26 Ocean Magnetic Observations, 1905-16 



in the ship's heading during observations, were eliminated as follows: If but one observer 

 was available, who likewise had to record for himself, directions were given to the helms- 

 man to hold a certain course as nearly as possible for an hour to an hour and a half, and to 

 call out "on" when he was on the course. During this period about 8 complete sets could 

 be made by a skillful observer, using two magnets, in all positions, embracing 32 inde- 

 pendent settings. In general this interval of time proved sufficient to justify treating aa 

 accidental the errors due to shiftings of course, and hence of lubber-Une, during settings, so 

 that the mean of all readings yielded a satisfactory result. Or still better, if a second 

 person was available, as was usually the case, who could record for the observer, he placed 

 himself at the standard compass and called out when ship was on the course, whereupon 

 the observer quickly made his setting, having previously made an approximate setting. 

 Owing to the damping effect of the liquid in the compass, as noted above, a set of four 

 readings, from which an approximate value of H could be derived, would be made even 

 on board ship under trying conditions of sea, within about 8 or 10 minutes. A method 

 more generally employed was to take simultaneous readings of the ship's head with the 

 standard, or other spare compass, close by, and then apply the necessary corrections to the 

 observed deflection-angles. Thus the agreement in the individual sets was improved, 

 though the final result was practically the same as by the first method. 



As is seen from the above description, the deflecting attachment was so designed that it 

 could readily be mounted on a compass for obtaining the required horizontal-intensity 

 observations, and readily dismounted when it was desired to make declination observations 

 with the same compass, or to use the latter in navigating the vessel. Thus, in an instant, 

 the same instrument used for navigation purposes, or for getting the magnetic declination, 

 could be converted into a horizontal-intensity instrument, and the design of not multi- 

 plying instruments unduly was carried out. 



Deflector 2. — When on her third cruise the Galilee reached Sitka, Alaska, it was possible, 

 on July 31, 1907, to replace experimental deflector 1 by a somewhat unproved instrument, 

 No. 2. For this new deflector a standard Ritchie azimuth circle, with sighting bows added, 

 served as framework for carrying the deflecting magnet. (See Plate 5, Figs. 2-4.) A 

 standard Ritchie U. S. Navy hquid compass (No. 33566) took the place of the Negus 

 compass used in deflector 1. The mounting and encasing of the deflecting magnet were also 

 improved. Values of the horizontal intensity and magnetic declination were again obtained. 

 No. 2 was in use during the balance of the Galilee's work, which closed in May 1908. 



Both deflectors 1 and 2 were of the type designated A , namely, that in which the sup- 

 ports for the deflecting magnet were carried by a framework rotating on the compass bowl, 

 and the deflection-angles were read directly on the card graduation. The improved type B 

 was introduced in the Carnegie work. In this type of deflector the supports of the deflecting 

 magnet form a pennanent attachment to the compass bowl, the bowl itself being rotated 

 when settings are made, and the angles being read by vernier on a graduation cut on the 

 edge of the bowl. For the description of this type, see pages 190-194. 



MOUNTING OF MAGNETIC INSTRUMENTS ON THE GALILEE. 

 Cruise I, August to December 1905. 



Figure 1, Plan A, shows the arrangement and spacing of the instruments as mounted 

 on the observing-bridge for use during the experimental work from San Francisco to San 

 Diego, California, August 2 to 23, 1905. (See also Plate 1, Fig. 1.) The position of the 

 center of the sea dip-circle was unaltered throughout the three cruises of the Galilee; it 

 serves, therefore, as the initial point of distance-reference for the various instruments. 



Sea dip-circle 169 {D. C. 169), with which the magnetic inclination and the total inten- 

 sity were determined, was mounted on the heavy, regular, cast-brass Dover stand with 



