78 MODERN SEISMOLOGY 
are those by Hecker, Orloff, and Schweydar. Hecker’s re- 
sults are to be found in ‘‘ Publications of the Royal Prussian 
Geodetic Institute,” No. 32, 1907, and No. 49, IgII. 
His observations were made at Potsdam with Rebeur- 
Paschwitz pendulums at a depth of 25 m. so as to secure 
constant temperature. 
The azimuth of Pendulum I was 42°E of N, and of Pendulum 
II 48° W of N. 
The semi-diurnal disturbing potential may be written 
ie COS Mae 
a m8 (1 — 8 é) cos? zi cos" cos 2 (¢+X- 4) 
where ¢ and 2 are the latitude and longitude, e and w the 
eccentricity and inclination of the orbit, and ¢ is the lunar or 
solar time, as the case may be, referred to some convenient 
29 a 
M2=? 
origin. 
It is usual to express the observations not in terms of the 
force 
oM2 
ox 
but in terms of the apparent angular change of the direction 
of gravity, viz. :— 
wee 
RuwOre 
Hecker’s latest results are as follows :— 
Solar Effect. Pendulum I. Pendulum II. 
* T OM, , Wa 9 
Theoretical = yeh 0’’-00399 cos (2t — 305°5°) 0/:00389 cos (2¢ — 48°77) 
5S 
Observed. 000353 cos (2¢ — 255°8°) 0'”"00448 cos (2t — 36°6°) 
Lunar Effect. Pendulum I. Pendulum II. 
rt OM, 
Theoretical Sy 0'’-00922 cos (2¢ — 305°5°) 0”’*00900 cos (2¢ — 48°7°) 
ox 
Observed. 0’’:00560 cos (2f — 293°6°) 000490 cos (2t — 59°7°) 
If the lunar effects are computed for the geographical 
directions we get 
Lunar Effect. N.—S. E.—W. 
Theoretical. 0’’*00788 cos (2t — 180°) _0’’"00ggg cos (zt — 270°) 
Observed. 0''°00355 cos (2 — 175°) 000665 cos (2¢ — 270°2°) 
