The Philosophy of Earthquakes. 363 



we omitted for simplification in our directions for diagram 

 drawing. What is called in mechanics the vis viva of the concus- 

 sion wave " must remain constant, and (in the same medium, 

 its dimensions being very great) the velocity of translation 

 also. The mass in wave movement, at any moment of its tran- 

 sit, is therefore the same, and the thickness of each successive 

 spherical shell decreases from the centre of impulse as the 

 square of its mean distance. "* We should therefore have to 

 make our circles less and less in the proportion specified, in 

 order to have our diagram correct. 



A very important fact to be remembered is thus stated by 

 Mr. Mallet in his great work, when he tells us that ' ' the distinc- 

 tion must be clearly borne in mind between the velocity of 

 transit of the wave, that with which the advancing form, or 

 seismal curve, is transferred from point to point of the 

 surface, and that of the earth particles moving within the 

 limits of the amplitude of each vibration.-" This will be plain 

 enough to most of our readers, but for others whom it may 

 perplex, it may be stated that when an elastic body is thrown 

 into wave motion, the force that compels that motion travels 

 from particle to particle faster than the particles themselves 

 move. In earthquake waves, Mr. Mallet considers the velocity of 

 the former to equal half the speed of a cannon ball, while the 

 latter, he tells us, " is often not greater than that which a body 

 acquires by falling from a height of two or three feet." Thus 

 the quake will visit various localities in succession immensely 

 quicker than it causes any body to move ; were it otherwise, the 

 stones of a building overthrown would be hurled about like 

 cannon balls, and, weight for weight, be as destructive as they 

 are when half their commencing velocity has been lost. 



When an earthquake reaches a building, or any object 

 whose dimensions are less than its own amplitude (or width), its 

 first proceeding is to impart to it a motion that tends to carry 

 the base forward, and thus, if the object be overthrown, it falls 

 backward, if free to fall in any direction. If we knew that an 

 earthquake was likely to come, we might arrange two rows of 

 wooden cylinders, all of the same height, but varying in 

 breadth, from the first in each row, broad enough to stand very 

 firmly, to the last in each row, so slender as to be easily over- 

 thrown. If these two rows stood upon two boards at right 

 angles to each other (as represented in the diagram, p. 98, 

 British Association Catalogue) no shock could reach the earth's 

 surface at that place without affecting them, and one set would 

 be in such a position as to be able to fall freely across the 

 jplanlv, without touching each other, provided they were far 

 enough apart. Suppose a bed of sand arranged for the cylinders 

 * British Association Catalogue, p. 111. 



