Page:Popular Science Monthly Volume 91.djvu/368

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352

��Popular Science Monthly

���Surveyors climbing to the top of a precipi- tous cliff in Alaska to take observations

��Risking Lives for the Sake of Pre- cision in Government Surveys

ONE of the most difficult and exacting tasks of an extensive land survey made by the United States Coast and Geodetic Survey, is the selection of points from which observations may be taken. In heavily wooded country it is necessary to climb the tallest trees or to raise poles taller than the trees, from which the distant horizon can be seen and such observations made as may be practicable. In mountainous country such as Alaska the surveyors have to face death for the sake of precision, climbing the highest peaks in order to obtain an unob- structed view of the hori- zon.

Why risk one's neck to make a simple survey " Because when great pre- cision is desired it is im- possible with chain or tape to survey a region in which there are bays, rivers, mountains and other natural obstruc-

��tions. To overcome these difficulties the? method called "triangulation" is employed. It rests upon the simple proposition taught in every school that if one side and the angles of a triangle are known the remain- ing sides can be computed. It is to obtain the first side of a triangle, or base line, that the surveyors climb the highest trees and mountains. So painstaking is their work that in a survey of one hundred miles the error is often less than five feet.

The accompanying photograph shows a Government surveying engineer climbing a steep cliff in Alaska, prior to establishing a base line. On his back he carries the necessary surveying instruments. On the top of the cliff stands a companion engineer who undoubtedly scaled' the cliff without assistance. The feat is made safer' for the second man because of the rope which has been lowered to guide him in his ascent of the nearly perpendicular^ surface.

If he is to measure a great distance from the top of the cliff the only instruments the engineer will use will be heliotropes (not helioscopes) by day and powerful lights by night. The heliotrope is a small mirror so arranged that it reflects the sunlight in a long line toward the observer.

��Keeping School Desks Presentable with Cardboard Covers

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���The cardboard cover keeps the school desks always presentable

��JANITOR of a school in Pasadena, Cal., has found way to keep the desks in his classrooms looking bright and presentable without subjecting them to frequent planing and var- nishing. He slips over the desk tops a pressed-card- board cover, which he calls an envelope. The enve- lopes are made to conform with the general shape of the desk tops, and the outer edges are bent down and back and riveted, so that the cover will slip on over the desk top and will remain in position after it has been adjusted.

There is a groove in- dented at the top of the cover to receiye pencils and pens, and a circular open- ing is provided through which the inkwell shows.

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