65 



The non-i"cotilincar character of the motion, ^\■lli(•ll can 1)0 dccluceil (as has 

 l)een ilcme above) liy compound iiig two rectangular coniponcnts of tlio liorizontal 

 motion when these are separately recorded, is most directly shown liy seismographs 

 ]iossessing two degrees of liorizontal freedom ami giving records of which Plate 

 XX is an example. Even in very small earthquakes the indices of these 

 instruments freijuently exhibit an immense number of movements in all 

 jMissible azimuths; so much so, that if the smoked-glass plate forming tlie 

 record receiver is stationary, the lamp-l)lack is sometimes completely rubbed oif 

 throughout a small area surrounding the point at which the index stands when at 

 rest. The writer has been dose to the duplex pendulum seismograjih (§ 41) 

 when a small (scarcely perceptible) earthquake began, and has watched the 

 marking jicintor draw circles, loops, figures of 8, and numberless other curves. 

 It appars, in fact, that the tangled character of the movements, which is striking- 

 ly shown in Plate XX, Ls by no means confined to such considerable shocks as the 

 one there ivcorded, but is, in general, equally present in the most feeble disturb- 

 ances of which records have been obtained. 



In the earthquake of § GO the princijial movements consist of wide loops, 

 nearly as broad as they are hjng. If we were to ascribe these to the simultaneous 

 arrival of two systems of rectilinear vibrations, we should have to conclude that 

 the principal movements in both systems reached the observing station together 

 — a thing in the highest degree improbable. Taken in conjunction witli the 

 small amount of vertical movement, these (and similar features in other records) 

 apjiear to admit of but one rational explanation, — that they are trans\crse waves 

 whose direction of emergence is not very far from vertical. 



