ON SEISMOLOGICAL INVESTIGATIONS. 



63 



III. On the Orientation of an Instrument with regard to the Building in 



which it is 2>laced, 



At Shide, in the Isle of Wight, one portion of the observatory runs 

 east and west. It has two north and south cross- walls ; it is therefore 

 very much more stiff in an east and west direction, or along its length, 

 than in a direction at right angles. Storms generally strike the buildino- 

 from the south or south-west — that is to say, in the direction it is most 

 likely to yield. In the building there are two very heavy horizontal 

 pendulums, one of which points to the south and the other towards the 

 east. At the time of strong winds, or even in light gales the latter 

 pendulum, even in spite of damping arrangements, responds to wind 

 impacts from the south or south-west. It does this to such an extent - 

 that during stormy weather it is useless as a recorder of earthquakes. I 

 may add, the building is very solidly constructed ; the support for the 

 pendulum is a heavy mass of concrete entirely free from the foundation 

 of the building. The pendulum which points north, south, or end on to 

 the direction from which gusts of wind come, remains quite steady even 

 during the heaviest of storms. Both pendulums are attached to the same 

 support, which is a lamp-post embedded in the concrete foundation already 

 mentioned. 



IV. The Large Earthquakes of 1907. 



On the accompanying map (Plate I.) v/e find the origins of large earth- 

 quakes which have occurred during 1907 indicated by their numbers as 

 given in the Shide Register. The accuracy of the position of an origin which 

 is indicated by the position of a number greatly varies, and is naturally 

 dependent upon the data available for its determination. In certain 

 instances a number only means that a particular disturbance originated 

 within a certain district. For the year 1907 we see that certain of these 

 districts were entirely free from megaseismic disturbances. The area which 

 suffered the greatest disturbance was district F. This and its Himalayan 

 continuation is one where irregularities of surface contour are pronounced, 

 and it is therefore a district in which seismic instability should be ex- 

 pected. The following table gives the number of large earthquakes which 

 have occurred in the principal earthquake-producing regions since 

 1899:- ° 



East 

 Pacific 



A 

 B 

 C 

 D 



1899 1900 1901 1902 1903 1904 1905 1906 1907 Total. Batio to D 



13 9 3 2 .3 2 2 34 IG 



5 4 10 7 6 4 3 10 49 23 



4 6 4 6 3 4 2 29 1-4 



9 113 10 4 2 21 1-0 



Total 



West 

 Pacifi 



«4f : 



Total 

 Grand total . 



31 20 18 18 13 6 13 14 13.S 



18 3 7 9 9 14 9 15 7 91 



4 2 8 22 22 5 15 14 6 98 



8 11 12 14 10 9 16 24 25 129 



30 16 27 45 41 28 40 53 38 318 



61 36 45 63 54 28 46 66 52 451 



43 

 4-7 

 61 



The first four districts are on the eastern side of the Pacific and in the 

 Caribbean Sea, while the last three refer to the western side of the Pacific 



