388 Dr. C. Davison—Founders. of Seismology— 
a combination of horizontal motion pendulums of the Ewing type 
and a Gray vertical motion seismograph, all recording in ink on a 
continuously moving strip ef paper, so that the register could be 
continued without interruption except for the brief interval during 
which the paper was being changed. 
In 1880 Gray had rediscovered the type of horizontal pendulum 
first devised by A. Gerard in 1851, the heavy mass being carried 
on a horizontal rod, the pointed end of which rests in a conical hole 
of the upright frame, while the mass is supported by a wire attached 
to a point of the frame nearly but not quite vertically above the 
pointed end of the rod. Realizing the value of such a pendulum for 
the registration of distant earthquakes, Milne, before he left Japan 
(1892), invented the instrument afterwards widely known as the 
Milne seismograph. With various modifications, it became for 
many years the standard instrument of the Seismological Committee 
of the British Association, and with it a vast amount of useful work 
was done. It is now being gradually replaced by the much-improved 
Milne-Shaw seismograph. 
Nature of Harthquake-Motion.—One can well understand the interest 
of the early meetings of the Seismological Society when the first 
precise records of earthquake- -motion were exhibited by Ewing and 
Milne. Most of the shocks in those days were comparatively slight, 
but they were strong enough to reveal the general character of the 
motion. Some of the results were unexpected, especially the small 
amplitude of the movement, which, in ordinary earthquakes is 
often less than one millimetre and seldom exceeds a few millimetres, 
while the maximum acceleration usually ranges from 10 to 100 mm. 
per second per second, and only in destructive earthquakes surpasses 
500 mm. per second per second. The main conclusions are perhaps 
too well known to require rehearsal here. Some reference must, 
however, be made to the many seismic experiments which Milne 
conducted, for the most part in conjuncticn with Gray, and to 
Milne’s study of the variations of the movement within a small area 
and under certain artificial conditions. 
The registration of earthquakes, it might at first sight be expected, 
would give all the information desired. But the earthquakes came 
at uncertain times and from different regions. The observing stations 
were limited in number and required arrangement with reference to 
the source of disturbance. Above all, that source was usually oi 
considerable size, and distortional vibrations from the nearer parts 
of the focus combined with condensational vibrations from more 
distant portions. It was necessary, therefore, to simplify the 
disturbance, and this was attained by explosions of dynamite and 
by dropping a heavy weight (17101b.) from various heights up to 
oo feet. 
The most interesting result of the experiments was the complete 
separation of the condensational and distortional vibrations. When 
the record was made with a single index on a fixed glass plate, the 
