ON SEISMOLOGICAL INVESTIGATION. 139 



the right and left of the floating-plate and through the slits in the same. 

 When the boom is steady, the resulting photogram on the moving bromide 

 paper will be, when developed, that of a white band equal in width to 

 that of the moving- plate, down the centre of which band are two very 

 clearly defined lines, one of which is thick and the other thin (tigs. 2, 3, 

 and 5). To the right and left of this white band the paper will have 

 been blackened by the light which entered at the two ends of the fixed 

 slit. On one edge of one of these black bands, at intervals of about 

 50 mm., there will be seen a series of white marks which have been 

 produced by the minute-hand of a watch, the broadened extremity of 

 which has hourly at the half -hour passed over the efid of the fixed slit, 

 and for a period of about one minute eclipsed the light. 



Should the clock at any time have failed to drive the bromide strip 

 with regularity this will at once be seen by differences in the distances 

 between successive time marks. 



Installation. — The instrument may be placed on any solid pier in an 

 observatory, on a specially constructed pier in the ground-floor of an ordi- 

 nary dwelling, or in a hut or shed in the open. The room should be dry, 

 which will generally be the case if means are provided for ample ventila- 

 tion. In order that the photographic paper may be examined or removed 

 at any time, the windows of the room should be provided with shutters, 

 through one of which red light can be admitted. A column or pier of 

 convenient size may be two bricks, or 18 inches (45 cm.) square, which 

 rises 2 feet 8 inches (80 cm.) above the floor. The base of this may rest 

 on a 6-inch (15 cm.) layer of concrete, which in turn rests on a bed of 

 gravel rammed in the natural earth. The top of such a column may be 

 made smooth by a thin facing of cement, whilst its sides should be oriented 

 N.-S. and E.-W. It is convenient to have space to pass round the pier on 

 three sides. The table, which projects from the column in a N.-S. direction 

 and carries the clock-box should be strong, 3 feet 8 inches (1*12 m.) long, 

 3 feet 7 inches (1'09 m.) broad, and rise 1 foot 8 inches (50 cm.) above 

 the floor of the room. The upper surface of this table is therefore exactly 

 1 foot (30 cm.) below the top of the column. If an existing pier is used 

 the height of the table must be increased or decreased to maintain the last 

 dimension. The table is made wide to gi%'e space for the clock-box, which 

 is run out upon it from its covering-case when removing a film. 



The installation may be on an alluvium plain or on solid rock. 



Adjustment of the Pendulitm. — The instrument is to be so placed that 

 the boom is in the meridian, or points N.-S. The balance weight is to be 

 placed at a distance of 3| inches (87 mm.) from the pivot, and the attach- 

 ment of the tie at a distance of about 5 inches (125 mm.). At the latter 

 point, but not shown in the sketch, there is a small upright, from the top 

 of which a thread is carried to within about 9 inches (22 cm.) from the 

 outer end of the boom. This is to prevent the boom from sagging. After 

 the bed-plate of the stand has been made approximately level, the boom 

 is suspended, as shown in the sketch, with its outer end about ^ inch 

 (3 mm.) above the top of the clock-box. To increase or decrease this 

 distance the tie, the last inch or so of which at its upper end is made of 

 unspun silk, may be shortened or lengthened by means of a screw at the 

 top of the stand. 



The next point is to give the boom a certain sensibility, which 

 increases as the period of its swing increases. Tlie sensibility which 

 must be arrived at is that which corresponds to an adjustment that 



