36 FORMATION OF THE EARTH 



as to record earthquakes produced in any part of the globe, 

 however distant, and even when they take place in the greatest 

 depths of the sea. A vertical pendulum, suitably constructed, 

 describes the horizontal components, and a horizontal 

 pendulum the vertical. The curves registered by the apparatus, 

 undulatory like those described on a revolving cylinder coated 

 with lampblack by a needle fixed to a freely vibrating scale, 

 are remarkably uniform. For each shock they fall into three 

 divisions, which differ solely in the length and amplitude of 

 waves and are inscribed in succession. From the time taken for 

 the registration of these vibrations and from what we know, 

 through the experiments of Wertheim, of the mode of trans- 

 mission of vibrations through solid bodies, Lord Rayleigh 

 concluded that the first and second parts of the curve represent 

 respectively transverse and longitudinal vibrations trans- 

 mitted across the barysphere, the third series of vibrations 

 and the last to arrive being transmitted across the earth's 

 crust or lithosphere. Now the speed of the transmission of the 

 first two series of undulations, 9, 6, and 5 kilometres per second, 

 indicates that these latter have been transmitted across a 

 medium more rigid than steel. The maximum density of this 

 medium, as calculated by Roche, would be 10, 6, and, at most, 

 a little greater than that of iron — 7, 7. These correspondences 

 are somewhat disturbing in view of the diversity of the con- 

 siderations involved, all bringing us back to the same 

 assumption — that the earth's core may well be solid and consist 

 essentially of iron. It must not be forgotten, however, that all 

 the metals heavier than iron, especially those that combine 

 with difficulty with metalloids, such as gold and platinum, 

 are very probably at least as largely represented in the bary- 

 sphere as in the lithosphere, and that, on the other hand, the mode 

 of transmission of luminous vibrations across the interstellar 

 ether equally leads us to assume a medium " more rigid than 

 steel ", without implying, however, that it is solid in the sense 

 in which we understand that term. The term " rigidity " simply 

 means that the molecules of the body considered can be 

 displaced only with difficulty and have an active tendency to 

 return to the position from which they have been dislodged. 

 As far as the barysphere is concerned, the rigidity of its sub- 

 stance appears to be due to the tremendous pressure to which 

 it is subjected and which maintains the molecules in place 



