The Philosophy of Earthqualces. 861 



the mountain ridges " are very large in relation to the basin, 

 the breadth of the seismic band may overlap the whole 

 surface." 



If, from these general considerations we proceed to study 

 the particular phenomena of earthquakes, we find that the 

 fundamental propositions laid down in Mr. Mallet's Seis- 

 mology are, that " the elastic wave or earth shock may reach 

 a given point upon the surface with any angle of emergence 

 (the angle contained by the horizontal plane with the wave 

 path at the point of emergence), or in any azimuth. The path 

 of the wave is a right line, joining that point with the centre of 

 impulse (or focus), the wave being assumed to be propagated 

 from thence in all directions in spherical shells. This is strictly 

 true only in a homogeneous elastic solid." We dare say 

 this will appear very unintelligible to persons unaccustomed 

 to such considerations ; but it will soon become plain. Draw 

 a large circle on a sheet of paper, and let this represent a 

 solid globe. Then draw a small circle in the middle of it, and 

 let this represent a hollow place. Then imagine a little gun- 

 powder ignited in the centre of the hollow. Its explosion 

 would strike the walls of the hollow equally in all directions. 

 Suppose the walls elastic, and too strong for the explosion to 

 break; they would still experience a shake all round, and 

 they would in turn shake the neighbouring particles all round, 

 and these again another set, and so on, producing concus- 

 sion waves, one outside the other, and forming what Mr. 

 Mallet calls " spherical shells." Thus a number of circles 

 drawn outside the circle bounding the hollow would represent 

 these " shells," and, if continued long enough, they would 

 extend to the large outer circle of all. Draw a horizontal line, 

 touching this circle, and then, from the same centre as all the 

 former circles, draw a great circle cutting the horizontal line, 

 and this will represent the concussion wave emerging at the 

 surface of the earth. 



Such a diagram, made without any difficulty, will explain 

 to the eye how concussion waves, starting in all directions 

 from a point below the earth's surface, widen and widen their 

 circles, until they reach that surface. 



The next essential thing is to learn what is meant by the 

 " seismic vertical," and we perceive its nature if we draw a 

 line from the centre of the hollow, or point from which the 

 concussion proceeded, up to the horizontal line representing- 

 the earth's surface, and falling 'perpendicularly upon it. To 

 obtain another illustration of this matter, give the under side 

 of a table a tap with a small hammer ; whatever is placed exactly 

 above the hammer on the table, so that the concussion would 

 throw it straight up in the air, is in the seismic vertical, and 



