178 



Ocean Magnetic Observations, 1905-16 



the magnet system while sighting on the Sun or star, hence he knows precisely to what 

 part of the arc the stellar azimuth appUes. In brief, practically the same method of obser- 

 vation can be used at sea with the marine collimating-compass as on land with a magnetom- 

 eter. In the latter case the magnetometer circle is set to some convenient point on the 

 magnet scale and then scale readings are taken of the positions of the magnet during the 

 interval of observation. The angle is next determined between the circle setting and some 

 mark, or the true meridian, and the declination is finally deduced. Similarly, with the 

 marine colUmating-compass, the angle between the magnet (say, middle of scale) and 

 some celestial body, as the Sun, is read with a sextant to the nearest minute of arc at a 

 given time, and then, with the sextant still clamped at the same angle, simultaneous read- 

 ings of the Sun's image on magnet scale and of watch are taken. With the aid of the time 

 readings, the motion of the Sun during the interval of observation is taken into account, 

 and the true azimuths detennined, whereas the scale readings give the varying positions 

 of the magnet system. 



General Formulae. 



In Figure 10, which represents the celestial sphere, stereographically projected upon 

 the plane of the horizon, let Z be the zenith of the magnetic station observed at with the 

 collimating compass; s the star sighted; A''^ the astronomic meridian; N'&' the mag- 



netic meridian. Suppose the observer, looking into one of the coUimators, sees the scale 

 apparently just above the western horizon. This scale is designated by the letter w. Sup- 

 pose further that the point of the scale brought into coincidence with the star's image is to 

 the right of the middle scale-division. Now let this point and the middle of the scale be 

 projected to the celestial vault, and then to the plane of the projection, at w' and at w, 

 respectively. The arc measured by the sextant is then projected to sw' . 



