424 TERRESTRIAL PHYSICS. 



will now comprehend my long silence, as since the 30th May I have been 

 busy at reorganization, drilling new hands, and pulling up heavy arrears 

 of work. 



As you ask me for particulars of earthquakes, I will tell you the 

 result of my long-continued observations. Their course here is invaria- 

 bly from the mountain range to the sea — in this district from east by 

 north to west by south. Walls built north and south — that is, across the 

 course of movement — are those that suffer most. Many have fallen and 

 all are more or less injured. Walls built east and west suffer little if at 

 all, except occasional!}' when they are at the corner of the street, and 

 then only a few of the last adobes fall. 



The movements appear to have the greatest intensity or rather effect 

 in sand. The walls of houses built on sand when they do not fall, as 

 most frequently they do, are left in a crumbled condition, crashed and 

 shattered in every direction. It is clear that the earthquake movement 

 imparts a variety of movements to sand. I think, too, the force that 

 causes the movements acts in the line of least resistance, or tries to lib- 

 erate itself where it finds its work easiest. 



You remember the " big cut" on the A. and T. line and the conglom- 

 erate that you so often anathematized. That conglomerate, however, 

 resists earthquakes, and I attribute the preservation of Tacua on the 

 13th August, 1868, to the town being built on it. Not a stone falls in 

 the big cut, and I have a hole in the yard of my house fifty-one feet 

 deep, all through this formation, with an old wall two feet from its edge, 

 that I was sure would have fallen in, but to my great surprise every- 

 thing remained sound and intact. This conglomerate is, as you know, 

 very tough and must offer immense resistance to the earthquake force. 



Our rock here is all trachyte tuffa, a few stages only removed from 

 pumice-stone, and offers little or no resistance to the earthquake shocks. 

 Houses built on it fall easilj'. 



The earthquake waves are low and only measure a few inches in 

 height. The damage they cause a^ipears to me more owing to duration 

 than to altitude of the movement. The earthquake of the 24th ultimo 

 lasted ninety seconds and left things standing ; while a duration of five 

 minutes would have brought them down. The effect of this last earth- 

 quake on vessels at sea you will find in the Valparaiso papers of last 

 mail in the case of the ship Payta. The water appeared to run away 

 from the vessel's sides, and the people in her feared being submerged. 

 She was fifty-seven miles south of Arica, in a direct line cast and west 

 with Islaya, now, as then, in a violent state of eruption, and must 

 have been caught in the very center of the movement. 



The effects of an earthquake on a train in motion arc worth mention- 

 ing. On the 13th of August, ISGS, the I^resideute (our very first and 

 last engine) took up the train, (regulation load one hundred and twenty 

 tons, cars included, but exclusive of locomotive and tender twenty-two 

 and one-half tons,) and was going at about sixteen miles per hour when 



