1896 - 97 .] Mr Thomas Heath on the Calcutta Earthquake. 483 
weight of a man standing on the rock, about 15 inches from the 
centre of the instrument, at a point north or south of the plane of 
the wire, caused the mirror to rotate towards the side at which the 
weight was placed. 
The instrument was fixed in position at Blackford Hill early in 
August of last year, and we have thus, so far, eleven months of its 
photographic record. Before this it was mounted at the Calton 
Hill Observatory from March 1894 to October 1895, but during 
this period we had no photographic recording apparatus, and the 
observations were made once a day by eye, and, of course, only 
those disturbances, naturally few in number, which happened at 
the time of observation were detected. The experience gained, 
however, in the working of so sensitive an instrument as this has 
not been without its use, as I believe it has enabled us to obtain a 
more continuous record at Blackford Hill than we would otherwise 
have secured. 
An examination of the photographic record for the eleven 
months just referred to shows at once that the disturbances which 
affect the instrument may be divided into at least three character- 
istic groups. 
In the first group I would place all those disturbances which 
show a widening and blurring of the photographic trace. This is 
the least numerous class, and the best marked examples of it show 
gaps in which there is no trace of effect on the paper. They are 
evidently caused by successive oscillations of the mirror; and when 
these are of great amplitude, the exposure to light at some points 
does not appear to be sufficiently long to produce photographic 
result. 
The first of these was found on the photograph of August 26, 
1896, which has two of them, and there is another on the record 
of Sept. 5. All three must, without doubt, be referred to the 
violent shocks of earthquake which were experienced in Iceland on 
these dates. The two disturbances of Aug. 26 were also observed 
in Paris. The next well-marked examples of these gaps occur on 
the 19th Feb. 1897, when there are again two of them, with an 
interval of about three hours between them, both of which were 
observed by Prof. Tacchini at Rome, and one of them by Prof. 
Milne in the Isle of Wight. The most remarkable record of all we 
