On Earthquake Measurements. 31 



reaches us is much modified by the media through which it 

 has been transmitted ; and, again, since there is a great want 

 of continuity at the surface of the earth, very important mo- 

 difications are introduced by surface conditions : for example, 

 ranges of mountains are well known to reflect earthquake 

 vibrations in a marked manner; and veins of good conducting 

 rock, by transmitting the vibrations more rapidly than less 

 conducting veins, set up transverse vibrations. 



Professor Palmieri and others have invented instruments 

 which record the date of the vibration, and give rough ideas 

 of the direction of propagation of the earthquake -waves, 

 together with what is called the strength of the vibration. 



Mr. Mallet, whose wide experience on the subject of earth- 

 quakes has necessarily caused his writing to be regarded with 

 great respect, describes the object of Professor Palmieri's in- 

 strument as follows : — " By means of this apparatus the time 

 of the first shock is recorded, as well as the interval between 

 the shocks, and the duration of each ; their direction, whether 

 vertical or horizontal, is given, as also the maximum of inten- 

 sity." He further says, however : — "It is not my intention 

 here to offer any criticism as to the construction or perform- 

 ance of this instrument, the rather as I must confess I do not 

 quite share the high opinion of its inventor as to the certainty 

 or exactitude of its indications." And this opinion of Mr. 

 Mallet with regard to Professor Palmieri's instruments is ours 

 with regard to all the seismoscopes of which we have read any 

 descriptions. Indeed it is well known that the instruments 

 hitherto invented have not satisfied even the modest hopes of 

 their inventors; whereas, even if these hopes had been fulfilled, 

 we should still hardly have made a step in this new science. 



A simple form of seismoscope, but by no means a perfect 

 one, would be a lamp suspended from a ceiling by a spiral 

 spring, of such a strength that the period of vibration of the 

 lamp in a vertical direction was nearly the same as that for its 

 vibrations when swinging as a pendulum. The vibrations of 

 such a lamp during an earthquake would contain motions due 

 to the motion of its point of suspension ; and an experienced 

 observer would be able during a shock, or very soon after it, 

 to tell the direction and strength of the shock with much more 

 accuracy than with any of the instruments previously described. 

 This lamp seismoscope, however, possesses the defects of all 

 slowly vibrating bodies : the main vibration of the lamp is (as 

 we shall presently show) executed in its ordinary periodic 

 time ; and the lengths of its swings depend on many other 

 things besides the strength of the shocks, which would show 

 themselves as small perturbations in the motion of the lamp. 



