200 COSMOS. 



commotion, the lake of Baikal, and the volcano of the Celes- 

 tial Mountain (Thianschan).* When the circles of commo- 

 tion intersect one another, when, for instance, an elevated 

 plain lies between two volcanoes simultaneously in a state of 

 eruption, several wave-systems may exist together as in fluids, 

 and not mutually disturb one another. We may even sup- 

 pose interference 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 elastic bodies, the stratum lying free on the one 

 side endeavours to separate itself from the other strata. 



Waves of commotion have been investigated by means of 

 the pendulum and the seismometerf 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 massive 

 edifices of several stories, I have often been astonished that 

 the violence of the nocturnal earthquakes so seldom causes 

 fissures in the walls, whilst in the Peruvian plains oscillations 



* Hurnboldt, Asie Centrale, t. ii. p. 110-118. In regard to the dif- 

 ference between agitation of the surface and of the strata lying beneath 

 it, see Gay-Lussac, in the Annales de Chimie et de Physique, t. xxii. 

 p. 429. 



+ [This instrument in its simplest form consists 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 

 in 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 vertical 

 metal rod, having a ball of lead moveable 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 directioa 

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

 pendulum. A self-registering apparatus is attached to the machine. 

 See Professor J. Forbes' account of his invention in Edirib. Phil. Trans* 

 voLzv., pt. 1.] Tr t 



