MAGNETIC INSTRUMENTS USED IN THE CARNEGIE WORK. 



The same general considerations were applied in the designs and construction 

 of the instruments for the Carnegie as in the case of the Galilee work (see p. 17). 

 With the improved facilities available when permanent quarters at Washington 

 were provided, it became possible to investigate, even more thoroughly than 

 before, the causes of instrumental defects and to remedy or avoid them in new 

 instruments. It also became possible, with the enlargement of the instrument- 

 shop equipment and personnel, practically to construct a complete instrument our- 

 selves. The standardizing observatory (PI. 21) erected in 1914-15 on the grounds of 

 the permanent quarters also added to the facilities for quickly testing an instrument. 



MARINE COLLIMATING-COMPASS FOR MAGNETIC DECLINATION. 



As the result of studies and experiments made by W. J. Peters on the final cruise of the 

 Galilee (see p. 19), a special form of compass, known as the "Carnegie Institution Marine 

 Collimating-Compass," was devised by W. J. Peters and J. A. Fleming, and constructed 

 with great care in the instrument shop of the Department of Terrestrial Magnetism by 

 J. A. Widmer, cliief instrument maker.' This instrument is the standard compass for 

 declination observations on the Carnegie; it has been in use since 1909 and is designated CI. 



Description. 



Many parts of the standard U. S. Navy 8-inch liquid compass, including the binnacle 

 have been used in the new instrument. These parts were supplied by the makers, E. S 

 Ritchie and Sons of Boston. Plate 11, Figure 3, gives a general view of the instrument 

 mounted in its binnacle. Figure 4 of the same plate shows the original buoyant ellipsoid 

 with the card rim removed, and also the magnets and two of the four concave mirrors of 

 speculum metal. There are also seen in this figure the four scales, each having nine divisions 

 and lying in the focus of the optical system formed by the corresponding mirror and window 

 lens. These alterations have changed the weight from 213 grams to 223 grams; that is, 

 the mass of the buoyant system has been increased one-twentieth of the original, but the 

 radius of gyration and the surface exposed to friction have been decreased so that the period 

 of oscillation in Washington is about 11 seconds instead of the usual 14 to 17 seconds. 



The axes of the four optical systems lie in the horizontal plane HPG (PI. 11, Fig. 6) 

 containing the point of support (the top of the pivot), and are directed to the four points 

 of the compass by the action of the system of magnets to which they bear a fixed relation. 

 Four windows in the bowl, two of which are shown in section at GG, pennit a view of any 

 scale after the bowl has once been turned to the proper position. These windows are seg- 

 ments of a spherical shell whose center is at the point of support of the optical systems, hence 

 the rocking of the bowl or the rotation produced by yawing does not alter the optical con- 

 ditions. The angle between a star, the Sun, or any other object and a selected scale is 

 measured with a sextant. The four optical systems are provided, so that when the star or 

 Sun observed upon is unfavorably located for one system another may be chosen. 



The bowl (PI. 11, Fig. 1) swings in a perforated gimbal-ring (PI. 11, Fig. 5), which in 

 turn is supported and inclosed in a metal cyUnder. This cyhnder (PI. 11, Fig. 2) is gradu- 

 ated on its lower edge so that the windows may be quickly turned to the desired position 

 when the course of the ship is known. The whole instrument is painted black, and the scales 

 are cut on small blackened silver bars. This arrangement shows the star and the illuminated 

 divisions of the scale against a dark background during the observations. 



From the foregoing general description, it will be seen that by the introduction of the 

 optical collimating system with scale the observer is enabled to note the arc of motion of 



'The first description of the iustrument by W. J. Peters appeared in Terr. MaO-, vol. 14, pp. 17-24, 1909. 



177 



