104 EARTHQUAKES. 



and there will be no relative displacements even if the 

 emergence of the wave be nearly or quite vertical. 



When we get a vertical wave acting upon an irregular 

 mass of masonry, the heavier portions of the masonry, by 

 their inertia, tend to descend relatively to the remaining 

 portions, and in this way vertical fissures will be pro- 

 duced. For this reason it would not be advisable to use 

 heavy materials above archways, heavy roofs, or heavy 

 floors. The vertical fissures, Mallet remarks, would have 

 their widest opening at the base. 



In considering cases of fracture produced by earth- 

 quake motion, it must be remembered that these are due 

 to stresses applied suddenly, and that if the same amount 

 of stress had been sloiuly applied to a building, fractures 

 might not have occurred. 



If a disturbance is horizontal, and has a direction 

 parallel to the length of a wall, the wall is carried forward 

 at its foundations. This motion is opposed by the inertia 

 of the upper portion of the wall and the various loads it 

 carries. The wall being elastic, distortion takes place, 

 and cracks, which are widest at the top, will be formed. 

 In a uniform wall the two most prominent fissures ought 

 to be near the ends. 



If the horizontal backward and forward movement has 

 a direction oblique to the plane of the wall, the wall will 

 be either overthrown, fractured, or have a triangular frag- 

 ment thrown off towards the origin from the end last 

 reached. 



Should the wave emerge steeply, diagonal fissures at 

 right angles to the direction of transit will be formed, or 

 else triangular pieces will be projected. 



The accompanying figures are reduced from Mallet's 

 'Account of the Neapolitan Earthquake of 1857.' 



Taking a 6 as the general direction of the fractures in 



