128 IGNEOUS AGENCIES. 



dium, and therefore they are rapidly quenched and dissipated by re- 

 peated refractions and reflections, and by repeated fractures of the 

 medium and thus changed into other forms of force, as heat, electricity, 

 etc. Were it not for this, the destructive effects of earthquakes would 

 be far more extensive. 



4. We have said the wave-length remains unchanged. This length, 

 therefore, represents the diameter of the original water-mound, aud 

 therefore of the original sea-bottom upheaval. In the Japan earth- 

 quake this was 200 miles across. This shows the grand scale upon 

 which earthquake-movements take place. 



5. Earthquake sea- waves differ from all other sea- waves in that 

 their great size makes them drag bottom even in open deep sea. In 

 their case, therefore, the velocity depends not only on the wave-length, 

 but also on the depth of the sea. Knowing the size (wave-length) of 

 these waves, and therefore what ought to be their free velocity, and 

 also knowing their actual velocity by observation, the difference gives 

 the retardation by dragging ; and by the retardation may be calculated 

 the mean depth of the ocean traversed. In this way it has been de- 

 termined that the mean depth of the Pacific between Japan and San 

 Francisco is 12,000 feet, and between Peru and Honolulu, Sandwich 

 Islands, 18,500 feet. The great importance of such results is obvious. 



Depth of Earthquake- Focus. 



The great obscurity which hangs about the subject of the interior 

 condition of the earth and the ultimate cause of igneous agencies ren- 

 ders any positive knowledge on these subjects of peculiar interest. There 

 can be little doubt that the phenomena of earthquake- waves, their form, 

 their velocity, their angle of emergence, etc., if once thoroughly under- 

 stood, would be a most delicate index of this condition, and a powerful 

 means of solving many problems which now seem beyond the reach of 

 science. Among problems of this kind none is more important, and at 

 the same time more capable of solution, than the depth of the origin of 

 earthquakes, and therefore presumably of volcanoes. 



Seismographs. — The most direct way of determining the depth of 

 an earthquake-focus is by means of well-constructed seismographs. 

 These are instruments for recording earthquake-phenomena. They 

 are of infinite variety of forms, depending partly upon the facts de- 

 sired to be recorded, and partly upon the mode of record. As examples 

 we will mention only two : 



An excellent instrument for recording slight tremors is one invented 

 arid used by Prof. Palmieri, of the Vesuvian Observatory. It consists 

 of a telegraphic apparatus with the usual paper-slip and stile. The 

 paper-slip, accurately divided into hours, minutes, and seconds, travels 

 at a uniform rate by means of clock-work. The battery-circuit is closed 



