EARTH MOVEMENTS AT BUKRINJUCK. 145 



changes in the water level and are no doubt connected 

 with such changes. The movements of larger amplitude, 

 however seem to have another origin. 



Mr. D. F. Campbell the resident engineer at Burrinjuck, 

 who has generously devoted much of his leisure to the care 

 and maintenance of these instruments, has recently called 

 my attention to the fact that these major changes are more 

 or less seasonal in character and that they follow the same 

 kind of variation as the temperature changes registered by 

 a thermometer which had been placed in the concrete dam 

 at a distance of 80 feet from the surface. Acting on this 

 suggestion I have worked out in detail the changes recorded 

 by the pendulum installed at Dale's tunnel, for the period 

 of four years from 1916 to 1919 inclusive. The results are 

 represented graphically in figure 1, in which the movement 

 of each boom is plotted and compared with the temperature 

 curve for the corresponding period. As the temperatures 

 are those recorded by a thermometer placed 80 feet within 

 the wall of the dam, the temperature variations therefore 

 lag some months behind the air temperatures at the surface. 

 This is, of course, due to the low heat conductivity of the 

 rocks. 



The degree of correspondence between the movements 

 of the pendulums and the temperature variations is very 

 remarkable, and casts a new light on the movements of the 

 outer earth's crust. 



The scale of the deflections given in figure 1 is different 

 for each pendulum boom as will be seen by reference to the 

 scale of seconds placed at the left hand side of the diagram. 



The actual amounts of movement measured in seconds 

 of arc may be readily obtained from figure 2, in which the 

 separate components of the two booms have been com- 

 pounded into a single resultant movement. In this diagram 

 the stations marked 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 

 correspond to the following dates: — 



J— August 3, 1921. 



