218 Ocean Magnetic Observations, 1905-16 



The series of observations with magnet 2L at distances 1 and 3, similar to that with 

 magnet 45, was made between the first half (first 40 readings, 1 to 5) of the observations 

 with magnet 45 and the second half (second 40 readings, 6 to 10), or during the time, 

 5^ 25™ to 5*" 59"", as given bj^ chronometer 256. During this period, magnet 45 was removed 

 from the observing-house and stowed at a safe distance. The local mean times of observa- 

 tions with each magnet and for each distance, as will be seen from the computations, are 

 the same within 2 minutes. 



The computations of H are given in the lower part of the same form. The formula 

 for computing H is given on page 236, and the values of log mC are obtained from Table 57, 

 page 238. It will be seen that the 4 values of H (0.2070, 0.2077, 0.2083, 0.2081), resultmg 

 from the observations at two deflecting distances with magnets 45 and 2L, are in fair 

 accord. The means in the last colmim give the mean readings of ship's heading during 

 the observations. The mean (1) of the deflected card-readings for sea deflector 4 is 202?57; 

 the mean (2) of the direct readings with compass 39670 is 202?46; the two independently 

 derived mean readings of ship's heading thus differ only 0?1. The mean reading of ship's 

 heading, by deflector and compass, is 202?5, which corresponds to the heading NNE, on 

 wliich it was auned to hold the vessel. A special form (25a), not given here, has also been 

 devised for disclosing readily any defective readings of deflector or of compass. 



A specimen determination of instrumental constants for the sea deflector will be found 

 on page 240. 



Total-Intensitt Observations, August 23, 1913. 



Specimens of total-intensity observations and computations for August 23, 1913, with 

 sea dip-circle 189, are shown in Form 28 (loaded-dip observations) and Form 28a (deflec- 

 tion observations), pages 219 and 220. 



The scheme of observation consisted of set I, loaded dips, next deflections with both 

 short and long distances, and finally set II of loaded dips. (See also p. 221.) It will be 

 seen that, in the case of the loaded dips, the extreme positions taken by loaded needle, as 

 it swings to and fro, are recorded^ to the nearest degree. In the deflection observations 

 it is necessary always to set the vertical thread of the microscope on middle of arc of 

 swing of suspended needle. Only the deflection observations for short distance are 

 given, the method of observing being the same for long distance. Before proceeding with 

 the computation of the horizontal intensity, H, from the total-intensity observations, it is 

 necessary to detennine the adopted value of the inclination, /. At the bottom of Form 

 28a, p. 220, will be found a summary of the values of I, derived from the various observa- 

 tions (deflected dip, needle 7, short and long distances; regular dip, needles 5 and 9; earth- 

 inductor dip). The adopted value of /, after the corrections on standard are applied, is 

 +65T20. This is used in getting the angle « = / — /', and in the computations of H. 



Referring to the formulae on page 247, the methods of computing H from loaded dips 

 and from deflections, given at bottom of form 28, page 219, will be readily understood. It 

 will be seen that values of H are: 0.2084 (deflections, short distance), 0.2102 (deflections, 

 long distance), and 0.2077 (loaded dips) ; the mean is 0.2088. The accord shown between 

 the 3 values of H, derived from the total-intensity observations with the sea dip-circle, 

 represents about the average case; sometimes the accord is considerably better, at other 

 times worse. The mean value of H agrees with that derived from the sea-deflector observa- 

 tions to within 0.0010 c.g.s., which must be regarded as satisfactory. 



'If arc of swing ia too large for field of microscope, a hand magnifier is used. As the needle, owing to the ship's motions, 

 is subject to discontinuous forces, causing sudden, spasmodic, and erratic displacements, it has not been found practicable to 

 follow the scheme of observation for a rhythmic swing — combining, for example, two readings of extreme position on the 

 right with one on the left, etc. 



