ON EARTH TREMORS. 159 



made for preventing tremor being conveyed to the instrument by the use 

 of these parts of the apparatus, the whole of such tremor being taken up 

 by the casing. 



The scale and small benzoline illuminating lamp are placed at the 

 east end of the hut. A frame, supported in a horizontal position by two 

 strong iron feet fixed by beds of cement laid on the rock, is traversed by 

 the lamp-stand, which carries an index along the edge of the scale. The 

 readings are taken when the image of a wire placed vertically in front of 

 a circular hole in the lamp- screen coincides with the vertical wire in a 

 fixed theodolite. The scale is divided into millimetres, and the effective 

 length of it traversed by the index is 382 mm., and it is distant 10 feet 

 ( = 3,048 mm.) from the centi-e of the instrument. If I be the distance of 

 the lamp from the point of the scale which is due east of the mirror, 

 the azimuth, north or south of east, of the normal to the mirror is 



•i tan"' . "When ^=191 this becomes 1° 47' 3.5'', and represents 



304o 



the rotation of the miri'or to each side of the north and south position 



within the range of the scale. When the rotation exceeds this amount 



the mirror has to be brought back to its north and south position by 



turning the long handle attached to the southern levelling screw. 



The plane of the suspending wire is in the east and west direction 

 with the longer section of the wire toward the east ; consequently, as the 

 face of the mirror is towai'ds the east, a tilt of the upper support of the 

 wire towards the north produces a corresponding deflection of the normal 

 to the mirror also to the north ; hence tilts in the north and south 

 direction only are measured by the instrument. 



From measures of the dimensions of the instrument which have been 

 supplied by Mr. Horace Darwin, it is computed that the movement of 

 the lever, attached to the micrometer screw against which the top of 

 the frame is pressed, through the fixed amount of 13 mm. for which the 

 apparatus is set, produces a tilt of 2-016 seconds of arc in the upper 

 support of the suspending wire. As soon as the pendulum was mounted 

 experiments were made to ascertain its sensitiveness, or the scale value of 

 this fixed amount of tilt. The mean of nine measures taken on March 26 

 gave 62'4 mm. of the scale ^= 2"'016. This was considered excessive, 

 and steps were taken to reduce it gradually. The mean of four measures 

 made between May 5 and 8 gave 21 '2 mm. — 2"'016. Since the last of 

 these dates the two levelling screws, whose combined movement alters 

 the sensitiveness, have not been interfered with. Observations of the 

 sensitiveness have, however, been made occasionally, four measures 

 between May 16 and June 9 giving 19*3 mm., 17'2 mm., 20'0 mm., and 

 23'7 mm. respectively, or an average of 20*0 mm., equal to 2"-016. Since 

 June 9 four measures have been made, but these give somewhat anomalous 

 and as yet unexplained results. 



In the absence of any photographic arrangement for giving a con- 

 tinuous record of the position of the mirror, it has been decided to take 

 readings at each full minute from five minutes before to five minutes 

 after Paris mean noon every day. This has been carried out from 

 May 26 up to the present time, with the exception of a few days, when 

 the readings were rendered impossible by a deposit of moisture on the 

 mirror window or other cause. These observations were made by Mr. 

 T. Heath and Mr. A. J. Ramsay. 



On no occasion has any unsteadiness or oscillation of the mirror been 



