38 



Ocean Magnetic Observations, 1905-16 



dredths, are entered for each heading, and the value of the horizontal intensity, H, computed 

 by the formula: H = mC/ sin u. The values of log sin u maybe conveniently looked up 

 in a table in which the argument is given to degrees and hundredths, such as Bremiker's 

 five-place table. The values of log mC„ corresponding to the observed mean temperature 

 on any heading, are derived as explained in the section giving instrumental constants 

 (p. 64). The mean value of H from the 8 equidistant headings is 0.2520 c. g. s.; the 

 corresponding value derived from the surrounding shore observations, for the same time, 

 is 0.2524; hence, the value of .4^ in the deviation formula (p. 39) is —0.0004. 



Computation of Horizontal-Intensity Observations {Swing) 



(Form 25) 



Station: San Francisco Bay 

 DaU: May 25, 1908, A. M. 

 InslTumenl: Sea Defl'r 2 (D2) 

 Weather: b Temy: 13° 



Sea: S 



C. 



Lat: 37° 51' N 

 Vessel: Galilee 

 Ohs'r: D. C. S. 

 Wind: 

 Roll: 0° 



Long: 122° 23' W 

 Com'd'r: W. J. P. 

 Comy'r: D. C. S. 

 Reviser: W. J. P. 

 Helm: Port 



Subtracting the mean value, 0.2520, from the individual //-values, the observed 

 deviations (without A^) are next derived. These quantities are then analyzed on Form 

 23a (Analysis of Horizontal-Intensity Deviations, page 39) in a manner precisely similar to 

 that followed in the analysis of declination deviations, page 36. The unit used on this form 

 is the fourth decimal c. G. s. The resulting deviation-coefficients in c. g. s. units are: 

 B^=-f 0.0000; C,= -H 0.0003; D^= + 0.0012; £,= -0.0004. With the aid of these the 

 computed deviations (without A^) are obtained in upper right-hand portion of the form, 

 and also entered on Form 25. 



The computed deviations with A", are appUed in Form 25, with signs reversed, to the 

 observed values of H, and thus the final or corrected values of H, with deviation corrections 

 applied, are derived. It wiU be seen that while the deviated or observed values of H vary 

 from 0.2503 to 0.2539, hence show a range of 0.0036, the undeviated or corrected values 

 exhibit a range of but 0.0015. This range might have been still further reduced had the 

 analysis of horizontal-intensity deviations been made separately for each magnet, instead 

 of for the two magnets (45 and 2L) together. The purpose, here, is merely to illustrate the 

 method of observation and of computation. These results must be regarded as satisfactory, 

 especially when the time consumed in the observations is considered, viz, 2 to 4 minutes 

 for a single value of H, and about an hour for the mean value. It is to be noted that 

 these results were obtained under ideal conditions. Later we shall have examples of obser- 

 vations made under severer conditions. 



