EARTHQUAKE MEASUREMENTS IN PIT AND ON SURFACE. 



255 



may he iningined their calculation is very difficult, especially in the 

 estimation of their periods, so that any great exactness is not to he 

 ohtained. The calculation will, however, give some approximate idea 

 as to the state of things. Hence, for some of the earthquakes, "large 

 waves " and " ripples " are separately calculated. " Large waves " are 

 those principal undulations for which calculation is usually made 

 in earthquake reports, ;and " ripples " are the irregular wavelets 

 superimposed on them. In douhtful cases the amplitudes only are 

 given. With respect to n, the numher of waves in 10 seconds, there 

 is no difference to he found hetween the large waves of earthquakes 

 observed on the surface and those observed in the pit ; l)ut, for 

 ripples, the number is often very much less in the pit diagram, because 

 of the reduction of amplitude and the consequent unification of some 

 of them amongst themselves. The quantity n is therefore given only 

 for ripples and not for large waves. The distinction between large 

 waves and ripples is often very doubtful and does not exist for small 

 earthquakes. 



We may here remark that the maximum acceleration, A, is a 

 quantity which approximately measures the overturning and fractur- 

 ing effect of the shocks. In the case of a ripple, whose period is very 

 short, this effect might probably be also measured by the total amount 

 of impulse communicated to a body during a semi-phase of the wave, 

 which is found to be proportional to the maximum velocity. 



Records. 



For the materials of the present paper we examined the records 

 of thirty actual earthquakes. Of these, tliree interesting shocks have 

 their diagrams shewn in PI. XXXVI. and IM. XXX\'II., and their 

 peculiarities are discussed. The other twenty-seven shocks were com- 

 paratively small and the different quantities, measured and deduced 



