THE RECENT ADVANCE IN SEISMOLOGY 4ol 
obtained the average value 2.54. The average of all the rocks tested 
is a value slightly less than that of the large or surface earthquake 
waves. 
The apparently uniform velocity of propagation of elastic earth- 
quake waves through the core of the earth is a revelation of the first 
order of magnitude, for it indicates for the earth a uniformity of com- 
position, and, moreover, a rigidity equal to one and a half times that 
of the hardest steel. The preliminary tremors from the great Indian 
earthquake of 1897, as received at Rocca di Papa, had an estimated 
period of 0.5 to 0.8 of a second, while their amplitude was but a 
fraction of a millimeter. 
Following the preliminary tremors from a macroseism,’ the seis- 
mogram indicates a second phase of larger disturbances; after which 
come the “‘large waves,”’ which in the case of the quake above men- 
tioned had a complete period of 22 seconds, a length of 34 miles, and 
a rise and fall of no less than 20 inches. ‘These waves appear, there- 
fore, to travel like a slow swell along the earth’s surface. 
Another fact of great interest is that the large waves gain in period 
of vibration and lose amplitude the farther they travel, so that an 
experienced observer can roughly estimate the distance of the dis- 
turbed area from the period of vibration of the waves. With fairly 
uniform rates of propagation established for both the direct and the 
surface waves which originate at any distant origin, the difference 
in time between the arrival of the preliminary tremors and that of 
the large waves gives a further measure of the distance of the origin 
from the observing-station. For example, a distance of 80 degrees 
corresponds to a time interval separating first preliminary tremors 
and first large waves of about 35 minutes. 
Laska has derived surprisingly simple formulas for fixing the 
distance of the seat of disturbance in the case of remote earthquakes. ? 
If V, be the time in minutes of the beginning of the preliminary 
t Great confusion exists because of the different uses of the terms ‘‘macroseism”’ 
and ‘‘microseism,” as well as of the adjectives derived from them. The usage here 
is that of both Milne and de Montessus, which makes ‘‘macroseism’”’ apply to the 
greater disturbance on the ground. 
2 W. Laska, ‘‘Ueber der Berechnung von Fernbeben,” Mittheilungen der Erd- 
beben-Kommission der k. Akademie der Wissenschaften zu Wien, N. F., No. 14 (1903), 
PP I-13; 
