78 REPORTS ON THE STATE OF SCIENCE. 



originated from faults which ran north and south and haded to the west, 

 which, however, is not likely, we should have a complete explanation of 

 what has been noted. 



2. Has the direction in which a primary impulse was delivered been 

 southwards towards the Equator, or in the direction of the latitude of its 

 origin 1 



3. Have the wave paths been sub-oceanic or sub-continental 1 In the 

 first case we have a ' tapping ofif ' of energy by a layer of water, and in the 

 second by a layer of air. 



4. The wave movement we are considei'ing has been recorded upon the 

 upper surface of the crust of our earth. To what depth beneath this 

 covering does this movement extend ? Are we dealing with a movement 

 which is merely superficial or one which extends to a considerable depth 

 in the material beneath the crust ? Are the factors which govern wave 

 speed within our earth the same in all directions ? 



VIII. A Catalogue of Destructive Earthquakes. 

 (Still iu preparation.) 



The catalogue to which I wish to draw attention only refers to earth- 

 quakes which have caused structural damage. The earthquakes which 

 are marked I. ai'e those which have had an intensity sufficient to crack 

 walls or break chimneys. Tliis implies that the acceleration or rapidity 

 of change of motion may have been about 1,000 mm. per second, and 

 the destructivity has usually been confined to a single town or village. 

 Those which are marked II. have had a destructivity so far in excess of 

 Class I. that a few buildings have fallen. For this class the acceleration 

 may be about 1,500 mm. per second, and the area may be represented 

 by a radius of twenty miles. Class III. refers to earthquakes which 

 have destroyed towns or caused widespread disasters. Although these 

 earthquakes have occasionally been local, it is usually found they have 

 been severe 50 or 60 miles round their origin and have produced effects 

 similar to those of Class I. at distances of 200 or 300 miles. It is 

 clear this classification is imperfect ; certain earthquakes under Class II. 

 might be included in Class III., whilst others under the head of Class I. 

 might be included in Class II. It is, however, probable there are few 

 mistakes about the entries in Class III. Earthquakes which have been 

 described as slight or feeble, which have wakened up a few people, which 

 have even rattled or broken a few windows, moved furniture, or have been 

 simply described as being very violent, are altogether excluded from the 

 catalogue. 



The object of compiling a register of this sort is that it will be more 

 uniform in its character and more nearly up to date than its predecessors. 

 If we turn to large earthquake catalogues which have hitherto been pub- 

 lished, as, for instance, those of Mallet and Perrey, in the first place we 

 notice that the material they contain is very heterogeneous in character, 

 and the heterogeneity is variable in different periods. The earlier records 

 they give refer to earthquakes which have devastated districts, or to 

 Class III., whilst as we approach modern times equivalent disturbances 

 are eclipsed by a large number of tremors which have not been sufficiently 

 intense to be included even in Class I. Another factor which tends to 

 disturb the uniformity of the older earthquake registers is the fact that 

 the entries in the same increase in number with the spread of civilisation. 

 For example, records of earthquakes greatly increased a.bout 100 years 



