OUR GUARDIANS ON THE DEEP 



663 



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 labori- 

 ous 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 ad- 

 hering 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 pres- 

 sure of 600 tons in a hydraulic press. 



For cheaper and more rapid work 

 aluminum plates, coupled with the litho- 

 graphic process, are used. A drawing is 

 made on tracing vellum and then photo- 

 graphed on a sensitized glass plate. Posi- 

 tive plates are made from these negatives, 

 and from these are transferred to the 

 aluminum plates, which have been sub- 

 stituted 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 vears' later the very man who made 

 the error could be located. 



100,000 CHARTS ARE DISTRIBUTED EACH 

 YEAR 



The vast importance of the hydro- 

 graphic 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 ma- 

 rine insurance rate is five-eighths of one 

 per cent, the harbor being surveyed. At 

 Kuskokwin, Alaska, the rate is from iy 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- 



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 fur- 

 nishes the same kind of charts for for- 

 eign 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 posses- 

 sions since the Coast Survey was organ- 

 ized. These range in importance from 

 the great charts of New York harbor, 

 where billions of dollars worth of com- 

 merce and millions of lives are safe- 

 guarded 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 be- 

 hind the mathematics involved. It deals 

 with conditions that very many people 

 do not know exist, and requires measure- 

 ments of a refinement that must elimi- 

 nate even the personal equation. 



When work must be so accurate that 

 it requires the elimination of the differ- 

 ence 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 level- 

 ing 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 Geo- 

 detic Survey for its finest surveys. 



It determines the longitude of a place 

 by use of a transit instrument and the 

 telegraph — the transit instrument show- 

 ing to 1/500 of a second the exact mo- 

 ment at which certain stars cross the 

 meridian of the place, and the telegraph 



