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SURFACE-TENSION

1851

SURVEYING

Sur'face=Ten'sion is a phenomenon exhibited when two liquids (or a liquid and^a fluid) which do not mix are brought into contact. For instance, a drop of oil placed in a mixture of water and alcohol, which has the same density as the oil, will neither sink nor rise; but it will gather itself into a sphere, i. e., will assume a shape in which it has the least possible surface. In other words, it acts as if it were covered with a thin elastic skin which was continually tending to contract to the least possible area.

Another illustration of this phenomenon is seen when mercury is placed, in air, on glass, as indicated in the figure. The mercury does not flatten and run over the glass as it would do if gravitation alone 'were acting upon it; but it gathers itself into a more or less spherical shape. This globule is exactly what we should expect if the mercury were covered with a thin elastic membrane tending, all the while, to contract into the smallest possible area. This tendency of liquid surfaces to become a minimum is known as surface-tension. There is no skin or membrane over the surface of liquids; but the particles lying on the surface and very near the surface are in a different physical condition from the particles in the interior of the liquid; hence results this surface-effect, the details of whose explanation would lead us too far. The numerical value of the surface-tension of any liquid is the force per centimeter which a single surface exerts across any line drawn upon it. The unit in which surface-tension is measured, therefore, is dynes per centimeter. Consult C. V. Boys' Soap-Bubbles and the Forces which Mold Them, the finest elementary and popular treatment in the English language.

Sur'rey, Henry Howard, Earl of, an early English poet, was born between 1516 and 1518, probably in Suffolk, and was the son of the duke of Norfolk. In 1532 he accompanied Henry VIII to France, and spent a year in study at Paris with Henry's son, the duke of Richmond, who was betrothed to his only sister. In 1524 he was sent to Fleet Prison for challenging a gentleman to a duel, and next year suffered another imprisonment for breaking windows in the streets at night. In 1544 Surrey went to France as marshal of the invading army, and distinguished himself at the siege of Montreuil, where he was badly wounded.

But next year he was defeated by a larger French force, and was superseded by the earl of Hertford. For his bitter speeches against Hertford he was imprisoned, and later, with his father, sent to the Tower, charged with high treason. His only offense was in having added to his own arms those of his ancestor, Edward the Confessor. In vain did he ably defend himself at his trial; he was found guilty and was beheaded on Jan. 21, 1547. His poems were published after his death, with those of Wyatt. They are sonnets, lyrics, elegies, paraphrases of Psalms and Ecclesi-astes and translations. He was the first English sonneteer and one of the best. Consult Morley's English Literature. See SONNET.

Surveying (sur-vd'ing), a branch of applied mathematics devoted to measuring land. There are two great branches: plane-surveying and geodesy. Plane-surveying is used where so small areas are measured that the surface of the earth can be looked on as plane. Where large areas are measured, as states arid countries, the curvature of the earth must be taken into account and the science of land-measurement becomes geodesy. Plane-surveying is divided into such branches as land-surveying, having to do with fixing property-lines and areas of tracts of land; topographical surveying, having to do with levels and general contours of the landscape; hydrographical surveying, having to do with areas and depths of rivers, lakes and other bodies of water; mining surveying, having to do with the direction and lengths of mining shafts and galleries; railroad surveying; etc. One of the most important geodetic surveys now going on is the American coast-survey. It was started in 1832 by F. R. Hassler, and has now covered nearly the whole coast of the United States, the results being embodied in a sectional map on a scale of one mile to the inch. Eventually the whole country will be mapped out on this scale. Surveying and geodesy are applications of geometry and trigonometry, and involve the measurement of the length and directions of lines. All plane-surveying could be done by the measurement of lengths, but it is most convenient to measure angles also. The chief instruments are the chain or tape, compass, level and theodolite. In geodetic surveying instruments for measuring latitude and longitude must also be used.

The survey of public lands of the United States is in charge of a surveyor-general. The land is divided into townships, each six miles square. The townships between two meridians six miles apart make a range. The townships are then divided into sections a mile square, these containing 640 acres. Land-titles in the western states mostly refer to the original United States survey, and in this respect differ from the land-titles in the older states, where boun-

A DROP OF OIL FREED FROM GRAVITY

MERCURY ON   GLASS