OUR GUARDIANS ON THE DEEP 



671 



with an epidemic of smallpox among the 

 Eskimos, and a few days later are down 

 in a little valley where the mosquitoes are 

 so thick that even a horse cannot graze 

 unless it be protected by nets. Now one 

 of them is at work with a party in the 

 Rocky Mountains, fording streams that 

 are bordered with quicksands, shooting 

 down rapids that threaten every moment 

 to swamp canoe, party and all. 



But whatever the task of the men, they 

 learn to live with the thought uppermost 

 in their minds that he who is willing to 

 bear the heaviest burden will find every 

 other member of the party ready to help 

 him bear it. 



Men go to Africa to hunt big game; 

 they go to South America to explore new 

 rivers ; they go to Switzerland to climb 

 high mountains; they go to the heart of 

 Africa and the interior of Tibet to find 

 strange people ; but the men who make 

 the basic surveys upon which our knowl- 

 edge of the shape of the earth and our 

 maps are based get all the thrills and all 

 the experiences that come to them all, and 

 get back to Washington safe and sound 

 and ready for the prosaic work of writ- 

 ing up their notes. 



A MAGNETIC SURVEY 



Accurate surveying such as is involved 

 in the triangulation work of the Coast 

 and Geodetic Survey would not boot 

 much on land or sea were definite infor- 

 mation not at hand concerning the be- 

 havior of the magnetic needle. The mag- 

 netic needle refuses to stay put anywhere, 

 and varies from the true north in differ- 

 ent degrees in different parts of the earth. 

 For instance, in London it changed its 

 direction 35 degrees in the 232 years pre- 

 ceding the outbreak of the War of 1812. 

 Then it began to swing back again, and 

 up to the present time has moved east 

 nearly 10 degrees. The change is by no 

 means as rapid in the United States as it 

 is in London, but at the same time it is 

 enough to interfere with every survey 

 if the rate of change were not known. 

 As the rate varies between London and 

 Washington, so it varies in different 

 parts of the United States. 



In order to determine the declination 

 of the magnetic needle innumerable ob- 



servations have been made all over the 

 United States, and the exact declination 

 for each place has been ascertained. In 

 the section east of the Rocky Mountain 

 States there is scarcely an area 50 miles 

 square where the variation of the needle 

 has not been measured and passed on to 

 the State and local surveyors, and in few 

 places are these points of needle obser- 

 vation more than 20 or 30 miles apart. 

 The method by which the variation at a 

 given point is found is to compare the 

 north of the compass needle with the 

 true north revealed by the theodolite. 

 The difference is the variation of the 

 needle. 



THE MAGNETIC NEEDLE IS NEVER STILE 



If the behavior of the needle is impor- 

 tant on land, it is even more so on the 

 sea. A boundary a mile long fixed by 

 the compass in Maryland in 1802 would 

 now have its one end some 525 feet away 

 from the original location if the survey 

 now did not allow for the change in the 

 variation of the compass since 1802. 

 But on land points can be located with- 

 out the use of the compass. On sea the 

 compass is yet indispensable, and the 

 navigator must constantly allow for its 

 continual changes, else some fine night 

 he would find himself high and dry on a 

 reef when he thought he was miles away 

 from it. For instance, the declination 

 of the compass at Key West is 2 degrees 

 east, while at New York it is 10 degrees 

 west. Did a skipper steer through his 

 voyage from Key West to New York 

 on the assumption that the declination 

 was the same at New York as at Key 

 West, he would run into the coast some- 

 where south of a point 19 miles west of 

 New York. 



The Survey maintains observatories at 

 Cheltenham, Maryland ; Baldwin, Kan- 

 sas ; Sitka, Alaska ; Vieques, Porto Rico, 

 and near Honolulu, Hawaii, where the 

 behavior of the magnetic needle is under 

 constant observation and study. There 

 are places where the compass needle 

 varies from forenoon to afternoon 

 enough to make a difference of from 

 5 to 20 feet in a mile-long line, and 

 places where there have been sudden and 

 unaccountable changes such as occurred 



