82 OLDHAM: THE CACHAR EARTHQUAKE OE 10TII JANUARY 18G9. 



neighbouring barrack or court-house as their only guide to the hours. 

 This is perhaps the only public announcement of the time, and as one of 

 the chief objects is to bring people together, it is of little consequence, 

 provided they do really meet, whether the meeting be a little later 

 or a little earlier than the true time stated. 



But with such sources of error, valuable as may be the indications of 

 time given by many observers, and especially valuable as placing the 

 identity of the shock felt at different points almost beyond the possibility 

 of doubt, still they are open to far too many and too serious sources of 

 error to admit of their being applied in the discussion of the .transit 

 velocity of the wave-form with any precision. The statements them- 

 selves are for the most part so vague and undefined, the errors of observa- 

 tion probably so great, and the errors in the time-measures so variable 

 and so large, that out of some hundreds or thousands such statements 

 of time, it is rarely indeed that we can find more than one or two, 

 perhaps not even this small number, which can be trusted. 



Nor will this pregnant source of error in any attempt to calculate 

 with precision the velocity with which such a wave-form is translated 

 from point to point be satisfactorily removed, until self-registering 

 instruments have been established at favourable points within the 

 areas subject to such disturbances. Even the possession of carefully 

 noted and timed chronometers will be a very inefficient substitute 

 for this. Observers cannot be always on the watch, and when such 

 a shock comes there is often too much astonishment or even alarm, 

 too much disturbance of all the elements that contribute to accurate 

 observation, to admit of a careful and calm noting of the precise time, 

 while further the moment of such observation is gone by before the 

 instrument is ready for observation. 



In the case of the great shock which we have now been describing, 

 I find only two, among all the observations, which can be supposed 

 to give even an approximation to data sufficiently accurate to admit of a 

 calculation of the transit velocity of the wave. One of these we owe to 

 the fortunate accident of Captain Godwin-Austin, an officer in charge of 

 ( 82 ) 



