454 L. A. COTTON. 



relatively small fraction of the total water load. The 

 variations in water level might thus be expected to affect 

 the booms less than in the case of either of the other 

 pendulums. 



In the case of No. 3 Pendulum (the Potsdam Pendulum 

 established at the River Tunnel), both booms are deflected 

 in a most marked manner. In this case also the amount 

 and direction of the deflection of the vertical is represented 

 on the map. This pendulum is twelve miles above the dam, 

 and the water load is not great, being represented by a 

 depth of about twenty-two feet of water at the commence- 

 ment of the records. Small variations in the water level 

 therefore represent large relative changes in the stresses 

 imposed. This is consistent with the nature of the curves. 



Thus in the case of each instrument there is a degree of 

 correspondence between the movement of the pendulum 

 booms and the variation in the water load ; and this corres- 

 pondence is so marked as to render a causal connection in 

 a high degree probable. 



There are at least four types of earth movements which 

 are being recorded by the pendulums. These are 



1. Earthtides. 3. Fault movements. 



2. Earthquakes. 4. Slow deflections of the vertical. 



The Earthtides. — As two of the pendulums were pre- 

 viously used for the detection of the earthtides in Europe, 

 it was to be expected that they would record this phe- 

 nomenon at their present stations. This expectation has 

 been fulfilled. 



As Burrinjuck is situated 125 miles due west from the 

 coast of New South Wales at Jervis Bay, it is possible that 

 the records may be slightly influenced by the load of the 

 oceanic tides. It will be of great interest to compare the 

 records of these instruments with those from the Standard 

 Earthtide Station at Oobar. This station forms part of the 



