130 TRANSACTIONS iSgg-'oO 



The only other seisniograms I will show 5^ou are those of a 

 severe quake which occurred in Mexico on January 20th of this 

 year ; the preliminary tremors reached Victoria at 6h 39m 48s 

 and Toronto 29 seconds later, while the larger waves reached 

 Victoria at 6h 53m 38s and Toronto 2m 19s later. These figures 

 indicate a somewhat different origin from the quake of the pre- 

 vious year on the 24th January; and it is probable that the centre 

 was near Colima near the west coast of Mexico. 



I have shown you a number of seismograms and read you a 

 lot of figures ; but what may we learn from them ? In the first 

 place we see that, in nearly every instance, disturbance is first in 

 dicated by a feeble movement of the pendulum, lasting during an 

 interval of from a minute to a quarter of an hour, to be followed 

 by a movement of much greater amplitude, which in most in- 

 stances began suddenly. It is fairly evident then that, in earth- 

 quake motion, certain preliminary tremors of small amplitude out- 

 race the larger movements, and analysis of these seismograms 

 seems to indicate that it is quite probable that these tremors may 

 be propagated through a very rigid interior of the earth, radiat- 

 ing from the origin in all directions ; while the larger waves are 

 surface waves in the crust of the earth. 



Rules which are likely to prove appoximately correct are : 



1 . The velocity in kilometers per second with which these 

 preliminary movements are propagated is equal to one quarter of 

 the square root of the mean depth in kilometers of the chord 

 or path along which we may suppose they travelled. 



2. The duration of preliminary tremors, or the interval of 

 time expressed in minutes by which the}^ outrace the longer period 

 waves as shown by the seismogram, equals the square root of the 

 mean depth of the supposed wave path expressed in kilometers. 



A very natural question is — " Your seismograph records the 

 passage of these earth waves, the magnets at your observatory 

 begin to swing — how is it then that we do not feel them ?' ' We 

 do not feel these waves because they are very long and slow un- 

 dulations, and each p ulsation»may be from 50 to 60 kilometers in 

 length . 



My own study of seismograms has led me to the conclusion 

 that there are many changes to be made before we can expect 



