210 Professors Perry and Ayrton on Structures 



terminate, and -which are comparatively small. It is not here 

 necessary to consider whether, as all the motions of a qnickly 

 vibrating body must be small, such a structure will be more 

 comfortable to live in, because it is doubtful whether the an- 

 noyance produced by rapidity of shock would not more than 

 counterbalance the annoyance of great but smooth motions. 

 It is only safety we are here considering ; and in this respect 

 there can be no doubt of the superiority of rigid structures, 

 or of structures having a sufficiently great viscous resistance 

 to motion. Some calculations which we have made of the 

 times of vibration of ordinary structures, such as well-built 

 houses of stone and brick, chimneys, lighthouses, &c, will be 

 found at the end of this paper ; and from these we see that 

 the periods are all much less than what we judge from our ex- 

 perience is the ordinary period of vibration of earthquakes in 

 Japan. Even two-storied houses built of wood, if framed in 

 the best way, have quick times of vibration ; such structures 

 are therefore, it seems to us, well capable of resisting the ordi- 

 nary Japanese earthquake-shock. As, however, we have not 

 yet experienced the effects of a destructive earthquake, and as 

 we presume that one of the most important ways in which 

 it may differ from ordinary earthquakes is in the suddenness 

 of motion, or change of motion, it cannot be said that any-or- 

 dinary structure has a quicker period of vibration than a 

 destructive earthquake ; consequently, if it be granted that 

 stability depends on the structure having a quicker period of 

 vibration than that of the earthquake, the stability of a 

 building will be only relative. We can of course be sure that 

 by making the walls of a building thicker and its height less, 

 we add to its safety ; but however far we may go in this direc- 

 tion, we cannot be certain but that after all the earthquake- 

 period may be less than that of our building. 



We must therefore content ourselves with saying that a slowly 

 vibrating structure will probably get broken in its connexions 

 with the foundations if these be rigidly fixed to the ground: 

 consequently (and we here oppose the practice of many archi- 

 tects and engineers) putting a heavy top to a lighthouse, the 

 chimney of a factory, or other high building, must certainly 

 take from its stability. 



And although w r e see from the calculations at the end of the 

 paper to which we have referred, that the times of vibrations 

 of ordinary brick and stone houses are very short, still, in view 

 of the possible great suddenness of a destructive earthquake, we 

 should advise that all buildings be kept as low and made as 

 rigid as possible. 



The argument used by engineers to support the practice 



