-. 22 



(see § 24 1 the ]>lati' i-< kept revdlvinix ('iiutiiuiDUsly in tlic dircctioii of tlic arrow 

 alu)iit a vertical axis fixed to tiie eartli, and sii loiii; as no earthquake oeeurs eaeh 

 pointer traces over and over a<;ain tlie sante eiiele on the revolvin<;- snrl'aoe. 

 J)uriiii>; a disturbance each records the component transverse to itself of all the 

 successive horizontal motions which may occur. The hase-plate of eaeli lever, 

 which inu>t of course be rigidly fixed to the earth, is clamped by means of three 

 levelling-screws and a holding-down bolt in the centre to the toj) of a wooden 

 |)ost, firmly driven into the ground and cut off a few inches above the surface, 

 "i'liree convergent Vs, or a jivraniidal hole, a V-slot and a plane surface aie cut 

 on the fo]) of the jiost for the levellin<>;-scrc\vs to ])ress into. Either arrangement 

 gives what Thomson and Tait * call a geometrical clamp, and, while affording 

 ]>erfect definiteness of su|)])ort without any nice fitting, allows the ai>i)aratus to 

 l)e removed and replaced in precisely the same position whenever that may be 

 desired. 



1\) nduee as far as jiossible the friction of the marking jioint on the glass 

 plate, a light spring h is added (adjustable by the screw 0, which carries a portion 

 of the .straw pointer's weight Ity means of the silk fibre j. 



§ 24. Improved fiinti af Hurizontid Penihihiiii Sei-<iuoff)-aj)h. 



Experience gained with fhe instrument <Iesciibcd above led to a eonsideial)le 

 modification of details, chiefly with the view of diminishing fri<'fion, and i)f 

 rendi'ring the apjiaratus h'ss liable fo get out of adjustment. The resulting form 

 is shown in I'lates If and 111. In Plate II, fig. 4 is a general ])laii of the 

 seismogra])h and driving clock com|ilete ; fig. .'") an i4evation of the glass plate 

 an<l sectional elevation of one of the ]»air of ])endulums; fig. 6 an I'levation of 

 the driving clock, sln)wing the sjieed-governor in section. In I'late 111, fig. 7 is 

 an isomefrical drawing of one of the penduhnns, and fig. 8 .shows the method by 

 which the glass plate receives motion from the clock. As before, each penduliuu 

 consi.-ts of a frame juvotted, about a vertical axis, to a fixed stand, and furnished 

 with a massive liob which is pivotted to the frame about a line which is the 

 instantaneous axis corres|)onding to the axis of su]>]iorf. A light |)rolongation of 

 the frame forms tlii' umltiplying lever, whose motions are recorded on a con- 

 finuonsly moving smoked-glass ])late. 



Each frame a (figs. 5 and 7) is a light triangle t)f steel, and the bobs are 

 Irnncated'cones of cast iron, ]iivotted to the iVanies in a manner which the figures 

 suliiciently exj)lain. The axis of support of each frame is defined by the points 

 of two hard steel screws b and c, of which the ujiper one li works in a A'^-slot in 

 a piece of agate, strongly suj>]'orted by f\\o wooden u|)iiglits fixed fo the base- 

 ]>late. One of the wooden uprights is removed in figs. .') and 7 to allow the 

 frame and other parts to be seen. The lower pivot c works in a conical socket 

 of hard steel and |)oints in the direction of the thrust (an improvement suggested 

 to the writer by Mr. T. (iray) -that is tosav its axis, if produced upwards, would 



* Nuhind Pliilosoiiliy, Vol. 1, § l'.)8. 



