424 EARTHQUAKE PHENOMENA. 



of the normal vibration, and by the nearly horizontal movements of 

 the two transversal vibrations in orthogonal planes. 



The observer must bear in mind that all these motions are due to 

 the inertia of the bodies at the moment of the wave transit. The 

 first tendency, therefore, of every body is to fall in a direction con- 

 trary to that of the wave's motion; but this is often perplexed by 

 mutually supporting bodies, as cross walls — by the direction of the 

 wave being one in which a fall is impossible, as when passing very 

 diagonally through a long line of wall — by disintegration from the 

 first wave, so altering the conditions of the bodies (walls, towers, 

 <fcc.,) though short of producing a fall, as that the dislocation and fall 

 produced by a succeeding one is not contrary, but in the same direc- 

 tion as the wave motion. When the shock emerges at a large angle 

 to the horizon bodies are often projected, as stones out of or from the 

 roping of walls: the size, weight, form, cement, sort of stone, distance 

 thrown, and all other conditions of projection should then be care- 

 fully noticed. Isolated bodies, such as bells from belfries, balls or 

 vases of stone, statues, &c., are often thrown from elevated points 

 on buildings, and reach the ground after describing a trajectory path. 

 The vertical height fallen through, and the horizontal distance thrown 

 from the original position, with the form, dimensions, substance, 

 weight, and mode of attachment of the body, being noted, afford 

 elements for calculating the velocity of the wave transit if its direc- 

 tion of emergence be otherwise known, or vice versa. 



Fissured or overthrown walls of buildings usually give approxima- 

 tions to the horizontal azimuth of shock, but may or may not give 

 any decided response as to the direction of transit, e. g. , with a N. S. 

 azimuth it may remain uncertain whether the transit was N. to S. or 

 S. to N. Objects overthrown, such as images, altar candlesticks, 

 pilaster slabs, pictures, that con fall only in one direction, may 

 generally be found, such as will decide the question. Space will not 

 permit of this part of the subject being treated systematically or 

 fully. The observer should train his mind, by solving for himself 

 various cases of the effects of shock on different sorts of buildings, 

 <fcc., and he will see from the hints here given how much the value 

 of his observations, in a recently shaken country, will depend upon 

 the "nous" and adroitness with which he seizes upon the fit objects 

 to afford him the best data. The note and sketch-books should be in 

 perpetual use — no conditions essential for after calculation must be 

 omitted — and the azimuths, directions, and emergence of the shock 

 at every observed point marked upon the best maps as soon as 

 possible. Azimuths must usually be taken with the prismatic com- 

 pass, or pocket sextant, but should be plotted to the true meridian; 

 and the magnetic variation should be determined at frequent intervals, 

 especially in volcanic countries. 



Bodies twisted on a vertical axis (such as the Calabrian obelisks, 

 see Lyell, "Geology") were formerly supposed to be due to a vorti- 

 cose motion of the earth. This movement arises from the centre of 

 gravity of the body lying to one side of a vertical plane in the line of 

 shock, which passes through that point in the base on which the body 



