Magnetic Instruments on the Galilee 19 



Practically every difficulty in securing magnetic results at sea, witli the desired degree 

 of accuracy, has been surmounted, as will be seen from the specimen results given later, 

 with the exception of this particular one — how to secure magnetic decUnations when no 

 celestial object is visible with the aid of which a true azimuth can be determined. On land 

 the magnetic meridian can be referred to some fixed object, the azimuth of which may be 

 determined at leisure and when the skies permit. At sea, in cloudy weather, no fixed object 

 is to be had. It is hoped that some time tests may be made as to how far a device based 

 on the gyroscope will solve this problem. 



While L. A. Bauer's attention was being devoted to the perfecting of appliances and 

 methods for inchnation and intensity, W. J. Peters was making a careful study of instru- 

 ments for measuring the magnetic dechnation at sea. Special experiments and studies 

 were carried on by him as opportunity afTorded, especially on the third and last cruise 

 of the Galilee. As the result, there was devised the "marine coUimating-compass," which 

 became the principal declination-instrument on the Carnegie. There have been eliminated 

 in this compass (see pp. 177-178) the chief instrumental sources of error in magnetic- 

 declination observations at sea described on page 18. 



Studies of the declination results with the instruments which were used in the Galilee 

 work showed that on land the magnetic declination could be obtained with the standard 

 Ritchie 7f-inch liquid compass, using either the cylindrical mirror or the dark plane mirror, 

 within 0?2, and with special care within 0?1. However, these devices did not afTord such 

 precision when used at sea; it was found, for example, that the results from 8 different sets 

 of 10 pointings each differed at times as much as 0?5. The Kelvin azimuth attachment on 

 the dry compass was frequently found to give even more discordant results, which, however, 

 in some measure, may have been caused by the near synchronism of the card oscillation and 

 the roll of the ship. 



SEA INSTRUMENT FOR INCLINATION AND TOTAL INTENSITY. 



The magnetic inclination and total intensity of the Earth's magnetic field were deter- 

 mined with the well-known "Lloyd-Creak dip-circle," modified as experience showed neces- 

 sary. This form of dip circle, designated hereafter as "sea dip-cLrcle," for use primarily 

 in observations at sea, replaced the well-known "Fox dip-circle," devised in 1835, which 

 had made possible the admirable ocean magnetic work in the fourth decade of the last 

 century on the Erebus, Terror, and Pagoda, and had been used in subsequent expeditions 

 (Challenger, 1872-76; Gazelle, 1874-76; and on various Arctic and Antarctic vessels). 



Briefly described, the new instrument applied the method of Lloyd's needles for the 

 purpose of determining the absolute inclination and the relative total intensity at sea; it 

 embodied a number of modifications of the Fox dip-circle, the chief improvement being in 

 the mounting of the needles, which greatly facihtated the various operations, reducing to a 

 minimum the possibility of injury to pivots of needle, permitting reversal of needles, etc. 

 The improvements were devised by Capt. Ettrick Creak before his retirement from the 

 superintendency of the Compass Department of the British Admiralty, and the expenses of 

 the initial experiments were defrayed by the British Admiralty. Several instruments were 

 constructed under Captain Creak's direction, and, after having passed the tests of the Kew 

 Observatory, were supplied to the Antarctic vessels of 1902-04, the Discovery of Great 

 Britain and the Gauss of Germany. Unfortunately, there were some instrumental defects 

 in these Lloyd-Creak dip-cu-cles, the German observer (Dr. F. Bidlingmaier) being highly 

 dissatisfied with the performance of the instrument on the Gauss. For some reason any 

 intensity observations which may have been made at sea on the Discovery with the new 

 instrument have not been published in the volume of results of the British expedition. An 

 instrument supplied about the same time to the United States Coast and Geodetic Survey, 

 and another one used on the first cruise of the Galilee, likewise required modification before 

 they could be used successfully. 



