192 REACTION OF THE INTERIOR OF THE EARTH 



of waves may exist simultaneously, and produce their 

 effects, as in fluids, without mutual disturbance. We may 

 even imagine interferences, as in intersecting waves of 

 sound. The magnitude of the waves propagated in the 

 crust of the Earth will be increased at the surface, accord- 

 ing to the general law in mechanics by which vibrations 

 transmitted in elastic bodies have a tendency to detach the 

 superficial strata. 



The undulations in earthquakes have been examined with 

 tolerable accuracy, in respect to their direction and intensity, 

 by means of pendulums and sismometers; but in their 

 characters of alternation and periodical intumescence they 

 have by no means attracted sufficient attention. In the 

 city of Quito, which is situated at the foot of a still 

 active volcano, the Rucu-Pichincha, and at an elevation 

 above the sea of 8950 (9539 English) feet, and which 

 possesses fine cupolas, high roofed churches, and massive 

 houses of several stories in height, I have been often 

 surprised in the night by the violence of the earthquake 

 shocks ; but these, though extremely frequent, very rarely 

 injure the walls, whereas, in the Peruvian plains, even low 

 dwellings built of reeds suffer from apparently far slighter 

 oscillations. Natives of those countries, who have expe- 

 rienced many hundred earthquakes, believe this difference to 

 depend less on the greater or less duration of the shocks, or 

 the slowness or rapidity ( 183 ) of the horizontal oscillation, 

 than on alternation of motion in opposite directions. 

 The circular (or gyratory) earthquakes are the most rare, 

 and at the same time the most dangerous. In the great 

 earthquake of Eiobamba in the province of Quito (4th 

 February, 1797), and in that of Calabria (5th February, and 

 2bth March, 1783), walls were changed in direction without 



