Page:Great Neapolitan Earthquake of 1857.djvu/59

Rh great in proportion to the range, which is also small, we are not disturbed by any consideration of resistance from the atmosphere.

These three classes of problems frequently are found combined in a single example—thus fracture and overthrow often occur together, or fracture and projection, and sometimes all three are united; a body (a gate pier for example) being broken off at its base, and overturned, but with a velocity more than sufficient for both, so that it is also projected, or thrown to a certain distance from its base. Although a less regular arrangement, it will tend to greater clearness, now to leave the further strict mechanical consideration of these questions, with the preliminary statements that have been made, and proceed first to describe pretty fully, the characteristics and details of structure of the buildings, &c., to which those principles will be applied in the present Report; and then to enter minutely upon the nature. of the fractures or fissures produced by earthquake shock in such buildings, &c, and describe the methods and conditions of observing them, and afterwards treating the observations; and finally to give in a connected form the equations referring to the treatment of all the classes of problems, as respects velocity, or direction obtained by calculation from velocity.

Almost all that follows with reference to the observation of the directions of fissures (or fractures), therefore is to be viewed as descriptive of the methods of arriving at direction only of wave-path, without reference to the velocity of the wave particles, or to any other mechanical conditions except those which determine the directions of such fissures as observed in buildings, velocity being determined,