SEISMOLOGICAL INVESTIGATIONS. 221 



The P phase of a seismogram sometimes, but not often, begins with a sharp 

 kick — denoted i P ; but sensitive machines show that much more frequently this 

 sharp kick is preceded by two or three waves of smaller amplitude and higher 

 frequency. When the frequency is distinctly quicker than that of the prevail- 

 ing microseisms, and the amplitude of the latter is not too great, it is easy to 

 detect the true P as a. superimposed wave, but if the period of these small 

 precursors approximate to that of the microseisms, then it is difficult to deter- 

 mine the true inception of the earthquake record. 



Machines which do not record the microseisms will not record these minute 

 waves. With such machines probably more uniformity, by reading the bigger 

 kick, will result, but misguided uniformity will not be conducive to obtaining 

 the true rate of propagation of the P phase. 



It is to sensitive machines and careful scrutiny of the record that we 

 must look for data for the perfecting of seismological tables. 



