Page:Popular Science Monthly Volume 92.djvu/424

 What Is Mean Sea Level?

��If there were no disturbing influences the ocean would be of one equilibrium

By William Bowie

Chief of the Division of Geodesy, U. S. Coast and Geodetic Survey

���speaks of the eleva- tion of a place, he has in mind the vertical dis- tance above an imagina- ry surface. The surface is generally that of the oceans imag- ined to ex- tend inland under the point con- sidered.

If there were no dis- turbing in- fluence by the sun and moon, the force of the winds and the varying

pressure of the air, then the surface of the oceans at all places would be one of equilibrium. If lines of precise leveling were extended inland from the Atlantic, Gulf and Pacific assumed to be quiet to Chicago, for instance, and if the leveling were done absolutely without error, the elevations of their junc- tion points would be absolutely the same for each line.

But the waters of the oceans are rising and falling in response to the forces acting on them, so in order to obtain an accurate starting point, a tidal station must be estab-

��How the Observations Are Taken

The leveling is done along t he railroads because of easy transportation. The engineers ride on motor velocipedes. Note the sunshade and wind- brake. The leveling instrument, which was designed by an official of the U. S. Coast and Geodetic Survey, is considered the most satisfactofi' one in the world. Its tripod is mounted on one of the motor cars. On the second car there is placed a listing adding-machine on which the observations are recorded. Wonderful speed is attained. A party working in Michigan in 1916 ran 340 single miles of levels in one month.

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into masonary or concrete

��lished, and the position of the wa- ter's surface with relation to a graduat- ed staff must be observed each hour for at least a year. The average po- sition of the surface of the water during the year will be almost ex- actly mean sea level.

Under the direction of Major E.Les- ter Jones, its superintend- ent, many such stations have been established along the coasts of the United States by the Coast and Geodetic Survey, and lines of accurate levels have been extended from them to furnish bench marks, which control the surveying, engineering and mapping done by the Federal Govern- ment, the States and cities and by pri- vate persons.

^ An elevation is the

vertical distance above what would be the sur- face of the water if a sea-level canal were ex- tended inland from the ocean to the bench mark.

Each line is run twice in opposite directionsto guard against errors.

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