance of sand-colored specimens as due to the action of natural selection. The hawks and owls which frequent the island, and are the only enemies the mice have to compete against, most easily capture the darkest examples, or those that contrast most strongly with the color of the sand. Thus a protectively colored race is becoming established. The island came into existence only about a hundred years ago. Consequently it is possible to fix a time limit within which the sandycolored race has been evolved. Its evolution also, as Professor Poulton observed in his comment on Dr. Jameson's paper, gives additional evidence to that afforded by the shore crabs described by Professor Weldon in his presidential address to the section, that the transmutation of species is not necessarily so slow as to be indiscernible.