1896 — 97 .] Mr Thomas Heath on the Calcutta Earthquake. 485 
that of the 12th till we go back as far as the 3rd June. Thus for 
nearly eight days the mirror was perfectly quiescent. On the 
other hand, between the remarkable disturbance of the 12th and 
the noon of the 16th there are four well-marked tremors to be 
seen, probably due to readjustment of the strata following the 
great shock of the 12th. 
I have now called your attention to all the more interesting 
examples of the first group of records. The second group or class 
comprises all those records which appear to have been produced by 
a single sudden tilt of the frame of the instrument, causing the 
mirror to rotate about its vertical diameter in one direction only, 
without any apparent trace of rapid oscillation in the opposite 
direction. The mirror returns to its normal position after some 
hours, but so slowly that this movement is quite masked by being 
mixed up with the slight swerving of the mirror which frequently 
takes place, and is due probably to seasonal changes of tempera- 
ture. Sometimes three or four of these tilts are found following 
one another in the same direction at intervals of several hours. 
More often they occur in opposite directions alternately, with 
intervals varying from several hours to five or ten minutes between 
them. In the case of the short intervals, they have an appearance 
on the curve like the tooth of a saw. 
An interesting example of three tilts in the same direction is 
found on the record of Feb. 6 and 7 (fig. 3). The first occurred at 
19 h 33 m on the 6th, a second at 5 11 35 m on the 7th, and the third at 
13 h 20 m . They are each towards the north, and are followed by 
a perfectly straight line sloping towards the south. Probably the 
sloping position of the lines is caused by the combined effect of the 
return movement of the mirror and the temperature swerving. On 
the same day records were found on the instruments at Rome and 
the Isle of Wight, but with this class of disturbance the identifica- 
tion of the records at different stations is not so satisfactory as it 
is in the case of the first group. The photograph of Jan. 28 (fig. 4) 
shows four sudden tilts towards the north, at shorter intervals 
than those of Feb. 6 and 7. Each tilt is again followed by a line 
sloping towards the south. There is no evidence, that I have seen, 
so far, of the disturbances in this case having been recorded 
elsewhere. 
