Form of Seismograph. 363 



lum-bob and strut is supported by a thin wire, v, attached at 

 the lower end to a stirrup, w, pivoted at x a little below and 

 in front of the centre of gravity of r, and taken at the upper 

 end over a small wheel, y, to a drum, z, round which the wire 

 may be wound, so as to adjust the level of the strut, s. The 

 position of the pivot, x, is so arranged that the knife-edge at 

 t has little or no tendency to rise or fall, no matter at what 

 part of the strut the cylinder r may be clamped. The wheel 

 y is provided with adjusting screws, a x and 6 1? by means of 

 which the top of the wire can be placed vertically above the 

 knife-edge, or as much in front of or behind that point as may 

 be necessary to make the period of free vibration of the pen- 

 dulum have any desired length. A light aluminium lever c\ 

 is hinged to the strut sat d ly and is provided at its outer end 

 with a small hollow steel cone e ly which may be placed over 

 one or other of a series of sharp points / l5 fixed to the vertical 

 arm of the cranked lever g v The lever g x turns round a 

 horizontal axis at h t in bearings fixed to the ink-well i } , and 

 the vertical arm is hinged at j u so as to be free to turn in a 

 direction at right angles to the plane of the crank. A siphon, 

 ki, is fixed to the horizontal arm of the lever g lf and, drawing 

 ink from the well i 1} writes a continuous line on the paper 

 ribbon. The horizontal arm of the lever g x is made very 

 flexible in a horizontal direction, and besides can be turned 

 round a vertical axis to such an extent as allows the pressure 

 of the point of the siphon on the paper to be adjusted until it 

 is only sufficient to give a record. 



The horizontal-lever pendulum used for actuating the 

 siphon which writes the vertical motion is illustrated dia- 

 grammatically in fig. 6. It consists of a horizontal lever, l ly 

 carrying at one end a cylindrical weight m l9 and free to turn 

 round knife-edges n 1 , fixed to the other end of the lever. 

 The lever is supported in a horizontal position by two flat 

 springs, clearly shown in fig. 1, and indicated at o 1? fig. 6. 

 A light aluminium index, p 1 , pivoted at q t , and connected by 

 a thin wire or thread to the end of the lever h, carries a fine 

 siphon, r l} which rests with one end in the ink-well, s x , and 

 the other end touching the surface of the paper. The end of 

 the index is weighted sufficiently to cause it to follow the 

 motions of the lever. This arrangement gives a period of 

 free vibration of about two seconds in the actual instrument ; 

 and in order to increase this period a second set of springs, 

 indicated at t ly are made to act on knife-edges, u 1} fixed ver- 

 tical^ above n ly so as to add negative stability to the arrange- 

 ment. When the lever is deflected downwards the pull on 

 the supporting spring is increased, but at the same time the 



