EARTHQUAKErJ. 207 



leat, and a misty horizon, are always the forerunners of this 

 phenomenon. The fallacy of this popular opinion is not only 

 refuted by my own experience, but likewise by the observations 

 of all those who have lived many years in districts where, as 

 in Cumana, Quito, Peru, and Chili, the earth is frequently 

 and violently agitated. I have ielt earthquakes in clear air 

 and a fresh east wind, as well as in rain and thunder storms. 

 The regularity of the horary changes in the declination of the 

 magnetic needle and in the atmospheric pressure remained un 

 disturbed between the tropics on the days when earthquakes 

 occurred.* These facts agree with the observations made bj' 

 Adolph Erman (in the temperate zone, on the 8th of March, 

 1829) on the occasion of an earthquake at Irkutsk, near the 

 Lake of Baikal. During the violent earthquake of Cumana, 

 on the 4th of November, 1799, I found the declination and 

 the intensity of the magnetic force alike unchanged, but, to 

 my surprise, the inclination of the needle was diminished about 

 48 '.t There was no ground to suspect an error in the calcu- 

 lation, and yet, in. the many other earthquakes which I have 

 experienced on the elevated plateaux of Quito and Lima, the 

 inclination as well as the other elements of terrestrial mag- 

 netism remained always unchanged. Although, in general, 

 the processes at work within the interior of the earth may not 

 be announced by any meteorological phenomena or any special 

 appearance of the sky, it is, on the contrary, not improbable, 

 as we shall soon see, that in cases of violent earthquakes some 

 eSect may be imparted to the atmosphere, in consequence of 

 which they can not always act in a purely dynamic manner. 



at the periods of the equiuoxes. It is singular that Pliny, at the end of 

 his fanciful theory of earthquakes, names the entire frightful phenom- 

 enon a subterranean stoi'm ; not so much in consequence of the rolling 

 sound which frequently accompanies the shock, as because the elastic 

 forces, concussive by their tension, accumulate in the interior of the 

 earth when they are absent in the atmosphere ! " Ventos in causa esse 

 non dubium reor. Neque enim unquara intremiscunt terrae, nisi sopito 

 mari, coeloque adeo tranquillo, ut volatus avium non pendeant, subtracto 

 omni spiritu qui vehit; nee unquam nisi post ventos conditos, scilicet 

 in venas et cavernas ejus occulto afflatu. Neque aliud est in terra 

 tremor, quam in nube tonitruum ; nee hiatus aliud quam cum fulmen 

 erumpit, incluso spiritu luctante et ad libertatem exire nitente." (Plin., 

 ii., 79.) The germs of almost every thing that has been observed or 

 imagined on the causes of earthquakes, up to the present day, may be 

 found in Seneca, Nat. Qucest.. vi., 4-31. 



* I have given proof that the course of the horaiy variations of the 

 Darometer is not affected before or after earthquakes, in my Relat. Hist.. 

 t. i., p. 311 and 513. 



t Humboldt, Relat. Hist., t. i.. p. 51.5-517. 



