183 REPORT—1 896, 
distance, say one quarter of the earth’s circumference, these may have 
periods of from 5 to 12 seconds. 
The latter near to an origin have periods varying between 4 and 
2 seconds, whilst at a great distance this period may be 20 seconds. As 
an average maximum velocity for the propagation of the preliminary 
tremors, we shall take 11 km. or about 7 miles per second. The large 
wave motion is propagated at about + of this rate. 
It has been found by trial that fifteen or twenty stations can be found 
on the globe, so that one of these shall be near to the antipodes of shocks 
originating on the west coast of South America, Japan or the Philippines, 
or the Western Himalaya, whilst six or seven other stations between one 
of these origins and its antipodes will lie at distances from each other of 
between one thousand and two thousand miles. 
Because such an arrangement of stations is possible, we may take one 
thousand miles as being the minimum difference in distance between 
observing stations relatively to important seismic centres. 
With the assumed velocity of propagation of 7 miles per second, the 
difference in times we expect to note will be about 143 seconds. 
Because some stations will be at shorter distances from each other 
relatively to origins, I shall assume that instruments are required to note 
differences in time of 100 seconds. 
Instruments. 
The instruments at our disposal are :— 
1, An Italian type like that of Vicentini which I call V. 
2. Von Rebeur’s Horizontal Pendulum 5 3 Re 
3. Milne’s i OKAYS 
” 3) bb) 
4, Darwin’s Bifilar Pendulum + a2 Pas 
Vicentint.—A pendulum of 100 k. at least 1-50 m. long. Light in- 
dices, multiplying motion eighty times relatively to the pendulum as a 
steady point, write on a moving surface of smoked paper. Two com- 
ponents of motion are recorded. 
Von Rebeur. —A light horizontal pendulum weighing 42 grammes and 
188 mm. in length, carrying a smallmirror. Light from a lamp is reflected 
from this back through suitable lenses upon a slit in a box containing a 
drum carrying a bromide film. 
Milne.—A. horizontal pendulum with a boom 2 ft. 6 in, long, the 
whole apparatus within a case 4 ft. x 1 ft. 3in. x 2 ft. The end of the 
boom is continuously photographed on a bromide film 2 in. wide. . Because 
the lamp is within 6 in. of the paper, the necessary light is small. 
Darwin.—A circular mirror with a bifilar suspension, so arranged that, 
a slight tilt causes the mirror to rotate. This is immersed in paraffine. 
The instrument is exceedingly sensitive to change of level, but not to 
elastic tremors.. The recording apparatus is photographic and very similar 
to that used by von Rebeur. 
Accuracy as time-recorders (important).—The accuracy depends upon 
the rate at which the recording surface is moved, the method employed. to 
mark time intervals upon its surface, and lastly the fineness or sharpness 
of definition of the record. 
