EFFECTS PRODUCED UPON BUILDINGS. 135 



done ; the only places on the low ground which suffered 

 were those near to the base of the hills. The evidence as 

 to the relative value of hard ground as compared with 

 soft ground, for the foundation of a building, is very con- 

 flicting. Sometimes the hard ground has proved the 

 better foundation and sometimes the softer, and the 

 superiority of one over the other depends, no doubt, upon 

 a variety of local circumstances. 



These latter observations open up the inquiry as 

 to the extent to which the intensity of an earthquake 

 may be modified by the topography of the disturbed 

 area. 



The swing of mountains, — If an earthquake wave 

 is passing through ground the surface of which is level, 

 so long as this ground is homogeneous, as the wave travels 

 further and further we should expect its energy to become 

 less and less, until, finally, it would insensibly die out. 

 If, however, we have standing upon this plain a mountain, 

 judging from Mallet's remarks, this mountain would be 

 set in a state of vibration much in the same way as a 

 house is set in vibration, and it would tend to oscillate 

 backward and forward with a period of vibration dependent 

 upon the nature of its materials, size, and form. The upper 

 portion of this mountain would, in consequence, swing 

 through a greater arc than the lower portion, and build- 

 ings situated on the top of it would swing to and fro 

 through a greater arc than those which were situated 

 near its foot. This explanation why buildings situated 

 on the top of a mountain should suffer more than those 

 situated on a plain, is one which was offered by Mallet 

 when writing of the Neapolitan earthquake. He tells us 

 that towns on hills are ' rocked as on the top of masts,' 

 and if we accept this explanation it would, in fact, 

 be one reason why the houses situated on the Bluff at 



