SEISMOLOGICAL INVESTIGATIONS. 215 
Let & denote the ratio of cos ED to cos EO, which is not very different from 
unity. Then 
(ka, —Ady)A +- (kb, —b.) B+ (key —Cy)C =I) 
Considered as a relation between (A, B, C) this represents a great circle on 
which (A, B, C) must lie. It has also the property of being at rignt angles to the 
great circle joining OD. For the pole L of this circle has co-ordinates 
by¢,—b C2, Cx4,—C 2, AyD,—aybo 
and these satisfy the above equation, which therefore represents a great circle 
through L, and consequently perpendicular to OD. 
We can find the point K at which this line cuts OD from the relation 
cos KD=k cos KO 
Since KD=KO~— 2d, this becomes 
cos 2d+sin 2d . tan KO=k 
or tan KO=(k—cos 2d)/sin 2d 
so that KO can be readily tabulated in terms of & for a given pair of stations. 
Or the positions of K for given values of & can be marked off on the projection. 
Microseisms. [By J. J. Suaw.] 
In the Report for 1920 particulars were given of some investigations made 
upon microseisms, and it was there shown how it was possible at stations two 
miles apart to identify the individual waves so precisely that there was little 
difficulty in determining the time of arrival at each station to within a fraction 
of a second. 
It was therefore proposed to extend the experiments, using three stations 
situated about ten miles apart, with the object of measuring more accurately 
the rate of propagation, and to confirm the previous observation that their 
direction was consistently from the North. 
The 1920 experiments were conducted at West Bromwich. 
Inquiries were made to discover two capable observers, 
(a) at suitable distances from West Bromwich, 
(6) with cellars or other accommodation which would provide a stable 
site for the instruments, 
(c) provided with telephones for synchronising the time circuits. 
By the kindness of Mr. Harry Walker, of Sutton Coldfield, and his brother, 
Mr. Sidney Walker, of Solihull, who gave much time to the work, the above 
conditions were fulfilled; and the thanks of the Committee are here placed on 
record for their valuable help in the experiments. 
The instruments used were three Milne-Shaw seismographs, timed by three 
closks with rates of about 1 sec. per day. 
Each time circuit was provided with an audible ‘clicker,’ which could be 
placed near the telephone and heard at West Bromwich each minute. By this 
means the time breaks were either synchronised, or the difference (if small) 
_ observed. 
One machine was installed at Sutton on 1921, January 18, and the other at 
Solihull on 1921, February 28; and both were oriented on a line 12° East of 
North to conform to the only convenient orientation at West Bromwich. 
All machines were used with 10 sec. period; 20:1 damping; and a magnifica- 
tion of 250 :1. : 
From the outset it was noted that it was not only quite impossible to 
identify individual waves, but ever the trains of waves would not bear com- 
_ parison. 
On February 28 West Bromwich and Sutton machines only were running. 
They showed an exceptional series of torpedo-shaped maxima for many hours; 
but it was rarely that the maxima were in agreement at the two stations, and 
then obviously by mere chance. 
