62 



It is intercsflnir to compare tlic rcoonls of the ];iiiici])al inovonionts givon liy the 

 two instiunicnts, and for tlii.s juirpoi^c the two eoniponcnts on Plate XIX have 

 been combined. 



It must ho liiirnc in mind that the diagram, as litliographed fnmi a photo- 

 graph, hears to the original tracing on the jilatc the relation of a 7niyror image 

 to its «ibject, and therefore the directions N.E.S.W. follow each otiier counter- 

 clockwise instead of clockwise. 



Lines, numbered 1, 2, 3 etc., have been marked on the two components of 

 the horizontal jjcndnlum record, to show corresponding instants of time ; and 

 these lines are drawn (by means of a template) so that each is the path wliich the 

 corresponding jointer would have described if it had been caused to oscillate 

 M-hilc the jilato stoc^d at rest. J'-arthcpiake displacements arc therefore to be 

 measured along them. 



It will be seen at once that the pliases of the two components do not agree, 

 in other words that the motion of the ground is not rectilinear. The actual jiath 

 of a surface jiarticle, got bv carefully compounding the disjdacements as deter- 

 mined by I'late XIX, from the jwints immbered 1 to IG, is shown in the upper 

 figure in the centit" of Plate XX. To make it immediately comparable with the 

 long pendulum record it is, however, magnified two and a half times, so that it 

 bears to the earth's motions tlu' ratio of 10 to 1. The path begins at the origin 

 and is traced in the dinction of the arrow hea<ls : it ends at a ]>oint correspond- 

 ing to the jx)ints numbered K! in I'lat'^ XIX. The time taken to trace this 

 j)ath was 3-7 seconds. 



The lower figure in the centre of Plate XX is a facsimile of the record 

 given by the long iicndulum duiing the same interval: it is inverted so as to 

 form a true fac-simili' ami not a mirror image. Its .scale is also t( n times 

 full size. The line marked on it E. .'50"N. is the direction of movement of the 

 plate under the pointer. Hence the lower diagram ought to agree with the 

 upper one, if we sup]iosc the upper one to be stretched out at a uniform rate, 

 during the time of its being de.-^cribed, in tlie dirertion W. ;^0'S. — the direction 

 shown by the dotted line in the up])cr figure. 



An inspection will show that tiie agreement between the two records is very 

 satisfactory, if we make allowance for slight movements on the part of the "steady 

 point", which, in such a violent disturbance and during so considerable an inter- 

 val, are in fact almost inevitable. By compressing the lower figure along the 

 tangent to the circle we get a path very similar to that in the ujjper figure. Tiie 

 two instruments which gave the records here compared are so different in all res- 

 pects that the agreement of the records, when thus examined, aflbrds very strong 

 evidence of the substantial accuracy of both. 



The greatest range of actual earth movement in this case was ö mm. The 

 greatest vertical motion was less than 1 mm. The movements were so far from 

 cyclic that we cannot, except very loosely, speak of their jieriod. For the prin- 

 cipal movements it was about O.S .«ec. 



