64 Ocean Magnetic Observations, 1905-16 



viz, "letters up" and "letters down" (designated respectively U and D), and at the short 

 deflection-distances are 



Magnet 45 {U) log mC = 9.00673 + 0.00019(20° - 1) 



Magnet 45 (D) log mC = 9.00673 + 0.00019(20° - 



Magnet NL(t/) log 7nC = 8.94312 + 0.00016(20° - t) 



Magnet NL(D) log mC = 8.94213 + 0.00016(20° - 1) 



Values of horizontal intensitj^ computed with those constants were reduced to C. I. W. 

 Standard (see p. 77) by applying a correction of — 0.00047i/. 



For Cruises II and III (to July 1907), the constants adopted on the basis of C. I. W. 

 Standard (see p. 77) for the short deflection-distances are 



Magnet 45 (both U and D) log mC = 9.00519 + 0.00l71Ar - 0.00068(Ar)2 



+ 0.00020(20° -0 



Magnet NL (both U and D) log mC = 8.94392 - 0.01627At + 0.00369(Ar)2 



-f 0.00015 (20°- 

 in which Ar = t - 1906.00. 



There is a periodic correction to log mC for sea deflector 1, which, however, has been 

 treated as a part of the ship's deviations, since it depends upon the orientation of the lubber- 

 line, i. e., the heading of the ship. The equation for Cruises II and III representing the 

 mean value of that correction for magnets 45 and NL is therefore given merely as a matter 

 of interest; it is 



5 log mC = -0.0002 sm f -f 0.0003 cos f - 0.0006 sin 2f - 0.0023 cos 2f 



where f is the compass reading of the forward lubber-line, reckoned continuously from north 

 through east. 



Sea deflector 2 {D2). — The constants adopted on the basis of C. I. W. Standard (see 

 p. 77) for the period August 1907 to May 1908, for the short deflection-distances are 



Magnet 45 log mC = 8.93126 - 0.00074At - 0.00107 (At)^ + 0.00020(20° - t) 

 Magnet 2L log mC = 8.80412 - 0.00276At + 0.00127(At)2 -|- 0.00017(20° - 



in which At = t — 1908.00. It should be noted that for sea deflector 2 each magnet had 

 only one possible position relative to the apparatus when mounted. 



As in the case of sea deflector 1, there is a periodic correction to log mC for sea de- 

 flector 2, which again has been treated as a part of the ship's deviations, since it depends 

 upon the orientation of the lubber-hne, i. e., the heading of the ship. The equation repre- 

 senting the mean value of that correction for magnets 45 and 2L is therefore given merely 

 as a matter of interest; it is 



5 log mC = +0.0001 sin f- 0.0001 cos f- 0.0005 sin 2f +0.0007 cos 2f 



where f is the compass reading of the forward lubber-hne reckoned continuously from 

 north through east. 



Inclination Observations. 



Sea dip-circle. — Specimen observations and computations for the determination of 

 inclination, I, by the sea dip-circle, from both the dipping-needle and the deflected-needle 

 methods, are fully shown on pages 50-54. Values for dip-needle corrections were deter- 

 mined at Washington and the various shore stations by comparisons between the sea 

 dip-circles and standardized dip-circles or earth-inductors. Since observations on board 

 ship were made frequently with one polarity only, it was necessary to determine the so-called 

 balance-error arising from any eccentricity of the center of gravity of the needle. For each 

 needle the correction due to that error is determined from graphs representing the quantity 



-(/^ — Ib), for different inclinations. I^ is the inclination observed when the end of the 



