EAR THQ UAKES. 3 7 1 



But they are also doubtless due to the reverberation of very distant 

 shocks. It is probable that there is not a minute of time without its 

 earthquake somewhere, and the vibrations may often be transmitted 

 to very great distances. In only a very few cases has it hitherto been 

 possible to identify a tremor with a distant shock, and even then the 

 identification is necessarily rather doubtful. One of the best authen- 

 ticated of these cases was when M. Nyren, an astronomer at St. Peters- 

 burg, noticed on May 10 (April 28), 1877, a very abnormal agitation 

 of the levels of his telescope, an hour and fourteen minutes after there 

 had been a very severe shock at Iquique, in Peru. 



Astronomers are much troubled by slight changes in the level of 

 the piers of their instruments, and they meet this inconvenience by 

 continually reading their levels and correcting their results according- 

 ly. Of course, they also take average results. These troublesome 

 changes are probably earth-tremors, with so slow a motion to and fro 

 that the term tremor becomes inappropriate. This kind of change 

 has been called a displacement of the vertical, since a plummet moves 

 relatively to the ground. Thus, we found at Cambridge that as the 

 pendulum danced it slowly drifted in one direction or the other. 

 There was a fairly regular daily oscillation, but the pendulum would 

 sometimes reverse its expected course for a few minutes, or for an 

 hour. During the whole time that we were observing, the mean posi- 

 tion of the pendulum for the day slowly shifted in one direction ; but 

 even after a voyage of six weeks the total change was still excessively 

 small. How far this was a purely local effect and how far general 

 we had no means of determining. 



This is a subject which M. d'Abbadie, of the French Institute, has 

 made especially his own. Notwithstanding his systematic observa- 

 tions, carried on during many years in an observatory near the Bay 

 of Biscay, on the French side of the Spanish frontier, hardly anything 

 has been made out as to the laws governing displacements of the 

 vertical. He has, however, been able to show that there is a tendency 

 for deflection of the vertical toward the sea at high tide, but this 

 deflection is frequently masked by other simultaneous changes of 

 unexplained origin. 



This result, and the connection between barometric variations and 

 earthquakes and tremors, should make us reflect on the forces which 

 are brought into play by the rise and fall of the tide and of atmos- 

 pheric pressure. Our very familiarity with these changes may easily 

 blind us to the greatness of the forces which are so produced. The 

 sea rests on the ground, and when the tide is high there is a greater 

 weight to be supported than when it is low. A cubic foot of water 

 weighs 62 pounds ; thus if high-tide be only ten feet higher than low- 

 tide, every square foot of the sea-bottom supports 620 pounds more 

 at high than at low water ; and 620 pounds to the square foot is 

 nearly 8,000,000 tons to the square mile. Again, the barometer ranges 



