HoGBEN. — The Tasmanian Earthquake. 597 



is felt, or only one direction is observed. To use the direc- 

 tions, then, we evidently have the following rule : Draw two 

 lines through each place, one in the direction named in the 

 report, the other at right angles to it ; one of these two lines 

 will be very possibly that of the direction of the shock. In 

 the diagram, for instance, the shock felt at might have 

 come from any of the four directions, AO, BO, CO, DO, but 

 it could not have come from X. 



The apparent directions were given for twenty-one places 

 in Tasmania as follows : Launceston, E. to W. (second 

 shock) ; Scottsdale, N. to S. ; St. Mary's, B. to W. (second 

 shock) ; Swansea, W. to E. ; Eagle Hawk Neck, N.E. to S.W. 

 (second shock) ; Boobyalla, S.E. to N.W. ; Cape Portland, 

 S.E. to N.W. ; Ormley, W. to E. ; Avoca, E. to W. (second 

 or third shock) ; Franklin, S.W. ; Beaconsfield, E. to W. ; 

 Sorell, S. to N. ; Geeveston, N.W. to S.E. ; George's Bay, 

 N.E. by N. to S.W. by S. ; Buckland, E. to W. ; Oaks, E. 

 and W.; Campbelltown, S.E. to N.W. ; Karoola, N.W. to 

 S.E. ; East Devonport N.W. to N.E. ; Blessington, from 

 N.W.; St. Helen's,'E. to W. 



The directions for Australia were given as follows : Omeo, 

 S.E. to N.W. ; Bairnsdale, W. to E. ; Grant, N. to S. ; 

 Forster, S.W^ to N.E. ; Genoa, W. to E. ; Cape Everard, 

 S.W. to N.E. ; Gabo, E. to W\ ; Walhalla, N. to S. 



Now, drawing lines in the directions indicated, we find 

 we can describe a circle to cut or touch lines through twenty 

 out of the twenty-nine places. The centre of this direction- 

 circle is D. Its radius is large (eighty miles) ; so that the 

 indication is only rough. The effect of the direction-lines of 

 two or three places of the nine whose lines do not cut the 

 circle would be to throw the origin somewhat further to the 

 east. We should therefore expect to find the origin either 

 within or somewhat to the east of the circle D. 



The indication, if any, given by the directions reported by 

 ordinary observers is only rough, and I should not have dwelt 

 so long upon it but for the circumstance that it is the custom 

 to despise it altogether, a prejudice which experience in the 

 calculation of a very large number of earthquake origins in 

 New Zealand has shown me to be wrong. One cause of error 

 in observing the direction must be noticed : When the method 

 fails the failure generally is not due to eccentricity in the 

 movement of the earth-particle, but to the neglect of the 

 observer to take into account any peculiarity in the method 

 of support of disturbed objects which would tend to make, them 

 move, whatever the direction of the disturbing force might be, 

 in certain directions only. 



I. (b.) The determination of the origin by means of the 

 observed times of the same phase of the earthquake is far 



