Redtjctions to Standard Instruments 65 



needle marked A is the north-seeking end, and Ig is the corresponding inclination observed 

 when the end marked B is the north-seeking end; inclination is reckoned as positive when 

 the north-seekmg end of the needle points below the horizon. Both shore and sea data 



were utiHzed for the determination of the graphs for | (/^ - 7^). In addition to the bal- 

 ance error there is also the error due to the irregularity of the pivots, which will vary in 

 general, with the magnetic field. To show the variation of the dip-needle correction A/ 

 with total intensity, F, and inclination, /, there was established for each needle, from all 

 available comparison-data, by the method of least squares, an expression of the 'following 

 general form (see Volume I, p. 45) : 



FAI = X + zcosi + ysinl 

 The adopted values of the coefficients, x, z, and y, for each needle are given on pages 66-69. 



Total-Intensity Observations. 



Sea dip-circle— As ah-eady stated (pp. 21-22) the Lloyd-Creak type of sea dip-circle 

 was modified after Cruise I to make possible the use of Lloyd's method to determine the 

 total intensity, F, as well as the inclination, /, in all magnetic latitudes. Complete speci- 

 men observations and reductions are shown on pages 40-42, and 50-52. The value of the 

 horizontal intensity, H, is obtained by the formula 



H = F cosi 



As the method employed is a relative one, it is essential that no change be made in 

 the_ weight used with the loaded-dip needle, and that its position be not shifted during a 

 cruise from one end of the needle to the other; furthermore, the magnetism of the loaded- 

 dip and deflected needles, except for the normal changes with time, must remain unchanged. 

 The reduction formulie for the total intensity, F, are as given below. Replacing F by 

 H sec 7, the corresponding expressions for H are derived. 



Loaded-dip observations only, F = Ci cos 7' esc u 



Deflection observations only, F = Cj esc Ui 



Both loaded-dip and deflection observations, F = C Vcos 7' esc u esc ui 

 where 7' is the loaded-dip angle, Mi is the single-deflection angle, u = I -I',Ci is the loaded- 

 dip constant = — , C^ is the deflected-dip constant = Kim, and C is the combined con- 

 stant = VK Ki. The constants C, and Cj involve the magnetic moment, m, of the loaded 

 dip-needle and are both, therefore, subject to change with temperature and with tune. 

 C, furthermore involves the induction correction, which is a function of F. d is aff'ected 

 also by changes in deflection distances caused by temperature changes, and by any changes 

 in the distribution coefficients. Two deflection-distances, designated short (5) and long 

 (L), are provided in the modified sea dip-circle (see p. 22), and thus there are two inde- 

 pendent sets of constants. There are also two positions, designated "direct" (D) and 

 "reversed" {R), for the deflected or suspended needle during deflection observations; 

 "direct" position means that the face of the deflected needle is towards the face of the ver- 

 tical circle; "reversed" position means that the face of the deflected needle is towards the 

 back of the vertical circle. Thus, since the deflection observations on board ship may be for 

 one distance and in one position only, the constants to be controlled by shore observations 

 are C,, do for S, dg for S, do for L, and CdR for L. Values for these intensity con- 

 stants were determined at Washington and at each shore station visited by means of 

 comparisons between the sea dip-circles and standardized land magnetometers and dip 

 instruments. 



UJ 



