300 



THE POPULAE EDUCATOR 



falls exactly along the path of the wave. If it be not thrown 

 down as the wave passes underneath it, it assumes its perpen- 

 dicular position as it stands on its crest. Then it slopes the 

 other way as it stands on its rear flank ; and when the dis- 

 turbance has quite passed, and 

 the surface has subsided to its 

 original level, the pillar, left 

 by the wave, suddenly totters 

 back to its perpendicular with 

 a kind of jerk. This motion 

 has a more destructive effect 

 than that which first threw 

 the shaft from its position ; 

 ftnd if the capital did not fall 

 off in the first instance, most 

 probably it will be thrown off by 

 the return of the pillar to its 

 upright posture. In any case 

 the line drawn from the fallen 

 object to the base of that which 

 supported it must be in the 

 direction of the wave. By pass- 

 ing through the visited district, 

 Mr. Mallet was enabled to map 

 very many of these lines, and 

 on their production he found 

 that they all intersected within 

 half a mile. Thus he determined 

 the position of the point E 

 (Fig. 13), directly under which 

 was the centre of the disturb- 

 ance. 



To find the exact depth at 

 which the explosion took place, 

 observations of fissured walls 

 were taken. It will readily 

 appear, on inspecting the dia- 



COLUMNS OP THE TEMPLE OP SKKAPIS. 



gram, that if the wave moved in 

 the direction of the line c s, the wall of the house, being bent 

 by the emerging wave, would be fractured at right angles to its 

 path. Therefore, a line perpendicular to the fracture would indi- 

 cate the angle 

 of emergence. 

 This being 

 found, the 

 depth of the 

 point c is at 

 once given. In 

 the instance of 

 thisNeapolitan 

 oarthquake,tho 

 centre of the 

 disturbance 

 was not more 

 than seven or 

 eight miles be- 

 neath the sur- 

 faoe. The 

 above state- 

 ment is but a 

 very crude out- 

 line of the sub- 

 ject, as so many 

 disturbing 

 causes inter- 

 fere with the 

 direct results. 

 For instance, 

 the origin of 

 the shock may 

 not be a point, 

 but the dis- 

 turbance may 



be distributed over an area. This has received confirmation by the 

 loud rumbling subterranean noises, which sound as if a number 

 of violent explosions had succeeded each other in vast cavities 

 far in the bowels of the earth. These frequently precede the 



earthquake-waves. Then, again, these latter waves have 

 different velocities in various rocks, and therefore the coseismal 

 lines that is, the lines which mark the emergence of the wave 

 are not always circular, but may extend much further in one 



direction than in another. Hence 

 it happens that areas disturbed 

 by an earthquake shock are fre- 

 quently of very irregular shape. 

 Varied and peculiar phenomena 

 are recorded as preceding and 

 accompanying earthquakes, 

 such as irregularities in the 

 seasons, deluges of rain, the un- 

 usual haziness of the air, sud- 

 den calms, etc. ; but as these 

 are not general, they must be 

 considered as accidental cir- 

 cumstances. It would be much 

 beyond the limits of our space 

 to attempt to chronicle even the 

 remarkable earthquakes which 

 have brought sudden destruc- 

 tion on thousands of human 

 beings. There is little or no 

 variation in the accounts, save 

 as to the amount of damage 

 produced by the shock. We 

 allude to the Earthquake at 

 Lisbon, which happened on the 

 1st of November, 1755, as an 

 example of all earthquakes. 

 The shock was preceded by no 

 premonitory symptoms, but 

 with a tremendous roar the city 

 reeled and fell. 



It seems from observations 

 made on the principle above re- 

 ferred to that the centre of 



disturbance was some eighty miles out at sea. The actual 

 scene of the gaseous explosion must have been deep-seated, for 

 the effects of the shock were felt over an area four times as 



large aa Eu- 

 rope. The 

 water rose sud- 

 denly twenty 

 feetintheWest 

 Indies. The 

 greatCanadian 

 lakes felt the 

 movement. In 

 Scotland, Loch 

 Lomond rose 

 on one beach 

 more than two 

 feet, the water 

 not participat- 

 ing in the lutcu 

 which the land 

 gave. To the 

 very north of 

 Europe the 

 waves of dis- 

 turbance ex- 

 tended. 



In six mi' 

 nutes 60,000 

 people in las' 

 bon perished. 

 Many had col 

 lected on the 

 wide expanseof 

 the new marble 

 quay, out of 



the way of the falling houses, when suddenly the quay, with its 

 living crowd, sank, with many ships in the harbour, and not a 

 body, nor the splinter of a wreck, ever rose up from the watery 



^ r depth. We can only suppose that a fissure opened beneath the 



shocks, the sound-waves being able to travel faster than the I harbour, and after e^gulphing the whole, as suddenly closed, 



