396 Special Reports 



error. Triangulation between stations for azimuths of marks is resorted to only when absolutely- 

 necessary. 



"To secure reliable results expeditiously, simultaneous observations with the instruments being 

 compared are preferred, as also an exchange of stations; in this way any possible station-difference may 

 be at once eliminated and the desired instrumental-difference be derived ivithout recourse to auxiliary 

 instruments, e.g., magnetographs. At observatories where the same piers used in determining the 

 magnetograph base-lines may be utilized and the required magnetogram-data be obtained promptly, 

 there may be no necessity for an exchange of stations and simultaneity of observations, though 

 this is found, in general, to be the better procedure. When tripods must be used, each instrument 

 is mounted each time on its own tripod. 



"When, for some reason, simultaneous observations are not possible, the observations are 

 carried out alternately at each station by the same observer with the two instruments 1 and 2, 

 and the stations A and B as follows: observations with 1 at A, with 2 at B; 2 at B, 1 at A; 1 at A, 

 2 at B; 2 at B, 1 at A; and so on; next, 2 at A, 1 at B; 1 at B, 2 at A; 2 at A, 1 at B; 1 at B, 2 at 

 A ; and so on. As little time as possible is allowed between determinations at the two stations in 

 order to minimize outstanding effects of corrections to common epoch. With the number of deter- 

 minations called for, this scheme of observation, while of course not as good as simultaneous inter- 

 comparisons, nevertheless yields good results when used with care. 



"Whenever possible, the practice is to secure with each instrument at least 12 complete deter- 

 minations of declination, 6 at each station; 6 complete determinations of horizontal intensity, 3 at 

 each station (one determination consisting of two sets of oscillations and two sets of deflections at 

 two or more distances); and at least 6 determinations of dip with each needle, 3 at each station. 

 The observations are made for different orientations of the footscrews of the instruments, prefer- 

 ably so that there will be an equal number of observations at each station for footscrew marked A 

 south, footscrew B south, and footscrew C south. The work for any one element is not completed 

 on one day, but distributed over several days in order to minimize a possible effect due to magnetic 

 perturbations. Where an exchange of stations is not practicable, the total number of determina- 

 tions for each element is at least as great as just stated. Particular care is used to see that the in- 

 struments are in good working order and the requisite caution is exercised to insure the absence of 

 disturbing influences of whatever character. Before leaving the station, the computations are 

 completed far enough to make sure, at least, that no observational blunders have been made." 



The instruments used by the Department observers are designated by their respec- 

 tive numbers, which will serve at the same time, by referring to Volume I (pp. 2-11), 

 to Volume II (pp. 5-15), and to the present volume (pp. 6-8) to identify and to de- 

 scribe them. The magnetometers are almost invariably of the design" of the Depart- 

 ment, in most cases constructed directly in its own instrument shop, or according to 

 its own specifications. The dip circles, with the exception of Casella Nos. 18 and 4655, 

 and Barrow Nos. 38 and 41, are of Dover make, with certain modifications in some 

 cases as specified by the Department, or of the universal-magnetometer type" as designed 

 and constructed by the Department. Earth inductors Nos. 2, 5, 6A, and 48 are of the 

 Wild-Eschenhagen type, the first two constructed by Toepfer, of Potsdam, and the 

 last two by Schulze, also of Potsdam; earth inductors Nos. 3, 4, 7, and magnetometer- 

 inductors Nos. 24 to 28 are of types 6 designed and constructed by the Department. 



The corrections applied to the magnetometers and dip instruments in order to refer 

 the results obtained to the provisionally adopted standards of the Department are as 

 enumerated in the present volume (pp. 9-18). 



The provisional standards of the Department of Terrestrial Magnetism for the com- 

 parison results obtained up to the end of 1914 were the same as for the results of the 

 field work during the period 1905-1913, namely: For declination, C.I.W. magnetometer 

 No. 3 without correction; for horizontal intensity, C.I.W. magnetometer No. 3 with a 

 correction of +0.00015// applied to observed values of H, the horizontal intensity; for 



See "Two new Types of Magnetometers Made by the Department of Terrestrial Magnetism of the Carnegie Institu- 

 tion of Washington," by J. A. Fleming, Terr. Mag., Vol. 16, 1911, pp. 1-12; also Res. Dep. Terr. Mag., Vol. I, pp. 2-7, and 

 Vol. II, pp. 6-12. 



' See "Description of the C. I. W. Marine Earth-Inductor," by J. A. Fleming, Terr. A/ay., Vol. 18, 1913, pp. 39-45; 

 also "Description of the C. I. W. Combined Magnetometer and Earth Inductor," by J. A. Fleming and J. A. Widmer, Terr. 

 Mag.. Vol. 18, 191'!, pp. 10.J 110; also lies. Dep. Terr. Mag.. Vol. II, pp. 9-12. 



