184 



Ocean Magnetic Observations, 1905-16 



Instrumental Constants. 



In a perfect instrument the axes of the four colUmators would he, two in the vertical 

 plane of the magnetic meridian and two in the vertical plane at right angles to it; and all 

 four would be in the plane of the horizon. There are meclianical difficulties, however, 

 which prevent the exact reahzation of these requirements, and even if the instrument were 

 found to be in perfect adjustment, it is questionable whether it would remain so. 



The determination of all the constants may be made with a non-magnetic theodolite, 

 at a station where the exact magnetic declination is known during the operation, together 

 with an approximate value of the vertical intensity. Usually at such a station the astro- 

 nomic azimuth of some mark is known. This mark may be used as the reference point 

 for each scale, provided it is m, or nearly in, the direction of one of the inter-cardinal 

 points, in which case it may be seen, unobstructed by the compass bowl, from any of the 

 four positions occupied by the theodolite in front of the 

 four scales. If the view of the mark is obstructed by 

 the bowl, another must be selected. 



The compass is mounted on its tripod and oriented 

 fully 5 minutes before observations. The theodoUte is 

 set up on the ami of the compass tripod, placed before 

 the selected scale, leveled, and adjusted to sidereal focus 

 (see PL 11, Fig. 7); it is then pointed upon the middle 

 division of the scale, with horizontal thread just touching 

 the tops of the shorter divisions. If the telescope now 

 points symmetrically thi-ough the window, the arm is 

 firmly clamped, the bowl is gently drimuned, and the 

 observations begin. Otherwise the arm must be shifted 

 for a lateral adjustment and all the footscrews turned 

 to produce a vertical adjustment as required. 



The constants, Ac and m, for each scale may be 

 determined from the same observations. Observations 

 begin with readings on the mark, theodoUte direct, or 

 vertical circle right, followed by a pointing on each visi- 

 ble division of the scale, and a reversal of the procedure 

 with vertical circle left. In the iniddle of the operation 

 it will be convenient to determine m by pointing on the 

 tops of the shorter divisions and reading the vertical 

 circle, both left and right. 



Reduction to center. — If there is an appreciable ratio between the distances compass- 

 theodolite and compass-mark, then the angles measured by the theodoUte must be 

 reduced to the compass center. In Figure 11 the relative positions, in plan, of compass, 

 theodolite, and mark are shown at C, T„ and M, respectively. The theodoUte is in posi- 

 tion to determine the constants of scale >S. The Une CS represents the direction of "scale 

 south," or approximately the magnetic meridian. Let 



T^C = d = the distance of the theodoUte from the compass. 

 CM = D = the distance of the mark. 

 ST,M = the angle measured by the theodoUte, always obtained by taking the scale 

 reading from the mark reading, 

 c' = the correction to this angle, in minutes of arc, to reduce it to the compass 

 center, always algebraicaUy additive to ST,M- 

 SCM = the angle required at the compass center. 



Then by the formula for reduction to center, 



, d sin ST.M .„. 



' = D sin I' ^^^ 



FlQ. 11. 



