Instrumental Outfit for the Carnegie Work 203 



until September 1912 with the spare Kelvin compass mounted in the deCk-house at a point 

 D, du-ectly below A; since that time a standard Ritchie liquid compass replaced the Kelvin 

 compass at D. The disturbing effect of either of the auxiliary compasses used at D is 

 negUgible^ at the distance of D from the compasses at A and C. Figure 13 represents a 

 vertical plane through the four magnetic instruments and the fore-and-aft line of the 

 vessel. The sides of the parallelogi-am are about 13.2 feet, and the distance from B (center 

 of forward observatory) to C (center of after observatory) is about 24 feet. The heights 

 of the various instruments above the main deck are as follows: Marine collimating- 

 compass 1 at A, 14.1 feet; sea-deflector 4 at C, 9.2 feet; marine earth-inductor 3 at B, 8.6 

 feet; and Ritchie compass at D, 3.5 feet. 



It will be seen that the fundamental principle of determining each magnetic element 

 in duplicate with different instruments is fully carried out. Thus the magnetic declina- 

 tion is determined both at A and C from simultaneous observations with instruments of 

 different design and by different observers. At B the inclination is determined not only 

 by different methods and with different needles, using the sea dip-circle, but also since the 

 latter part of 1912 by means of the marme earth-inductor. The horizontal intensity is 

 determined directly at C with the sea deflector, and indirectly from the simultaneous 

 observations of F and I at B. Originally it was intended to make also //-observations 

 with the marine colUmating-compass at A, by the same method embodied in the sea 

 deflector at C; the instrument was accordingly constructed for deflection observations, as 

 well as for declination observations. However, experience showed that sufficient controls 

 on the //-results were already being obtained by the methods described. 



LAND MAGNETIC INSTRUMENTS. 



As in the case of the Galilee work, at practically every port visited the ship magnetic 

 instruments were compared with a standardized land-magnetometer and a standardized 

 earth-inductor. The land instruments used in the Carnegie work were standardized at 

 Washington by direct comparisons with the Department's standards whenever oppor- 

 tunity presented itself — certainly always before and after a cruise. In order to supplement 

 the du-ect comparisons at Washington and to control any possible changes in the constants, 

 frequent additional comparisons were secured at the ports visited, either with reserve land 

 instruments belonging to the vessel's outfit, or with the instruments used by the Depart- 

 ment observers who happened to be engaged on field work near a port of call. 



The various instruments will be foimd mentioned specifically in the inventory, pages 

 203-21 1 . The types of land instruments used are fully described and illustrated in Volumes I 

 (pp. 2-11) and II (pp. 5-15). 



INSTRUMENTAL OUTFIT FOR THE CARNEGIE WORK. 



CRUISE I. SEPTEMBER 1909 TO FEBRUARY 1910. 



Magnetic iNSTRtrMENTs. 



I. For magnetic declination at sea. (1) Marine collimating-compass 1, designed and 

 constructed by the Department of Terrestrial Magnetism, provided with brass binnacle- 

 stand and deflector attachment for use on board ship, and tripod 2 for use on shore; (2) 

 deflector 3, designed and constructed by the Department of Terrestrial Magnetism, pro- 

 vided with brass binnacle-stand by E. S. Ritchie and Sons, for use on board ship, and 

 tripod 1 for use on shore; (3) Kelvin dry compass-bowl (13, Pat. 5892) with cards 8127 and 

 13845, provided with Kelvin azimuth instrument 3619, all by the Kelvin and James White 

 Company, mounted on board ship in wooden binnacle by T. S. and J. D. Negus. The 

 designations adopted, respectively, for the 3 compasses with appurtenances are: CI, D3, 



'Bpfore the Keh-in compass was replaced at P by the Ritchie compass, tests were made on shore at Papeete, which 

 showed that any possible disturbing effects were entirely negligible. 



