Sea Dip-Circle 195 



be required by the conditions of sea, is made to extend over nearly an hour's time, during 

 which simultaneous dip and intensity observations are made in the forward observatory with 

 the sea dip-circle. The sea deflector is mounted in the after observatory. Plate 12, Figure 2, 

 shows observations being made with the instrument mounted aboard the Carnegie. 



A specimen of sea observations and of computations will be found on page 217. For 

 specimen illustrating the determination of instrumental constants at shore stations, see 

 page 240. 



SEA DIP-CIRCLE FOR INCLINATION AND TOTAL INTENSITY. 



The modified form of sea dip-circle, described in connection with the Galilee work 

 (pp. 21-23), was used throughout the cruises of the Carnegie, 1909-1916. A new, rever- 

 sible gimbal-stand, however, on which the sea dip-circle is mounted, was designed and the 

 new portions were constructed in the instrument shop of the Department (for description, 

 see pp. 196-197, and PI. 14, Fig. 5). The new stand was installed on the Carnegie during 

 Cruise II, at Tahiti in 1912. With this stand, the effect of any lack of level of dip circle 

 during observations may be more effectively eliminated or minimized than previously. 

 Before the introduction of the new stand, as well as since, the same precautions were taken 

 to control level of dip cii-cle as were observed in the Galilee work (see p. 22). For views 

 of the sea dip-circle used, see Plate 4, Figure 3, and Plate 14, Figure 1. 



Both deflection distances, as provided in the modified sea dip-circle, have proved avail- 

 able for all the Carnegie cruises, with the exception of a small portion on Cruise II in 1913, 

 when the vessel was off the Brazilian coast and near the magnetic equator; in this region 

 the short deflection-distance could not be used, but no difliiculty was experienced with the 

 long distance. During the period November 18, 1910, to March 7, 1911, when deflections 

 at short distance were not possible, double the usual number of intensity observations 

 with the loaded needle were made. 



Various investigations have been made with respect to the improvement of the loaded- 

 dip observations. In quiet waters, intensity results of value may be obtained from these 

 observations, if made with care. Whenever there is any considerable motion of ship, 

 however, especially of roUing and pitching, then dynamic effects enter as the result of the 

 comparatively large leverage of the eccentric load on the needle, the load or weight being 

 inserted in the blade of the needle near one of its ends. At such times but little use, 

 if anv, can be made of the observations. Accordingly, throughout, chief dependence has 

 been placed on the intensity results from the deflection observations, though the loaded- 

 dip results have been used whenever observing conditions warranted doing so. 



In the original design of the sea dip-circle it was intended that for the loaded-dip 

 observations the load or weight inserted in the needle should be shifted from one end of 

 needle to the other when passing from one magnetic hemisphere to the opposite one. In 

 the northern magnetic hemisphere, for example, the suitably selected weight was to be 

 inserted in the south-seeking end, or in the end of the needle which was above the horizon. 

 In the southern magnetic hemisphere, on the other hand, the weight was to be inserted in 

 the north-seeking end of needle. It was even intended that the weight should be varied 

 from tmie to time according to the magnetic latitude of the region of work. For this 

 purpose spare weights are provided, some of which, because of lack of marking, could 

 readily be confused with one another. Accordingly, a simple brass case was designed to 

 contain these weights, with such appropriate designations as to avoid possible confusion. 

 Referring to Plate 14, Figure 2, 6 holes will be seen in which the various weights are mserted. 

 A weight placed in the hole between the figures 1 and 1 is designated weight 11; weight 

 12 is the one in the hole between the figures 1 and 2. The lengths of the weights, which 

 were made of German silver, were also measured in order to help to identify them. 



