SEISMOLOGY, from (mews, an earthquake (that is, a movement like the shaking of a sieve), and Ad-yos, a discourse, the science of earthquakes ; and SEIS310METRY, from the same, and oerploato measure, the science of the mensuration of all the phenomena of earthquakes which can be expressed in numbers, or by their relation to the co-ordinates of space. The origin of these new departments of science has been stated in the article EARTUQUAKES, in which the physical definition of an earthquake was also given, agreeably to the researches of Mr. Robert Mallet. We now proceed to state some of tho elements of seismology, and the nomenclature applied to them, premising that the basis of this is formed by the adjectives seismal and seismic, both signifying what has relation to an earthquake. An earthquake being the transit of a wave of elastic compression through the earth, there are three elements of this earthquake-wave, namely, tho velocity of transit, the velocity of the wave-particlo (or wave itself [WAvs)), and the direction of motion at each point of the seismic area, or tract on the earth's surface within which the earthquake is felt. From the last element may be deter mined, 1st, the point upon the earth's surface vertically over the centre of effort, or focal point, whence the earthquake impulse was delivered; and 2nd, the depth below the surface (or rather sea-level) of the focal point itself. The lino passing through both these points is the seismic vertical. The wave starts from the focal point with one normal and two transversal vibrations, and may be imagined transferred outwards, in all directions, in concentric spherical shells, whose volume at the same phase of the wave is constant. The shock reaches the surface directly above the focal point vertically; hut, for all points around that, it emerges with angles becoming more and more nearly horizontal as the distance measured on the surface increases. The intersecting circle of any one shell with the surface, which is that of simultaneous shock, is the coseismal line, or crest of the earth-wave : circular if in a homogeneous medium, more or less distorted if in a hetero geneous one (such as constitutes the various geological formations of tho earth's crust), but always a closed curve. The meitoseismal area
is that in which the shock is experienced in the greatest degree, and isoseismal areas are those in which it is felt at the same time. A seismic region is a tract of the earth's surface, the earthquakes of which have some community of origin, manifested by their simultaneous occurrence. Seismic energy is the total amount of force exerted in earthquakes at a given period, or in a given space, or with relation to any Other element of the subject. A Orono-ammo curve, laid down from the records of earthquakes, the ordinate of which is that of epoch, and the abscissa that of seismic intensity (in this case tho force of a given earthquake), expresses the progression of seismic energy in time.
Every object displaced by an earthquake shock is in fact a seismo meter, and in the selection and arrangement of suitable objects for such displacement, so as to give the required evidence of the velocity and directions of the motion, consists the practical science of seismometry. For particulars of the construction and principles of complete self registering seismometers, we must refer to Mr. Mallet's Fourth Report to the British Association; but we proceed to notice some extemporaneous seismometers with which important observations may bo made, under circumstances in which fixed instruments cannot be appealed to. The elements to be recorded, Mr. Mallet states, in his article On Observation of Earthquake Phenomena,' in the third edition of the Admiralty Manual of Scientific Inquiry,' are such as will enable us to calculate, in order to obtain finally the actual elements of the wave itself, as stated above—I, 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—that is, its amplitude. and altitude.