Magnetic Instruments on the Galilee 17 



MAGNETIC INSTRUMENTS USED IN THE GALILEE WORK. 

 GENERAL CONSIDERATIONS. 



The available space on board ship for magnetic work is necessarily restricted and in 

 fact is never as large as one would like — not even on the Carnegie. Hence it becomes 

 essential to arrange the instruments so that what is aimed at can be accomplished without 

 bringing them so close as to have an effect on one another, thus again introducing devia- 

 tion corrections. There are three elements to be determined: the magnetic declination, 

 inclination, and intensity of the Earth's field. 



The general experience in magnetic work has abundantly shown the need of getting, 

 whenever possible, a totally independent check on each element. Hence each element 

 should be determined twice, preferably by simultaneous observations, which would require 

 6 different instruments or the measurement of more than one element with the same instru- 

 ment. The first is rarely practicable because of the limited space for observing and the desir- 

 ability of taking advantage of the best possible conditions regarding steadiness of ship, etc. 



Our developments have accordingly been along the second line, viz, that each instru- 

 ment should be capable of measuring, as far as practicable, at least two different magnetic 

 elements. Thus an instrument primarily intended for magnetic declination was arranged, 

 by a suitable deflection device, to measure also the horizontal intensity; one arranged 

 chiefly for horizontal intensity was so made that declination could be observed with it, 

 and finally the adopted dip circle measured both the inclination and the total intensity. 

 Thus it was possible to apply all needful checks, and the instrumental equipment was such 

 that the three magnetic elements could be determined wherever the vessel might happen to 

 be. In regions of low horizontal intensity it is better to employ a total-intensity method; 

 hence the need of appliances for measuring both. 



Next, a sjTnmetrical development of all instruments was striven for. It was recognized 

 as a mistake to pick out any one instrument, e. g., an intensity instrument, and devote 

 exclusive attention to it, disregarding the way it would fit in with the other appliances. 

 Hence, from the start, equal attention was bestowed on all three elements, various methods 

 and instruments being studied and thoroughly tried out under actual sea conditions. 



The improvements in instruments and the new principles which were developed for 

 the work of the Carnegie are described on pages 177-20.3. 



SEA INSTRUMENTS FOR MAGNETIC DECLINATION. 



The magnetic declination was determined on board the Galilee chiefly with a Ritchie 

 United States Navy standard Uquid compass and azimuth circle of latest pattern. This 

 well-constructed compass is made by E. S. Ritchie and Sons of Brookhne, Massachusetts, 

 and in only one size, viz, 7|-inch card curved mward, graduated to every degree, and is 

 provided with four cylinders of needles with central buoyancy, the keel-Unes being enameled 

 on copper. The azimuth circle is the Ritchie type III, as used in the United States Navy; 

 it is carefully fitted to the top of the compass, and carries the optical parts. The rays of 

 the Sun are received directly upon a cylindrical mirror and reflected through a right-angle 

 prism on the opposite side of the ring, appearing on the card as a bar of light upon the 

 graduation. (See Plate 3, Figs. 1-4.) 



Some declination results were also obtained with a Negus liquid compass, bearing the 

 imprint, "sold by T. S. and J. D. Negus." It was made by E. S. Ritchie and Sons, and is 

 catalogued by them as a "flat-card compass with central buoyancy"; the card is 8 inches in 

 diameter, is graduated to single degrees, and supports six cyhnders of magnetic needles, these 

 cylinders being somewhat smaller than those in the Ritchie standard compass. This 

 compass is not so well constructed as the "standard," and is Usted, size for size, at about half 



