EARTHENWARE (erth"n-war). See POT TER.
gARTHQUAKE (erth-kwak), (Heb. rah' ash, vibration; Gr. efLegds, sice-mos').
The quaking of the earth ; a vibratory or un dulatory movement extending superficially over a wide area, and downward, it is believed, from a mile or two to more than thirty miles. The vi brations are, perhaps, produced by contractions of portions of the earth's crust. Earthquakes and volcanoes are connected, and are confined to par ticular regions constituting continuous lines.
1. Proximate Causes. (1) The proximate cause of earthquakes, though by no means accu rately defined, seems referable to the action of in ternal heat or fire. That the earth was once subject to the action of a vast internal power springing probably from the development of subterranean or central heat, the elevations and depressions, and the generally scarred and torn character of its exterior make sufficiently evident. A power sim ilar in kind, but more restricted in degree, is still at work in the bowels of the earth, and occasion ally breaks down all barriers and devastates cer tain parts of the world.
(2) But although subterranean disturbances may be the true cause of all great earthquakes and eruptions, there can be little doubt that the oc casion of those subterranean disturbances is often, if not always, to be sought outside the earth's crust. In times of great storm the mercury changes rapidly in height, and this corresponds to the rap id addition or removal of many thousands of millions of tons to and from the areas of rising and falling barometer. When we hear that the barometer has risen or sunk half an inch, we do not commonly attach much importance to the change, nor, in most parts of the earth, is such a change likely to produce any remarkable effects. Even in regions where the crust of the earth is notably unstable, a change of half an inch in the height of the mercurial column is not ordinarily of great importance. Yet it might under certain conditions make such a change in the conditions of equilibrium as to bring about an earthquake. Consider what it really means. When the barom eter rises half an inch over an area of to,000 square miles, less than a sixth of the area of Missouri, the pressure on that area is increased by 4,26o, 000,000 tons. If a wave of atmospheric pressure passed over the United States in such sort that over tb-i eastern half of the states the barometer were first half an inch lower than in the western half, and then half an inch higher, the effect would be as though a mass of about seven hundred thou sand millions of tons were shifted from the west ern to the eastern half of the United States.
(3) \Ve need not be surprised to find, then, that earthquakes have very often been preceded by re markable atmospheric phenomena. Usually great earthquakes have not followed tremendous storms. but a condition of portentous calm. The air has been found oppressive for hours, perhaps days, be. fore the earthquake occurred. Remembering aft erward the sense of oppression which had pre ceded the subterranean disturbance, the ordinary observer has been apt to infer that the dull, heavy calm, the unrestful stillness, was nature's pause before the mighty throes in which her imprisoned energies found vent. But in reality the oppressive
stillness has been simply the result of increased at mospheric pressure, and this increased pressure brings about the earthquake as its direct conse quence.
(4) But while the action of atmospheric pressure in helping to excite subterranean activities must not be overlooked, the varying pressure exerted by seas and oceans is a more potent disturbing factor. Atmospheric pressure is distributed in such a way that though the weight of air on any given area is continually changing, there are no sharply defined lines, at any time, which separate regions of less pressure from regions of greater pressure. It is otherwise with the sea along a shore line. Here we have the sea acting with constantly varying in tensity, as its level changes, on the seaward side of the shore line, while on the landward side there are no such variations of pressure. Let us con sider what this means. Take a tolerably straight shore line 50o miles in length, and suppose that along this shore line a region of ocean too miles broad rises through a height of three feet under the combined action of sun and moon raising a tidal wave, and favoring strong winds urging the water shoreward. Then we have so,000 square miles of sea-water, three feet deep, added as so much dead-weight to that part of the earth's crust which underlies the seas along that shore. Each square mile contains in round numbers 3,000,000 square yards, or 27,000,00o square feet. The ad ditional weight corresponds, then (as the added layer is three feet deep), to 50,00o times 81,000, 000 cubic feet of water, each weighing 64 1-3 pounds, or to 116,000,000,000 tons. It is clear that the addition of so enormous a weight as this to the submerged part of the earth's crust, outside the shore line, may well produce strains too great to be resisted. (Richard A. Proctor, LL.D.) 2. Regions of Most Frequent Earth. quotes. (1) The manifestation of the awful phenomena which accompany earthquakes is generally restricted in its range. Accordingly geologists have laid down certain volcanic regions or bands within which this manifesta tion most frequently takes place. Over these regions various traces of volcanic agency are found, such as either gaseous vapors or hot springs; also bituminous substances, and in some instances there are volcanoes in eruption. Several sources of bitumen are found on the Tigris, in the Persian mountains, near the Kharoon, and at Bu shire, as well as along the Euphrates. At Hit, especially, on the last-mentioned river, it exists on a very large scale, and, having been much used from the earliest times, seems inexhaustible. Abundant traces of it are also to be seen amid the ruins and over the entire vicinity of Hillah—the ancient Babylon. Syria and Palestine abound in volcanic appearances. Between the river Jordan and Damascus lies a volcanic tract. The entire country about the Dead Sea presents indubitable tokens of volcanic agency.