92 



WORK OF THE CARNEGIE AND SUGGESTIONS FOR FUTURE SCIENTIFIC CRUISES 



and permanent or subpermanent magnetism of the hard 

 iron of the ship. The term X is defined and discussed 

 in a later paragraph. The equations of the mathematical 

 theory of ship -deviation in which these parameters are 

 found are discussed in detail in reference (4) at the end 

 of the table. 



The mean Ad for each ship, that is the nonharmon- 

 ic part of the declination-deviations, does not exceed 

 0.3 in this list of vessels. It is constant according to the 

 mathematical theory. Practically it has been found to 

 vary within a range of about 1 ° , but some part of such 

 changes probably has been due to changes in the instru- 

 ment corrections. Experience with the declination com- 

 passes on the Galilee has revealed possibilities of 

 changes in the instrument corrections alone of more 

 than 1° (1). For future work it might be noted that the 

 instrumental constants of the marine collimator lost 

 with the Carneerie were remarkably stable (2). A simi- 

 lar instrument might show that the ship's constant part 

 of the declination-deviations indeed is more stable than 

 has been found when using the ordinary compass. 



The nonharmonic part of the horizontal-intensity 

 deviations Ah theoretically is equal to a constant frac- 

 tion of the horizontal intensity H of the earth's undis- 

 turbed field expressed by (X - 1)H in which X repre- 

 sents the ratio of the horizontal component of the earth's 

 undisturbed field to that of the ship's and earth's com- 

 bined. The value of X differs little from unity in wood- 

 en hulls. Excepting the values for the Discovery andthe 

 Gazelle , the greatest difference from unity occurs in 

 the value 0.991 for the Erebus which would give about 

 three in the third decimal place of H for the nonharmon- 

 ic part of the deviations in H in a region of high H. It 

 would seem that even a mediocre control of the changes 

 in X should make the values of the correction dependa- 

 ble at least to within a unit in the third place. 



The nonharmonic part of the deviations in dip or 

 inclination, I, is more complicated. It depends on the 

 vertical components of the ship's magnetism at the point 

 of observations and on the inclination. On the Challen- 

 ger it amounted to 6° or 7° in some parts of her cruise. 

 On the Gauss and Gazelle apparently it did not exceed 

 0.°3 to Of 6. 



To calculate the probable magnitude for any given 

 inclination and vertical force, the parameters for the 

 vertical components of soft and hard iron magnetization 

 at the -instrument would have to be determined. Their 

 combined effect can be determined very readily with a 

 vertical-force instrument, but to separate the effects of 

 soft from hard iron, which would be necessary to enable 

 some estimate to be made of the probable values of the 

 nonharmonic part of the dip-deviations, observations 

 would be required in localities with widely different 

 values of the vertical component of the earth's field. 



All the vessels above referred to, except the Gali- 

 lee and Carnegie , had steam boilers with long funnels. 

 The funnels have been regarded in some cases as con- 



tributing the major portions of the vertical components 

 of the disturbances and the irregular changes in these 

 components have been ascribed to changes in funnel 

 temperatures. The nonharmonic part of the dip-devia- 

 tions is given by 



2 sin 1/2(2 - n) 



Al 



and 



1 + k + R/Z 



in which R represents the vertical magnetization of hard 

 iron and k the coefficient for soft iron magnetized by 

 the vertical component Z of the earth's field. 



Data for modern steel hulls usually do not contain 

 values for the vertical parameters. There are, how- 

 ever, published values for early English warships (3) 

 giving /x by which the nonharmonic part of deviations in 

 dip may be computed for the localities in which the given 

 Al has been determined. These values are found to range 

 from a few tenths of a degree up to 10°. It is inferred 

 that steel hulls also would give values of the same order 

 of magnitude and it is reasonably certain that magni- 

 tudes of this order could not be controlled to the re- 

 quired degree of accuracy. 



Details in the methods of observing by swinging 

 ship, etc., at every station belong to specific instruc- 

 tions but it might be well to note some of the departures 

 from former practice that would be permissible. As it 

 is proposed to eliminate the harmonic part of deviations 

 by swinging, it is not necessary to determine the coeffi- 

 cients. Four headings are sufficient to eliminate the 

 harmonic part for declination, horizontal intensity, and 

 dip. Two are sufficient to eliminate the harmonic part 

 in vertical intensity. For the same reason the headings 

 need not be restricted to cardinal or intercardinal 

 points. It is necessary, however, and it is sufficient 

 that they be equally spaced around the compass points. 

 This is worth considering especially when swinging 

 under sail only. The time taken to make a swing proba- 

 bly can be reduced still more by the adoption of electri- 

 cal methods for determining inclination and intensity. 

 Experience on the Carnegie confirms both the practica- 

 bility and the rapidity of electrical methods for deter- 

 mining inclination and the method for horizontal inten- 

 sity had reached a very promising stage of development. 



It appears from these considerations that satisfac- 

 tory determinations for secular variation can be made 

 on a vessel not especially constructed for magnetic 

 work and that the reduction need not involve so much 

 labor in observing and computing as was required for 

 such vessels on past expeditions. But trial is necessary 

 to substantiate these conclusions. Some vessels, even 

 of wooden construction, may be too highly magnetic for 

 reliable work. After one has been found that is suitable, 

 every precaution should be taken against large changes 

 in her magnetic condition. 



LITERATURE CITED 



1. Researches, Department of Terrestrial Magnetism, 



vol. 3, p. 61. Carnegie Inst. Wash. Pub. No. 175. 

 1917. 



2. Researches, Department of Terrestrial Magnetism, 



vol. 3, p. 187. Carnegie Inst.Wash. Pub. No. 175. 

 1917. 



3. Magnetism of ships and deviations of compass. Wash. 

 D. C, Navy Dept. Bur. of Nav. 1867. Papers by 

 Evans and Smith, 1865. 



