The region within which the displacement occurs is sometimes called the _hypocentre_, but more frequently the _seismic focus_ or simply the _focus_. The portion of the earth's surface which is vertically above the seismic focus is called the _epicentre_. The focus and epicentre are often spoken of for convenience as if they were points, and they may then be regarded as the centres of the region and area in which the intensity was greatest. This is not quite accurate, but to attempt a more exact definition would at present be out of place. An _isoseismal line_ is a curve which passes through all points at which the intensity of the shock was the same. It is but rarely that the absolute intensity at any point of an isoseismal line can be ascertained, and only one example is given in this volume. As a rule, the intensity of a shock is determined by reference to the degrees of different arbitrary scales. These will be quoted when required. In every strong earthquake there is a central district which differs in a marked manner from that outside in the far greater strength and complexity of the phenomena. As this district includes the epicentre, it is sometimes referred to as the _epicentral area_, but the term _meizoseismal area_ is more appropriate, and will be employed accordingly. The district over which an earthquake is perceptible to human beings without instrumental aid is its _disturbed area_. In like manner, that over which the earthquake-sound is heard is the _sound-area_. A great earthquake never occurs alone. It is merely the most prominent member of a group of shocks of greater or less intensity, and is known as the _principal shock_ or _earthquake_, while the others are called _minor_ or _accessory shocks_, and _fore-shocks_ or _after-shocks_ according as they occur before or after the principal earthquake. When the sound only is heard, without an accompanying tremor being anywhere perceptible, it is more accurately called an _earth-sound_, but is frequently for convenience numbered among the minor shocks. [Illustration: FIG. 1.--Diagram to illustrate simple harmonic motion.] The movement of the ground during a vibration of the simplest character (known as simple harmonic motion) is represented in Fig. 1. The pointer of the recording seismograph is here supposed to oscillate along a line at right angles to AB, and the smoked paper or glass on which the record is made to travel to the left. The distance MP of the crest P of any wave from the line AB represents the _amplitude_ of the vibration, the sum of the distances MP and NQ its _range_, and the length AB the _period_ of the vibration. From the amplitude and period we can calculate, in the case of simple harmonic motion, both the _maximum velocity_ and _maximum acceleration_ of the vibrating particles of the ground.[1]