414 EARTHQUAKE PHENOMENA. 



Observations of each of these classes of waves which we have 

 thus briefly described may be made either directly by the aid of 

 instruments, specially provided or extemporaneously formed, or indi- 

 rectly by proper notice of certain effects which they produce on 

 objects upon the earth's surface. 



Direct observations by complete self-registering seismometers do 

 not come within our present scope. They will be found treated of 

 at large in Mallet's 4th Report, Brit. Ass., 1857-58, where the princi- 

 ples, roust ruction, methods of observation, and applications of the 

 best known instruments are described. 



Whatever instruments be employed, however, it is found that per- 

 turbations in the main directions of emergence at the surface, of the 

 normal earth-wave, due principally to heterogeneity of structure in 

 depth, and to inequality of surface, are such as to render a special 

 choice of district necessary, in attempting- any seismometrical re- 

 searches (even with perfect instruments) having- in view the determi- 

 nation of the position of the origin or focus of disturbance. This 

 choiee. according to our present knowledge, must be determined by 

 the following conditions: 



1. The whole surface-area of observation must be, as far as possi- 

 ble, uniform in geological structure, and so to as great a depth as 

 possible. If of stratified rock, not greatly shattered and overthrown, 

 but (viewed largely) level or rolling only. The harder and more 

 dense and elastic the formation the better, but neither intersected 

 by long and great dikes or igneous protrusions of magnitude, nor 

 suddenly bounded by such formations. 



2. The surface must not be broken up into deep gorges, and rocky 

 ranges, and valleys. Seismometry, in a. high and shattered moun- 

 tainous country, can scarcely lead to any result but perplexity. If 

 the surface be deeply alluvial all over, it is less objectionable than 

 valley-basins and pans of deep alluvium, with rocky ribs between 

 them. 



3. The size of the area chosen for observation must bear a relation 

 to the force of the shocks experienced in it. Moderate slwcks are 

 always best for observation, and in large areas of the most uniform 



'formation and surface will give the most trustworthy 

 indications. 



-!. [f several seismometers are set up in the area they should be 

 all placed on corresponding formations, either all on rock, or all on 

 deep alluvium. The rock, when attainable, is always to be preferred. 

 Three seismometers, at as many distant stations, will be generally 

 found sufficient, if the object he chiefly to seek the focal situation 

 and depth. 



We. therefore, proceed to observations with extemporaneous in- 

 struments on the earth-wave or shock. The elements necessary to be 

 recorded are such as will enable us to calculate: 1. The direction in 

 azimuth of the wave's motion upon the earth's surface: and also its 

 direction of emergence at the points of observation. 2. Its velocity 

 of transit upon the surface. 3. Its dimensions and form — i. e.. 

 amplitude and altitude. 



