S'2G Burbanh — Apparent Variations of the Vertical. 



arc occurred between 10 a. m. and 5 p. m. Professor Oiuori 

 considers that these remarkable variations are in large part 

 due to an abnormal rise of the sea under the storm center in 

 excess of the amount necessary to compensate the barometric 

 pressure. 



This barometric effect has been studied in this country by 

 Denison, Varnc}' and Ivlotz. Denison's results at Victoria, 

 B. C, extend over a period of 11 years and present some very 

 interesting and valuable information. 



In 1908 the records of the seismograph at this observatory 

 for about two years previously were very carefully examined 

 for indicatioris of changes of level due to the passage of areas 

 of high and low pressure across the eastern part of the United 

 States and no satisfactory relation was observed. , 



The more or less pronounced variations during this period 

 showed a fairly systematic agreement with the temperature 

 changes accompanying the passage of these low or high areas. 

 These temperature changes were often such as to cause a 

 pronounced tilting away from the storm center (low) and at 

 other times sucb as to cause an equally pronounced tilting 

 toward the center of low pressure. The direction of the tilt- 

 ing did not change appreciably as the center advanced from 

 west to east, except as the temperature changed. 



While the barometric variations produce an effect which can 

 \>e readily measured by sufficiently sensitive instruments, it is 

 in many locations entirely obscured by changes of level due to 

 causes of a local nature. This yielding of the earth's crust to 

 variations of barometric pressure is discussed by Sir G. H. 

 Darwin.* 



A general formula for the tilting shown by a horizontal 

 pendulum can be derived as follows : Let M be the steady 

 mass at the end of the horizontal strut x, see fig 1. This strut 

 ends at A in a hardened steel, cup and point bearing. The 

 weight of M is supported chiefly by two guy wires, BM, 

 attached at opposite sides of M, and joined, near B, in a yoke 

 which terminates at B in a point and cup bearing, similar to 

 that at A. 



Let 6 be the angle between AB and the vertical. 



X distance A to M (center of mass of suspended- 

 system). 

 h vertical distance of B above horizontal plane 

 through A. 



Since x is usually horizontal and 6 is always a very small 

 angle, the distance x can be taken as the approximate length 



* Scientific Papers, vol. i, pp. 444-450; or Phil. Mag., vol. xiv, 1882, pp. 

 409-427 ; or British Association Eeport for 1882, pp. 106-119. 



