52 PROCEEDINGS OF SECTION A. 



C. WAVE-ELEMENTS, UNITS, ETC. 



T = complete Period of earth-particle (in seconds). 



A = Amplitude ,, ,, measured from median position, in 



microns, jn ; (ly« = iTiW mm.). 

 Ag = maximum acceleration of earth-particle (in milligals) — 

 I milligal = Tihrn Gal (C. G. S. unit of acceleration). 



= 10^-' cm/sec- (or approximately, g x 10 '^, since 

 1 gal =5rl7. nearly), 

 i ( = impetus) = abrupt commencement, clearly defined. 

 e { = enter sio) = gradual ,, not clearly defined. 



E' = styhis-displacemeiit for 1" of tilt (inclination), in mm. 

 E, N, Z = EW, NS, and Vertical components of earth-oscillation, respectively 

 Ep. = approximate distance from Epicentre (or epicentral area), in km. 

 E. Q. = earthquake. 



N.B. — Phase-symbols, numerical values, etc., in brackets ( ) are 

 to be considered as probable only. 



INSTRUMENTS. 



No. 1 Wiechert 1000 kilo. Horizontal-Seismometer (" Astatic Pendulum ") 

 2 Components— EW and NS. (G. Bartels, Gottingen). 



No. 2 Wiechert 80 kilo. Vertical-Seismometer. (Spindler & Hoyer, Got- 

 tingen). 

 [Nos. I and 2 in use since March and April, 1909, respectively. No. 3 to 

 be erected immediately (April, 1910.)] 



No. 3 Mainka 500 kilo. Horizontal-Seismometer (" Bifilar Conical Pendu- 

 lum "), 2 Components — EW and NS. (J. & A. Bosch, Strassburg.) 



Working-Constants (re-determined at short intervals) : last determination: — 

 No. 1 — EW To = 8.7s. V =. 207 e = 5.8. 



No. 1 — NS T„ = S.5s. V = 217 e = 4.7. 

 No. 2 T„ = 3.5s. V = 57.5 t = 2.6. 



The determination of Constants, calculation of A, Ag, Ep., etc., are 

 carried out strictly according to accepted methods. For A, a graphic method. 

 (Dr. L. Geiger, 1908) is employed, and for Ep. (in most cases) the " Lauf- 

 zeiten " of Dr. Zoeppritz. 



Site. — The Seismological-cellar (half underground) is situated in a 

 secluded portion of the College Grounds, remote from any artificial source 

 of vibration therein, 5 km. distant from the city of Sydney, and 3 km. from 

 the nearest railway. It is about 300 m. from the shore of one of the numerous 

 indentations of Sydney Harbour, at a height of about 25 m. above mean 

 sea-level. The seismometers are mounted on massive concrete piers, with 

 rock-foundation. A gap of several centimeters surrounds the piers of Nos. 

 1 and 3 to the full depth. 



Geological Structure. — The foundation is solid Triassic " Hawkes- 

 bury Sandstone," a formation of remarkably uniform character. The sand- 

 stone has a dip of about 1° westwards, and a thickness of about 270 m. 

 Weathering renders the stone soft to a depth of a metre or two from the 

 surface. Under the Hawkesbury Sandstone there are about 600 m. of Triassic 

 " Narrabeen Beds " (sandstone and shale), and probably at least 1500 m. of 

 Permo-carboniferous rocks, the whole thickness being approximately in 

 horizontal layers. 



