ON SEISMOLOGICAL INVESTIGATIONS. 85 
Colombo, Ceylon (Observatory at the Technical College). 
Lat. 6° 54’ N.; long. 79° 51’ E.; alt. 13 feet above mean sea-level. 
Foundation.—A brick-in-cement pier built on a cement-concrete base rising from 
a bed of laterite. 
Topographical Situation.—The Observatory is situated on low ground, quite close 
to a canal, abont 50 feet from the bank of it. There is a lake about 150 yards away 
towards the south-west, with which the canal communicates direct. The sea is 
about a mile distant (south-west) from the Observatory. The canal above mentioned 
is on the north of the Observatory. 
Geological Structure.—The ground surrounding the Observatory to a considerable 
distance is alluvium with outcropping laterite. 
Rating.—The rating of the time-keeper attached to the instrument is done by 
comparison with the chronometer of the Master Attendant, Colonibo Harbour, at 
intervals of two to three days, by means of a portable time-keeper carried backwards 
and forwards in a locked box. Periodic time of instrument varied between 
18 seconds and 15 seconds. During the period August to December the periodic 
time did not fall below 17 seconds, and for the months of September and October 
the periodic time was nearly 18 seconds. 
E., HUMAN, Superintendent. 
III. Photographic Record-receivers. 
If two similar and similarly adjusted seismographs are installed on 
sites which are geologically and topographically different it is to be 
expected that the records they yield will show certain differences. If, for 
example, we compare the seismograms obtained at a station on rock with 
one on alluvium we find that at the former, within a given period, more 
records have been obtained, and earlier times of commencement, than at 
the latter! The probable explanation of this is that thick beds of 
alluvium, in consequence of their non-elastic nature, do not transmit the 
waves of small amplitude which constitute small earthquakes and the 
preliminary tremors of larger disturbances. 
Another condition on which the recording of very minute waves is 
dependent, and which does not hitherto appear to have been recognised, 
is the speed of the film on which the record is received. 
In connection with the Milne horizontal pendulums adopted by the 
British Association, two types of recording surfaces are now employed. 
In one the photographic film moves beneath a slit about 0:25 mm. in 
width at the rate of 60 mm. per hour. In the other the photographic 
surface passes beneath a similar slit at a little over four times that rate.? 
In the first type of receiver the paper as it passes the slit is exposed 
to light for fifteen seconds, whilst in the second the exposure is between 
three and four seconds. Experiment shows that the line obtained from 
the long exposure may be double the breadth of that from the short 
exposure. From this it would seem that minute tremors with a short 
period which would show as deviations of the narrow line might be 
eclipsed if the same became broadened by halation. This, however, would 
not be the case if the tremors had a very long period. 
This probably explains the observation that earlier commencements are 
more frequently noticed on a rapidly moving surface than on one which 
moves more slowly. For example, for the year ending June 1906, for a 
series of sixty-one disturbances, a pendulum, at Shide recording on a quickly 
moving surface was on forty occasions from one to six minutes in advance 
' See Brit. Assoc. Rep., 1902, p. 74, and 1908, p. 82. 2 Tbid., 1904. 
