87 



Plate XXII gives a miiuher nf example^' of this test, .'is applied to the 

 horizontal pentlulum of § 24. The oun"es marked aa n were given by the 

 pointer whose bob was fixed : they represent the true displacements of the table, 

 magnified 4 times. The curves hbh were given by the pointer whose bob was 

 free. The general agreement of the two records with each other shows that the 

 steady line of tlie free seismograph did in fact remain very nearly steady, during 

 movements of much variety. The pointers were set facing each other, and so 

 would have dcRn-ibed arcs of opposite curvatures on a stationary plate — a fact 

 which must be taken into account in comparing the records. 



