190 Ocean Magnetic Observations, 1905-16 



distances on each side of the middle or long mark. The scale value is thus practically 

 eliminated. Care is taken to read along the upper portion of the scale, since the inclina- 

 tion, m, is determined for an imaginary line just touching the upper ends of the small 

 divisions. This becomes more important as the star's (Sun's) altitude increases. In 

 Figure 12 the correct reading is 4.5, whereas the reading 4.0 at the bottom of the scale would 

 be erroneous. The observer should shift his eye and the sextant so as to keep the middle 

 division in the center of the min-or. The Sun's image is kept close to the edge of the silvered 

 portion of the horizon glass of the sextant. 



It may be noted that, since opposite scales are so nearly 180° apart in any plane com- 

 mon to both, the star's (Sun's) image for the second observation may readily be found by 

 setting off the supplement of the sextant reading of the first observation, providing opposite 

 scales are used. If opposite scales are employed symmetrically in a series of observations, 

 constant errors of the sextant are eliminated, as may be seen from equation (6), since, by 

 convention, A is positive for one scale and negative for the other. 



A "set" consists of 10 or 11 readings of the scale, taken at extremes of the oscillations, 

 or at precisely equal intervals of time. The observer calls out "mark" at each reading, 

 and the time is noted. During a "set" the index arm or screw is not touched, so that the 

 sextant reading corresponds to the mean of the set. The observer "standing by" to 

 measure altitudes notes the altitude at the fifth or sixth reading, or between the fifth 

 and sixth, according to the proposed number of readings. (See PI. 15, Fig. 2.) 



Specimens of observations and computations are given on pages 213-215. 



SEA DEFLECTOR FOR MAGNETIC HORIZONTAL INTENSITY AND DECLINATION. 



Early in 1905, in order to supplement the sea dip-cu-cle for obtaining magnetic intensi- 

 ties at sea, a deflecting apparatus was devised by L. A. Bauer which could readily be 

 attached to an ordinary Uquid compass, and make possible the direct detennination of the 

 magnetic horizontal intensity, as well as of the magnetic dechnation.^ The ' ' sea deflector 

 has been used throughout the ocean work accomphshed on board the Galilee, 1906-1908, and 

 on the Carnegie, 1909-1916. The following paragraphs briefly describe the instrument in 

 its later improved forms as used on the Carnegie, and as constructed in the Department's 

 instrument shop, under the direct supervision of J. A. Fleming, who is responsible for many 

 of the unprovements. The special requirements of the instrument were simphcity of 

 construction, of observation, and of computation, and availability both for observations 

 of decUnation and horizontal intensity. The earlier forms of the instrument used in the 

 Galilee work will be found described on pages 24-26. 



Declination Observations. 



As a check upon the decUnation results with the standard compass, the sea deflector, 

 while designed chiefly for horizontal-intensity observations, has been steadily improved, 

 so that with it good declination values also may be obtained. This has been accomplished 

 by constructing the compass part of the deflector practically ourselves and embodying 

 the improvements described later. Three instruments of this type (D3, D4, and D5) 

 have been successively constructed in the Department's instrument shop, and supplied 

 to the Carnegie. The instrument in its final form is no longer a mere attachment to a 

 compass supplied by mercantile makers, but is now entirely a distinctive product of the 

 Department of Terrestrial Magnetism. Had the original name, "sea deflector," Mt 

 aheady been used, it might now be more appropriately termed a "sea magnetometer," as 

 with it both the magnetic decimation a nd the horizontal intensity are determmed at sea . 



iior first descriptions of the instrument, see articles by L. A. Bauer and J. A. Fleming in Terr. Mao-, vol. 11, pp. 78-83, 

 1906; vol. 14, pp. 167-169,1909; vol. 18. pp. -57-62, 1913. See also this volume, pp. 24-26 and 191. 



