575 
1898 - 99 .] Prof. Knott on Earthquake Vibrations. 
detect seismic movements, tlie more delicate the instrument must 
he; and, as pointed out by Milne in 1883 (see Earthquakes 
p. 226), the detection of vibrations which have come from an/ 
earthquake occurring at the other side of the earth is a merer 
question of having sufficiently delicate instruments. It was not, * 
however, till 1889 that a record taken in Europe was identified ‘ 
with an earthquake occurring in Japan. At the present time: 
Milne obtains at his seismological observatory in the Isle of Wight 
some 70 records per year of vibrations which have travelled over 
or through the earth from true earthquake origins. 
These records of vibrations having a true earthquake origin have 
certain characteristics which at once distinguish them from other 
types of tremors and oscillations recorded from time to time. It 
is generally possible to distinguish in each seismogram two and 
perhaps three types of vibration. There is first of all a series of 
small motions or tremors ; and after these have lasted for it may 
be 20 or 30 minutes, much larger motions assert themselves, and 
these gradually die away into a series of small tremors again. It is. 
usual to call the first set of small motions the Preliminary Tremors, 
and the subsequent larger motions the Large Waves. It must 
be remembered, however, that the so-called Large Waves have 
no effect on our senses, and are large only when compared with 
the preliminary tremors ; also that the term £ preliminary ’ has 
reference simply to the fact that the tremors come first to the 
front. Exactly similar tremors may continue throughout the 
whole disturbance. (See B.A. Report , 1898, pp. 208-218.) 
On the assumption that this type of motion has come from an 
earthquake origin, the conclusion is inevitable that the Preliminary 
Tremors have outraced the Large Waves. As a general rule, the 
interval of time between the first appearances of the Tremors and 
Large Waves increases with the distance of the earthquake origin 
from the place where the record is taken. In the B.A. Report for 
1898 (pp. 222-3) Milne has given interesting details in this connec- 
tion. In considering these, we must bear in mind that all the records 
were not made with the same type of instrument, and that con- 
sequently the comparison of intervals can be only approximate. , 
Still, there is not more discrepancy among the results as a whole 
than among the results obtained at any one place, and presumably 
