I EARTHaUAKES. 205 



ence to exist here, as in the intersecting waves of sound. The 

 extent of the propagated waves of commotion will be increased 

 on the upper surface of the earth, according to the general law 

 of mechanics, by which, on the transmission of motion in elas- 

 tic bodies, the stratum lying free on the one side endeavors to 

 separate itself from the other strata. 



Waves of commotion have been investigated by means of 

 the pendulum and the seismometer* with tolerable accuracy in 

 respect to their direction and total intensity, but by no means 

 with reference to the internal nature of their alternations and 

 their periodic intumescence. In the city of Quito, which lies 

 at the foot of a still active volcano (the Rucu Pichincha), 

 and at an elevation of 9540 feet above the level of the sea, 

 which has beautiful cupolas, high vaulted churches, and mass- 

 ive edifices of several stories, I have often been astonished 

 that the violence of the nocturnal earthquakes so seldom 

 causes fissures in the walls, while in the Peruvian plains os- 

 cillations apparently much less intense injure low reed cot- 

 tages. The natives, who have experienced many hundred" 

 earthquakes, believe that the difference depends less upon the 

 length or shortness of the waves, and the slowness or rapidity 

 of the horizontal vibrations,! than on the uniformity of the 

 motion in opposite directions. The circling rotatory commo- 

 tions are the most uncommon, but, at the same time, the most 

 dangerous. Walls were observed to be twisted, but not thrown 

 down ; rows of trees turned from their previous parallel direc- 



* [This iustrument, iu its simplest form, cousists merely of a basin 

 filled with some viscid liquid, which, on the occurrence of a shock of 

 an earthquake of sufficient force to disturb the equilibrium of the 

 building iu which it is placed, is tilted on one side, and the liquid made 

 to rise in the same direction, thus showing by its height the degree of 

 the disturbance. Professor J. Forbes has invented an instrument of 

 this nature, although on a greatly improved plan. It consists of a vert- 

 ical metal rod, having a ball of lead movable upon it. It is supported 

 upon a cylindrical steel wire, which may be compressed at pleasure by 

 means of a screw. A lateral movement, such as that of an earthquake, 

 which carries forward the base of the instrument, can only act upon the 

 ball through the medium of the elasticity of the wire, and the direction 

 of the displacement will be indicated by the plane of vibration of the 



Eeudulum. A self-registering apparatus is attached to the machine, 

 ee Professor J. Forbes's account of his invention in Edinb. Phil. Trans., 

 vol. XV., Part i.] — Tr. 



t " Tutissimum est cum vibrat crispante aedificiorum crepitu ; et cum 

 hitumescit assurgens alternoque motu residet, iunoxium et cum concur- 

 rentia tecta contrario ictu arietant; quoniam alter motus alteri renititur. 

 Undantis inclinatio et fluctus more quaidam volutatio infesta est, aut cum 

 in unara partem totus se motus impellit." — Plin., ii., 8*2. 



