May. 1913. 



KNOWLEDGE. 



169 



crack to be seen. They withstood the great earth- 

 quake in 1892, when thousands of houses fell in 

 Nagoya and Gifu, and in the smaller places round 

 about, and when all the new brick telegraph and 

 post-offices and other European buildings came 

 crashing down like ninepins. On that occasion, 

 Japanese houses did not fall, unless they were old 

 and frail, when in many cases the supports gave 

 way and the roof came down, imprisoning the in- 

 mates until they were rescued, sometimes from a 

 house in flames. The walls of the Castle of Tokyo 

 show the same remarkable state of preservation, the 

 blocks of cvclopean masonry, there also uncemented, 

 being neither cracked nor displaced in the least 

 degree. 



Figure 15.3 represents an earthquake-proof 

 structure erected in the grounds of the Imperial 

 University, Tokyo, which has been built according 

 to mathematical calculation on a solid concrete 

 foundation, and is intended for use as a Seismo- 

 logical Observatory, and as a standard with which 

 to compare the effects of a shock on ordinary brick 

 buildings. In it most interesting investigations 

 into the stability of various structures against earth- 

 quake shocks are carried on, artificial earthquake 

 motion being produced by means of a " shaking 

 table," which can be made to move with independent 

 horizontal and vertical motions by the use of steam 

 engines. (See Figure 158). 



Another remarkable fact in Japan is that pagodas 

 (see Figure 155), built hundreds of years ago embody 

 the principle of the modern seismograph, which is 



union of a stable and an unstable structure, to 

 produce a neutral stability which renders the whole 

 building least sensible to earthquake shock. In the 



Figure 155. A typical Japanese Pagoda. It is a remarkable 



fact that these pagodas, built hundreds of years ago, embody 



the principle of the modern seismograph. 



to minimise the effect of earthquake motion by the 

 combination of an inverted pendulum with an 

 ordinary pendulum; or, in other words, by the 



Figure 156. A Japanese bell-tower, wherein the suspended 

 bell acts as a safeguard against earthquakes. 



hollow well of every five-storeyed pagoda a heavy 

 mass of timber is suspended freely, like an 

 exaggerated tongue, from the top right to the 

 ground, but not in contact with it, and at the shock 

 of an earthquake this large pendulum slowly swings, 

 the structure sways, and then settles back safely to 

 its base. This is also the principle followed in the 

 construction of all bell-towers throughout Japan, 

 where the bell acts as pendulum, and the roof, 

 supported by posts, forms an inverted pendulum, 

 as in the seismograph. When an earthquake occurs, 

 a pagoda or a bell-tower may be rotated or dis- 

 placed, but it cannot be overturned as a whole. 



Although seismologists have not yet succeeded in 

 finding out any means of definitely predicting the 

 occurrence of an earthquake, the} - are very hopeful 

 of finally arriving at this desired goal ; and already 

 Professor Omori, with his deflectograph and vibra- 

 tion measurer, can discern danger by careful 

 observation of the pulsations which are always 

 gently agitating the surface of the earth, and can 

 usually give ten or twelve hours' notice of a shock. 

 A sudden cessation of the regular heart-beats or 

 pulsations of the earth's crust is a danger signal, 

 extreme stillness invariably preceding an earthquake, 

 whereas constant tremors are a good sign. 



A great earthquake is almost always followed by 

 weaker ones, and when it is violent and destructive 

 the number of minor shocks following it may 

 amount to hundreds, or even thousands, and 

 continue for several months or years. The 

 occurrence of after-shocks is quite natural and 

 necessary for the settling down into stable equili- 



