63 



EARTHQUAKES. 



movement such as is produced by the firing of 

 a mine — the perpendicular action, from below 

 upwards — was displayed most conspicuously on 

 the occasion when the town of Riobamba was 

 destroyed (1797), when the bodies of many of 

 the inhabitants were thrown upon the hill of 

 La Culla, which is several hundred feet high, 

 and rises on the other side of the Lican rivulet. 

 The propagation of the motion generally takes 

 place in a linear direction, in waves, and with 

 a velocity of from five to seven G. geographical 

 miles in a minute. Sometimes it is in circles, 

 or in great ellipses, from the centre of which 

 the vibrations are propagated with decreasing 

 force towards the circumference. There are 

 districts which belong to or fall within two mu- 

 ' tually intersecting circles of concussion. In 

 North Asia, which the father of historyC^^"), 

 and, after him, Simocatta(^^') characterize as 

 *' the Scythian territories free from earth- 

 quakes," I found the southern part of the Altai 

 Mountains, so rich in mineral treasures, sub- 

 ject to the influence of the concussive foci both 

 of Lake Baikal and the volcanoes of Thian- 

 Schan, or the Celestial Mountain^"). When 

 the circles of concussion intersect each other — 

 when, for instance, a lofty plain lies between 

 two simultaneously active volcanoes — then 

 may several systems of waves exist at once, 

 and not interfere with each other, just as in the 

 case of fluids. Interference, however, can be 

 conceived here, as in mutually intersecting 

 waves of sound. The magnitude of the trans- 

 mitted wave of succussion is increased at the 

 surface, in conformity with the general laws of 

 mechanics, according to which, when motion 

 is communicated in elastic bodies, the outer- 

 most free-lying stratum tends to detach itself 

 from the others. 



The waves of succussion can be pretty accu- 

 rately measured in their direction and total 

 strength, by the pendulum and the sismomcter 

 bowl, but in no way investigated in the intimate 

 nature of their alternations and periodical intu- 

 mescences. In the city of Quito, which stands 

 at the foot of an active volcanic mountain — the 

 Rucu-Pichincha, 8,950 feet above the level of 

 the sea, and boasts of beautiful cupolas, lofty 

 fanes, and massive houses several stories high, 

 I have frequently been surprised at the violence 

 of the earthquakes by night, which neverthe- 

 less very rarely occasion rents in the walls ; 

 whilst in the plains of Peru, apparently much 

 weaker oscillations injure lowly houses built of 

 cane. Natives who have stood the shocks of 

 many hundred earthquakes, believe that the 

 difference of effect is less connected with the 

 length or shortness of the waves, with the 

 slowness or rapidity of the horizontal oscilla- 

 tion("3), than with the equality of the motion 

 in opposite directions. Circular or rotatory 

 concussions are the rarest, but they are the 

 most dangerous of all. Twistings round of 

 walls without throwing them down ; planta- 

 tions of trees, which had previously stood in 

 parallel rows, deflected ; the direction of the 

 ridges of fields covered with various kinds of 

 grain altered, were observed on occasion of the 

 great earthquake of Riobamba, in the province 

 of Quito (February 4th, 1797), as well as of those 

 of Calabria (February 5th and March 28th, 1783). 

 With the latter phenomenon of rotation, or the 



transposition of fields and cultivated plots of 

 ground, of which one has occasionally taken 

 the place of another, there is connected a trans- 

 latory motion, or mutual penetration of several 

 strata. When taking the plan of the ruined 

 city of Riobamba, I was shown a place where 

 the whole of the furniture of one dwelling-house 

 had been found under the ruins of another. The 

 loose earth of the surface had run in streams 

 like a fluid, of which it must be conceived that 

 it was first directed downwards, then horizon- 

 tally, and finally upwards. Disputes about the 

 property, in those instances where things were 

 carried many hundred toises from their original 

 stances, were adjusted by the Audiencia, or 

 Court of Justice. 



In countries where earthquakes are compar- 

 atively much rarer, in the south of Europe for 

 example, a very general belief, grounded upon 

 an imperfect induction, prevails(^'*) ; viz. that 

 calms, oppressive heats, and a misty state of 

 the horizon, are always preludes to an earth- 

 quake. The erroneousness of this popular be- 

 lief is not, however, shown by my own experi- 

 ence only ; it is farther gainsaid by the obser- 

 vations of all who have lived long in countries 

 where earthquakes are frequent and violent, as 

 in Cumana, Quito, Peru, and Chili. I have ex- 

 perienced earthquakes when the air was clear 

 and a fresh east wind was blowing, as well as 

 during rain and thunder storms. Even the 

 regularity in the horary variations in the decli- 

 nation of the magnetic needle, and in the press- 

 ure of the air(^"), remained unaffected within 

 the tropics on the day of the earthquakes. The 

 observaRons which Adolphus Erman made in 

 the temperate zone on the occasion of an earth- 

 quake at Irkutsk, near lake Baikal, on the 18th 

 of March, 1829, agree perfectly with my expe- 

 rience. During the violent earthquake of Cu- 

 mana which happened on the 4th of November, 

 1799, I found the declination of the needle and 

 the magnetic intensity unaffected ; but to my 

 astonishment the dip was diminished by 48'("*). 

 I had no suspicion of any error ; yet in all the 

 other earthquakes which I have experienced in 

 the high lands of Quito and in Lima, the dip of 

 the needle remained equally unaffected with 

 the other elements of the terrestrial magnet- 

 ism. If in a general way the acts that proceed 

 deep in the interior of the earth are annoimced 

 beforehand by no special meteorological phe- 

 nomenon, by no peculiar aspect of the heavens, 

 it is on the contrary not improbable, as we shall 

 see immediately, that in certain very violent 

 earthquakes the atmosphere has sympathized 

 or partaken in some measure, and that these, 

 therefore, do not always act in a purely dy- 

 namical manner. During the prolonged trem- 

 blings of the ground in the Piedemontese val- 

 leys of Pelis and Clusson, extreme changes in 

 the electrical tension of the atmosphere were 

 observed, whilst the heavens were free from 

 storm. 



The strength of the dull noise which gener- 

 ally accompanies an earthquake does hot by 

 any means increase in the same measure as the 

 strength of the vibrations. I have satisfactorily 

 made out that the grand concussion in the 

 earthquake of Riobamba (Feb. 4th, 1797), one 

 of the most awful catastrophes in the physical 

 history of our earth, was accompanied by no 



