332 Burhank — Apparent Variations of the Vertical. 



lOOS is due partly to solar diurnal eli'oct and partly to lari^e 

 temperature variations occurring at this time, when tlie niglits 

 were very cold. Of this period Jan. 31st and Feb. 1st were 

 l^artly cloudy and Feb. 5th was cloudy. During the period in 

 Ai>ril and May, April 27tli was cloudy in the v. m., April 28th 

 was cloudy, April 30th cloudy, and May 3d, 4th, atii, and 

 6th cloudy. April 29th was clear. 



Ai> interesting effect which has been frequently noticed 

 during the winter of 1908-9 ap}>ears as a very minute tilting 

 of the pier back and forth of very small amplitude and irregu- 

 lar period of about a minute. This occurs at times wlien the 

 ground is frozen and is especially noticeable when it is freezing 

 or thawing. 



jMo complete explanation of the diurnal tilting is evident, 

 yet certain facts are apparent. The tilting is not due to any 

 change in the temperature of the seismograph itself, or of the 

 pier, since temperature changes inside the variation observatory 

 are insignificant. While the temperature changes in the new 

 siesmograph house are often as great as 1° C. per day, yet on 

 two occasions a change of 2° or 3° C, produced by a lamp in the 

 room, had no appreciable effect on the tilting of tiie pier. The 

 actual tilting differs in amount from place to place, yet it 

 appears to be a comparatively common effect. 



Tilting due to rainstorms. — Sudden and heavy downpours 

 of rain do not appear to cause any decided tilting effect on the 

 piers in the variation building. A heavy rain has, however, a 

 decided effect in tilting the pier in the new seismograph house. 



It is interesting to note that this pier always tilts toward the 

 northeast, the E component of motion being much greater than 

 the N component. This is possibly accounted for by the sur- 

 face formation at the south and west of the house. A slight 

 depression beginning about 50 yards southeast of the building 

 extends around the south and west sides, being about 50 yards 

 wide on the west side of the observatory. Up to this point its 

 slope is very gradual and its surface is here on a level with the 

 bottom of the pier. From this point the depression falls away 

 more rapidly toward the north. Aboiit 50 yards northwest of 

 the house are several springs and the soil beyond this point 

 may be regarded as saturated with water. 



The house is placed in the edge of a gravel bank, the top of 

 which is about 15 to 20 ft. above the level of this depression, 

 and the movement of the pier is the same as if the bottom of 

 this depression was expanded by the water and pressed against 

 the side of the hill. 



P'igure 5 gives some curves of tilting due to sudden rain- 

 storms. That on August 5th is the best example. Between 

 3:30 p. M. and 5 p. m., 2"95 inches of rain fell. The ground 



