1J« REPORT— 1891. 



The ripples referred to appear amongst the waves in the early part of 

 a disturbance, and, as Mr. Omori suggests, may be the continuation of 

 the minute motions which are sometimes recorded in diagrams before the 

 true earthquake itself has commenced. 



A conclusion of some importance, which is confirmed by the above 

 observations, is that buildings which rise from a basement or which are 

 surrounded by an open area receive less motion than those which rise 

 from the surface. 



Obsei'vations on the vertical component of motion are now being 

 made in the pit. 



The Overturning and FRACTaRiNG of Brick and other Columns. 



During the past year a long series of experiments was carried out to 

 determine the accelerations necessary to overturn or fracture columns of 

 T«rk)tis diescriptions. The columns were placed or fixed upon a truck 

 which could be moved back and forth through a range and with a period 

 comparable with what might occur in a severe earthquake. Each back 

 and forth motion was recorded on a band of paper running at a uniform 

 speed in a direction at right angles to the direction of motion of the 

 truck. At the instant the column overturned or was fractured a mark 

 was made on the paper, so that the particular wave which was being 

 drawn when overthrow or fracture occuri'ed could be identified. 



On the assumption of simple harmonic motion, calling the period of 

 this wave T and its amjalitude a, which were quantities measurable on 



the diagram, the maximum velocity V, or —^~ , and the maximum ac- 



celeration, or — , could be calculated. These quantities were compared 



with quantities dependent on the dimensions, density, and strength of 

 the columns experimented upon. The object of the experiments was to 

 furnish those who have to build in earthquake countries with data 

 respecting the quantity of motion which certain forms of structure 

 might be expected to withstand. 



On October 15, 1884, we recorded in Tokio a maximum acceleration 

 of 210 mm. per sec. per sec, whilst on February 22, 1880, when Yoko- 

 hama was considerably damaged, such records as were obtained apparently 

 indicated 360 mm. per sec. per sec. A maximum range of motion of 

 100 mm. and a period of 2 seconds implies a maximum acceleration of 

 450 mm. per sec. per sec. As it is possible that this quantity might be 

 exceeded, structures in earthquake countries ought at least to be able to 

 withstand three times as much. 



For various reasons, of which the following are important, it seems 

 impossible to absolutely determine the quantity of motion necessary to 

 overturn a body of given dimensions. 



1. The body may be set in motion and be rocking with a definite 

 period and amplitude when it receives the final impulse which 

 determines its overthrow. 



2. Bodies, like columns, standing on end have a period of oscillation 

 varying with the arc through which they rock. 



3. An earthquake seldom, if ever, consists of a single sudden move- 



