SEISMOLOGY
I7IQ
SELEUCIA
main branches are the Aube, Marne, Oise, Yonne, Loing, Essonne and Eure; and canals join it to the Somme, Scheldt, Meuse, Rhine, Saone and Loire. Deep dredging from Rouen, through the inlet at its mouth, has made that city a seaport and has reclaimed 28,000 acres of land. The situation of Paris on the Seine has made this, like the Hudson and the Thames, one of the famous rivers of the world. See Vernon-Harcourt's The Seine.
Seismol'ogy. That branch of physical science which is concerned with the investigation of earthquakes,— their causes and effects, their distribution and the various circumstances attending their occurrences. Professor John Milne suggests that, in addition to what are generally known as earthquakes, seismology should investigate earth-tremors which escape attention by the small-ness of their amplitude; earth-pulsations overlooked on account of the length of their period; and earth-oscillations or slow and quiet changes in the relative level of the sea and land, which geologists speak of as elevation and subsidence. But seismology is a very new science, having been developed almost wholly during the nineteenth century and being regarded as yet in the early stages of its development. Its pursuit is regarded, however, as of the utmost importance to geological and other forms of terrestrial research. It already has a great and growing body of material and literature. It is hoped that soon practical applications of the science will be developed; one of these, it is confidently predicted, will be the warning to dwellers in unstable regions when earthquakes or similar disturbances impend. Milne is sanguine that earthquake warnings will be as common and as trustworthy as the storm warnings at pur seaports. The value of such warnings is beyond estimate. In Japan, where earthquakes are frequent, an earthquake-investigation committee was formed in 1892 and has been investigating the practical aspects of seismology very carefully. Professor Gerland, director of the imperial German chief station for earthquake investigation at Strassburg, now issues an annual catalogue of earthquakes, and in 1903 organized The International Seismological Association. The interest is growing in America, and the broad extent of our outlying territory gives us fine advantages in collecting data. These earth-movements are recorded by seismographs, instruments found in many modern observatories. They are, essentially, a pillar extending well into the earth to gather the vibrations, with an additional device for recording them. There is wide variation in detail of construction; but the instrument traces a continuous curve, the variations of which indicate the intensity, distance, time, and duration of the shock. Professor Milne's instrument on the Isle of Wight detects
disturbances in Japan, Borneo or South America.
Seis'mograph. See SEISMOLOGY.
Seismometer (sis-mom'e-ter), an instrument which is used to measure the amount and direction of displacement at any point on the earth's surface during an earthquake. The name is also applied to those devices by which the slow and gradual tilting of the earth's surface is indicated.
For the first of these two purposes the one thing essential is to place a body in neutral equilibrium and without friction. To realize these two conditions perfectly is of course impossible. But they can be very nearly approximated. Thus an open gate, with a heavy mass at its outer end, rotating about a strictly vertical axis with very minute friction, would fairly represent a seismometer. Three instruments of this kind, one free to move east-and-west, one free to move north-and-south and one free to move in a vertical direction, will give the three components of any motion produced by an earthquake; and by compounding these three motions we can obtain the original motion. In the modern instruments, as perfected by Prof. John Milne, these records are automatically photographed.
Seismometers which are employed to measure the tilting of the earth's crust generally use a beam of Hght which has been reflected, part of it from a mirror rigidly attached to the earth, and part of it from a surface of mercury, which remains horizontal whether the earth's crust is tilted or not. The variation in angle between these two beams measures the tilting of the earth. See Milne's Earthquakes in the International Scientific Series and Geo. H. Darwin's Tides, popular lectures before Lowell Institute, Boston, Mass., in which earth-til tings are treated.
Seja'nus. See TIBERIUS.
Selaginella (sel'd-ji-neVla}, the genus of plants to which the "little club-mosses" belong. See LYCOPODIALES.
Selenium (se-le'ni-um) is a very rare element which exists in two forms. In the glassy form at ordinary temperatures it is a solid of a dark-brown color and a non-conductor of electricity. When it is slowly cooled from its melted condition, it does not appear vitreous or glassy, but crystalline, of a dull leaden color, and at ordinary temperatures is a conductor of electricity. It is very sensitive to light, and its electrical resistance is much less in the light than in the dark. This property led to the discovery of the photophone in 1880. Selenium is found chiefly in combination with lead, silver, copper or iron; but it has also been found in sulphur and certain iron sulphides. Selenium was discovered in 1817 by Berzelius in the waste of a sulphuric-acid factory.
Seleucia (se-lu'shi-a), the name of se \reral early cities.
