74 MODERN SEISMOLOGY 
The following table gives the average amplitude and period 
observed at Eskdalemuir on the Galitzin horizontal seismo- 
grams. 
: Amplitude of 2 Amplitude of 
IgII Period IQII Period 
Month. seconds, CR Month. seconds. ee 
Jan. 6°6 2°2 July 4°3 03 
Feb. 6:0 2°1 Aug. 4°2 0°3 
Mar. 555 Te Sept. : 0'6 
B 8) 
Apt. 555 o'7 Oct. 4°9 o°5 
May 5:2 06 Nov. 5°4 r°8 
June 4°5 04 Dec. 5°3 18 
Average for year: Period = 5:2 seconds; Amp. = Io. 
The vertical movement is quite as pronounced as the hori- 
zontal movement, and this suggests that we are dealing with 
Rayleigh waves propagated over large continental areas. 
The general phenomena and the periods presented by 
these microseismic movements correspond so closely with what 
one observes of the sea waves on the coast, that one can 
hardly doubt that the two things are closely connected. Dr. 
Schuster has devised and set up an apparatus near Newcastle 
for obtaining a continuous register of the sea waves, but de- 
tailed results are not yet available for comparison with the 
movements shown by seismographs. 
It has been suggested that the land effects are due to the 
actual breaking of waves on the coast, but this can hardly be 
maintained as an explanation of effects observed so far inland 
as central Europe or central Canada. It seems more probable 
that, and it is at least worth while investigating theoretically 
whether, the motion observed far inland is due to Rayleigh 
waves set up at the bottom of the sea by water waves set up 
and maintained over large ocean areas by the wind. To take 
a simple example: we know that a travelling wind sets upa 
train of waves following after it. On deep water, such as mid- 
Atlantic, we should get a period of 5 seconds, wave length 40 m., 
with a wind velocity of 8 m. per second, or about 20 miles per 
hour; while a period of 10 seconds, wave length 160 m., requires 
a wind velocity of 16 m. per second. Such waves advancing 
