NOTES ON THE PROGRAM FOR FUTURE MAGNETIC MEASUREMENTS AT SEA 



In considering the possibilities of measurements of 

 the magnetic elements at sea in the future, approach 

 must be made from one of two angles; the ship either 

 shall be specially constructed so as to be nonmagnetic, 

 or shall have only certain modifications which will make 

 it available for magnetic work. At present it appears 

 that the second arrangement--modifying a ship not es- 

 sentially nonmagnetic --will offer, perhaps, the earlier 

 opportunity for the resumption of measurements at sea, 

 and this paper will be devoted to considering what kind 

 of program will be required on such a ship. 



In accordance with the suggestions of the first re- 

 port of this section, the major requirements of a wood- 

 en hull and of a region eight meters in radius free from 

 magnetic materials around the proposed site of the mag- 

 netic instruments will be considered as having been ar- 

 ranged for, since the work would prove of little value 

 unless that were the case. 



The special construction of the Carnegie made it 

 possible to make a series of magnetic observations 

 which could be accepted as correct, without the need for 

 additional observations to eliminate the deviations in- 

 troduced by magnetic materials. A ship not specially 

 constructed, but modified as indicated in the preceding 

 paragraph, will require swinging ship for each set of 

 magnetic observations. For the Carnegie magnetic 

 work, swinging ship was carried out at comparatively 

 infrequent intervals and practically always in port, and 

 thus did not constitute a very large part of the work. 



On cruises I to VI of the Carnegie , declination ob- 

 servations were made twice each day if weather permit- 

 ted, and measurements of horizontal intensity and in- 

 clination at least once each day. On cruise VTI this 

 program was reduced, horizontal intensity and inclina- 

 tion measurements being made only on alternate days, 

 though declination measurements were made twice each 

 day as previously. On cruise VII, declination observa- 

 tions made with the marine collimating compass (1) re- 

 quired twenty to thirty minutes each morning and even- 

 ing, with three observers; horizontal intensity and incli- 

 nation required fifteen to thirty minutes and thirty to 

 sixty minutes, respectively, with the earth inductor (2) 

 and three observers; horizontal intensity with the de- 

 flector (3) and two observers required sixty to one hun- 

 dred and twenty minutes. The earth inductor and deflec- 



tor observations always were carried on simultaneously 

 on the Carnegie . For all these measurements the ship 

 was put on some cardinal or intercardinal heading and 

 was kept there throughout the work on each element. 



When swinging ship on cruise VII, the various meas- 

 urements were curtailed on each heading, the periods 

 being five minutes for declination, twenty minutes for 

 horizontal intensity (simultaneously with deflector and 

 earth inductor), and twenty minutes for inclination. Re- 

 peating the measurements on eight headings made the 

 time for a declination swing forty minutes, for horizontal 

 intensity two hours and forty minutes, and for inclination 

 two hours and forty minutes. The declination swing was 

 taken either at sunrise or sunset, the five-hour period 

 for the other two elements being arranged to occupy 

 either morning or afternoon. 



Since every set of magnetic measurements on a 

 modified ship will require swinging ship and since such 

 observations will consume at least one-half day's time, 

 it is realized that the work probably could not be done 

 oftener than once a week. In addition to the time re- 

 quired for observation, time needed for computation will 

 amount perhaps to two or three hours with two men en- 

 gaged. In attempting to fit magnetic work into the pro- 

 gram of an expedition, probably it would be well to es- 

 timate that the time of three men would be required for 

 one entire day each week. 



The measurements of horizontal intensity with the 

 earth inductor on cruise VII of the Carnegie indicate 

 that the degree of accuracy that may be obtained with 

 that instrument is sufficient for marine measurements, 

 and it is possible that deflector observations can be 

 omitted in future work. If deflector observations are 

 not discontinued, but are to be taken simultaneously with 

 earth inductor measurements, five men would be needed 

 for the work instead of three as mentioned above, al- 

 though the time required for swinging ship would be 

 about the same. 



The marine collimating compass, the earth inductor, 

 the deflector, and the many pieces of apparatus used 

 with these instruments, all destroyed with the Carnegie , 

 would have to be duplicated in the instrument shop of the 

 Department and would have to be requested very consid- 

 erably in advance of the date set for the beginning of an 

 expedition as the work would be a large item. 



LITERATURE CITED 



1. Researches, Department of Terrestrial Magnetism, 



vol. 3, p. 177. Carnegie Inst. Wash. Pub. No. 175. 

 1917. 



2. Soule, F. M. Earth-inductor measurements aboard 



the Carnegie cruise VII. Terr. Mag, vol. 35, June 

 1930, pp. 103-109. 



3. Researches, Department of Terrestrial Magnetism, 

 vol. 3, p. 191. Carnegie Inst. Wash. Pub. No. 175. 

 1917. 



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