The Philosophy of Earthquakes. 365 



eating that a measurable quantity of masonry lias been moved 

 in one or more given directions. 



Such cracks are invaluable to the seismologist, and if he 

 can find enough of them, he interprets their silent language 

 into a veritable history of the shock. By the direction of the 

 cracks he learns which way the concussion came; by the 

 quantity of displacement work done he estimates its force, and 

 from a sufficient collection of details he draws his curved lines 

 of meizoseismal or greatest overthrow, and isoseismal or equal 

 overthrow. A few points on a co-seismal line will also enable 

 him to determine the seismic vertical, or the line perpendicu- 

 larly over the focus of the disturbance. For reasons already 

 explained, none of the isoseismal lines will be circles, nor will 

 the focus of concussion appear as a single point. If, however, 

 we may judge from the success with which Mr. Mallet investi- 

 gated the Neapolitan earthquake of 1857, the practical appli- 

 cation of seismological principles is easier than might be 

 supposed, and bids fair, when extended to a sufficient number 

 of concussions, to throw a valuable light upon these commotions 

 in the crust of our globe. 



We cannot attempt to indicate all the important questions 

 discussed with great ability in Mr. Mallet's work, but we will 

 endeavour to supply a sufficient quantity of elementary material 

 to assist those who might, without such aid, find the work 

 itself somewhat beyond their grasp, and to afford to others, 

 who do not desire to fathom the subject, as much information 

 as they will need. We must, however, recommend all to 

 consult the book for themselves, because those who do not 

 desire to study it will gain a great deal by reading portions 

 and carefully examining the large number of beautiful and 

 instructive plates. Proceeding with our necessarily imperfect 

 sketch, let us inquire into the velocity of earthquake shocks. 

 Mr. Mallet informs us that " the power of the wave-shock to 

 produce overthrow depends upon the intensity of the wave, 

 that is upon its amplitude, and upon the elasticity of the 

 medium in its passing through, conjointly ; and as all free dis- 

 placed bodies must be displaced to the extent due to the 

 greatest velocity impressed at the centre of gravity, the measure 

 of such displacement is always that of the velocity of the wave 

 particle at its maximum.'''' In the endeavour to determine 

 these velocities, Mr. Mallet found the cases divided into three 

 groups : the first, in which the true wave velocity and its 

 result were not much interfered with by surface oscillations ; 

 the second, in which the velocity ascertained " was made up of 

 the true wave velocity plus that of the elastic oscillation of the 

 surface at the displacing point/-' and thus was greater than the 



