134 Ocean Magnetic Observations, 1905-16 



33. The new commander of the expedition, W. J. Peters, having the party in readiness, arrived 

 in San Diego on February 9, 1906. The relative positions of the instruments remained unchanged 

 from what they were between August 24 and December 20, 1905. For plan of the extended bridge see 

 Fig. 1, Plan C, page 27, the initial point being as in all previous conditions the center of the sea dip- 

 circle. Before swinging ship, I personally adjusted the instruments with much care for alinement. 

 The change in the forward one was quite considerable, as the bridge, in splicing it out, had taken up 

 some wdnd. The weather during February proved to be unusually bad (for San Diego) for swinging 

 ship, there being a great deal of rain, interspersed with cloudy and foggy weather. The vessel was 

 swung on February 14, 15, and 26, 1906, in the same place where the previous swings had been made. 

 During these swings the vessel was in a normal condition, with the exception that on the last day the 

 iron stern-davits were not in place, as they were ashore at a shop serving as patterns for making 

 heavier ones to carry a gasohne launch. On February 27 I left San Diego, returning to my duties 

 in the Coast and Geodetic Survey on March 1, 1906. 



W. J. Peters: Discussion of Alidade Correction for Standard Compass R3C.' 



There are many different styles of compasses and compass devices for obtaining the 

 magnetic bearing of celestial and terrestrial objects. This discussion is based upon con- 

 siderations of the Ritchie azimuth-circle with peep-sight, vertical thread, and dark mirror, 

 referred to in this volume as "alidade method" (see AB, PI. 3, Fig. 2). The principles 

 are, however, applicable to any compass device which depends upon reflection by mirror or 

 prism mounted on a horizontal axis to obtain the desired bearing. 



Let D„ be the observed magnetic declination, obtained by the alidade method at some 

 station where the standard declination at the same instant is D. Furthermore, let — A„ 

 be the alidade correction to be applied to Z)„ to obtain D. Then 



A„ = D,- D 



If, when observing, the mirror axis of rotation is not truly horizontal, or if it is not 

 perpendicular to the line of sight (from lower part of peep-sight to lower end of vertical 

 thread of azimuth circle),^ or again, if the axis does not lie in the plane of the mirror surface 

 or is not parallel to it, then Aac, as determined from observations with the mirror, will contain 

 the combined efTect of the conditions mentioned. If the separate effects are represented by 

 a, b, c, respectively, then 



A^ = a + b + c + x = D, -D 



In this equation x is the part of Aac which in no way depends upon the position of the 

 mirror surface. 



If the altitude of the Sun or object sighted is very small, the mirror is nearly horizontal 

 and the line of sight from the eye, striking the mirror at an exceedingly small glancing 

 angle, is but little affected by a faulty installation of the mirror, so that, for an altitude, 

 h = 0,it may be assumed that a + b + c = 0. 



For an altitude, h = 180°, the mirror is vertical, the first effect, a, disappears, and b 

 and c attain their maximum values, a, b, c are functions of the altitude h, which are 

 found from geometric considerations to be as follows: 



a = y tan h b = z tan h tan ^ c = w tan h sec „ 



The expression for A„ becomes, accordingly 



Aac — X -\- y tan h •]- z tan h tan ^+10 tan /i sec ^ = D^ — D (1) 



'This designation applies to the standard Ritchie liquid compass 29499 provided with azimuth circle 481-III. R3C 

 was used on Cruise III of the Galilee (see pp. 31 and 62). 



'To avoid circumlocution, the expression "peep-sight and vertical thread" is used to denote this line, and "plane o( 

 apparent bearing" defines the vertical plane that contains this line. 



