484 Proceedings of Royal Society of Edinburgh. [sess. 
have yet secured, however, appears on the photographic trace of the 
12th of June (figs. 1 and 2), the date of the great earthquake at 
Calcutta. Special interest attaches to the record of this disturbance, 
because of its long duration and great amplitude, and also because 
of the distinctness with which several of the separate oscillations of 
the mirror appear on the paper. The duration of the earthquake 
at Calcutta is stated to have been, according to one account, only 
“some seconds,” according to another “ fully five minutes.” How- 
ever this may be, the undulatory movements arising from it cover 
a period at Blackford Hill of nearly two hours. Very slight pre- 
liminary tremors commenced at 11.18 in the forenoon and lasted 
ten minutes. The more violent oscillations commenced at 11.28 
and lasted up to 0 h 33 m , thus covering a period of 1 hour 5 minutes, 
and after this lighter tremors can be traced up to 1.12. The 
amplitude, when at its maximum in the distinctly photographed 
part of the record, I found to be equivalent to an angular move- 
ment of the frame which supports the wire of about 20 seconds of 
arc, and we are justified in assuming that in the unphotographed 
part, or gap, the amplitude must have been still larger. 
The first newspaper reports of the earthquake gave 5 o’clock p.m. 
as the time at which the shock was felt at Calcutta, and I have not 
since seen any more precise statement of the time of its occurrence, 
— 5 p.m. at Calcutta corresponds to 11.6 of the forenoon in Green- 
wich mean time. The first oscillation of large amplitude occurred 
here at 11.28. If we put aside the lighter tremors, and suppose 
for the moment that the first large oscillation shown here was the 
result of the first violent shock at Calcutta, we have twenty-two 
minutes as the time taken by the seismic wave to travel through 
the earth’s crust from Calcutta to Edinburgh, a distance in round 
numbers of 4970 miles on a great circle, or 226 miles per minute. 
At Grenoble the record of this earthquake was registered on the 
seismograph at 11.28. If I am right in supposing this to be Paris 
mean time, it would correspond to 11.19 Greenwich mean time, or 
one minute later than the beginning of the preliminary tremors at 
Blackford Hill. 
It may be interesting to note that, with the exception of two 
very slight tremors on the afternoon of the 11th, there is not a 
vestige of any kind of disturbance on the photographs preceding 
