330 Burhanh — Apparent Variations of the Vertical. 



Tlie curves in fisjure 3 show that shortly after sunrise, on 

 clear days, the W-E pier began to tip toward the east, reaching 

 its maximum east deviation about 10 a. m. ; it then reversed 

 its motion and tipped toward the west, reaching maximum west 

 deviation about i p. m. (K! In-s.), and then slowly returned to 

 its normal })osition. It shonld be noticed that all the curves 

 show the slow tilting due to gradual cluuiges of temperature, 

 on which these diurnal tiltings are snperposed. Tiie tilting 

 begins earlier in summer than in winter, but the times of max- 

 imum east and west deviation do not vary greatly with the 

 season. The range of motion is greatest in winter, the yearly 

 average being about one second of arc. Any cloudiness or other 

 condition which diminishes the intensity of the sun's radiation 

 tends also to diminish the range of tilting. Tliis is especially 

 noticeable in cases of fog in the morning. 



The results of this tilting of the E-W pier have been recently 

 confirmed by a similar seismograph adapted to show only the 

 slow tilting of its pier. The heavj' mass, M, carried a vane 

 dipping in a dish of glycerine, so that all rapid oscillations were 

 damped out. A pointer fastened on M was in contact with a 

 small arm extending from the mirror frame of an Eschenhagen 

 magnetic variometer. The mirror was carried by a quartz fiber 

 and the torsion head was turned so that the arm and the pointer 

 remained in contact. Any movement of M caused the mirror 

 to turn. The record was obtained on bromide paper in the 

 same way that the magnetic elements are recorded. The sen- 

 sitiveness was such «that any motion of M was magnified 300 

 times. Eeeords were obtained from Aug. 28 to Oct. 12, 1908, 

 and they confirm the general resiilts stated above. They show 

 clearly the great variation in amount of tilting on different 

 clear days, depending apparently on the intensity of the solar 

 radiation. 



Since October, 1907, the seismograph has been mounted on 

 a concrete pier in the new seismograph house. Here the tilt- 

 ing due to external temperature changes has been larger than 

 in the former location. When the external temperature rises 

 the pier tilts toward the east and south, directly opposite to 

 the effect in the variation observatory. The diurnal tilting is 

 inappreciable on the N-S component, but is well marked on 

 the E-W component. It differs essentially from that observed 

 in the variation house, consisting of a tilt toward the east, 

 beginning about 10 to 11 hrs. and reaching a maximum east 

 deviation about 16 to 18 hrs., and then returning slowly to its 

 normal position. The diurnal oscillation is in tliis case a 

 deflection to the east and return. The building is placed in 

 the side of a gravel bank so that the west side only is exposed, 

 all other sides being banked to the eaves with gravel. There 



