THE WIRE DRAG

The lead-line method of sounding suffices to record the lay of the bottom with sufficient accuracy where there are no extraordinary obstructions; but in a region like the coast of Maine and that of Alaska, where there are many isolated pinnacle rocks and ledges under water, or along shores like those of Florida, Porto Rico, and the Philippines, where coral reefs abound and coral heads fringe the coast, special investigations have to be made. The lead line might be cast all around a pinnacle rockmight even strike it a giancing blowand still fail to discover it.

An instance of this kind occurred in Buzzard's Bay, Massachusetts, in 1902. Although more than 91,000 soundings had been made, more than 16,000 angles

observed, and 1,462 miles of sounding lines run, a rock whose head was 18 feet below the surface was run upon by the cruiser Brooklyn during the naval maneuvers of that year.

In order to discover such obstructions in much frequented waters a new instrument, the wire drag, has been devised. It consists of a long wire, sometimes more than a mile long, weighted down at intervals with sinkers and supported at any desired depth by surface buoys. Several power-boats. are hitched to it, usually one at each end and one in the middle, and with these. it is drawn around a harbor much as a farmer drives his binder around his field of standing wheat. If it strikes no obstruction the hydrographers know that the harbor bottom is clear to the depth of the drag (see page 656).

Another line of information the mariner must have is about the movement of currents, so that his ship may not be

carried around by currents whose presence he does not suspect. Information concerning them is gathered by means of current rods, as a rule. A current rod is an instrument made to float vertically beneath the water, with only its tip showing above the surface, so that it is not disturbed by the wind. Its movement is observed, and the observations give definite information concerning the currents.

MAKING THE CHART

When all the work in the field has been finished, including observations which show the deflection of the magnetic needle, the force goes back to headquarters and begins the task of "writing up" its notes. And a bunch of notes they are to be written up! To make a single chart of a single harbor may require as many as 12 sheets of field hydrographic drawings, 16 sheets of field topographic drawings, 57 volumes of soundings, 22 volumes of tidal records, 20 volumes of trigonometric rec

ords, 7 volumes of altitude records, and 8 volumes of magnetic records.

Obviously the proposed chart cannot be as big as the area it is to represent, so the first thing to be determined is the scale to which it is to be drawn. In representing a harbor the usual scale is approximately half a foot to the mile, or exactly one foot to ten thousand feet. For ordinary navigation along the shore, where ships keep outside the 100-fathom line, a foot on the chart may represent nearly 80 miles, or, to be exact, 400,000 feet. On charts used to approach the coast the usual scale is about a foot to 16 miles, or one foot to 80,000, to be exact again. In all cases a foot on the chart represents just as great a distance on the land or on the water as a clear exposition of the facts to be placed on the chart will permit.

After the map is drawn it is reproduced either on copper plates or on aluminum plates. By photography a copy is made from the drawing, and this is laid down on the copper to fit the latitude and

longitude marks. The impression is then chemically fixed on the plate, and the engraver, who must have a high degree of skill and accuracy, proceeds to work. The figures showing the depth of the water at each point are put on the plates by a machine, one man being able to do with the machine the work of several by hand, cutting uniform figures and putting them in with mechanical accuracy.

Printing from copper plates is a laborious process. The ink is first placed on the plate and then wiped off with the palms of the hands of the operator and his assistant, leaving only a tiny bit adhering in the engraved lines. It is then run through the press, in contact with wetted paper, after which the prints are calendered by being subjected to a pressure of 600 tons in a hydraulic press.

For cheaper and more rapid work aluminum plates, coupled with the lithographic process, are used. A drawing is made on tracing vellum and then photographed on a sensitized glass plate. Positive plates are made from these negatives, and from these are transferred to the aluminum plates, which have been substituted for stones in the lithographic process. One of the old stones weighed 640 pounds and cost $185; the aluminum plates weigh 5 pounds and cost $4.50.

All the original information upon which every chart is made is filed away, and if one little detail were found wrong 10 years' later the very man who made the error could be located.

100,000 CHARTS ARE DISTRIBUTED EACH

YEAR

The vast importance of the hydrographic work of the Coast and Geodetic Survey is illustrated by insurance rates in harbors where it has done its work and in those in which it has done nothing. In Nome, Alaska, for instance, the marine insurance rate is five-eighths of one per cent, the harbor being surveyed. At Kuskokwin, Alaska, the rate is from 11⁄2 per cent to 5 per cent, with the agents not eager for business even at such rates. The charts of the Survey are kept on sale at all times, and are sold at a price which covers only the cost of the paper and the actual printing. They are fur

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nished to all the ships of the American navy, and the chart-room of one of the big superdreadnoughts contains a series of cabinets where the charts are always kept, ready for reference at a moment's notice. The Hydrographic Office furnishes the same kind of charts for foreign coasts that the Coast and Geodetic Survey furnishes for home coasts.

Nearly a thousand different charts have been prepared of the coasts of the United States and its outlying possessions since the Coast Survey was organized. These range in importance from the great charts of New York harbor, where billions of dollars worth of commerce and millions of lives are safeguarded every year, to the charts of places in Alaska, where a ship may not be seen more than once a week, or even once a month. The charts are in great demand by navigators at home and abroad, and the annual sale amounts to more than a hundred thousand copies.

ASTONISHING PRECISION

The geodetic work of the Coast and Geodetic Survey forms a fascinating part of its activities when one looks behind the mathematics involved. It deals with conditions that very many people do not know exist, and requires measurements of a refinement that must eliminate even the personal equation.

When work must be so accurate that it requires the elimination of the difference of the speed with which the eye telegraphs to the brain and the brain to the hand, in two men; when it must be so exact that a line a mile long may turn to the one hand or to the other no more than 1/33 of an inch; when it must reach that standard when the average error is less than one inch in 500 miles in leveling work, it is apparent that the most. delicate instruments and the most refined measurements are required. Yet such is the standard set by the Coast and Geodetic Survey for its finest surveys.

It determines the longitude of a place. by use of a transit instrument and the telegraph-the transit instrument showing to 1/500 of a second the exact moment at which certain stars cross the meridian of the place, and the telegraph