337 



the completed portion is indicated by dotted lines in figures A and B. 

 It does not seem improbable that in southern latitudes the figure 

 will be inverted, and that it will bo a simple ellipse near the 

 equator. 



Monthly Motions. — The motion corresponding to the moon's vary- 

 ing phase has not been projected, chiefly because of the irregularities 

 still existing in the result of the four years' observations for the mag- 

 netic declination, the epoch of minimum being ill-determined ; it is 

 conceived that the figure is a simple ellipse with its major axis in the 

 astronomical meridian, the northern extremity being at conjunction, 

 the epoch of minimum dip, and the southern extremity at opposition, 

 the epoch of maximum dip ; this, however, is doubtful. 



The motion for the moon's position in declination has been ob- 

 tained in the following manner : — Having first projected the means 

 of magnetic declination for each three days of the moon's position 

 in declination, as obtained from the Tables for the years 1843-6, 

 the day after the farthest northerly position being the abscissa, a 

 curve was passed freely among the points ; the values of the ordi- 

 nates at the points of intersection by the curve were then taken as 

 the interpolated value of magnetic declinations for the corresponding 

 abscissa) : a similar operation was performed for the magnetic dip. 

 In both cases very satisfactory curves, agreeing nearly with the true 

 points, were obtained. These values are projected in figure C, 

 Plate VI., Edin. Trans., Vol. xix., Part 2. From this figure the 

 north end of the dipping-needle commences its ascent about two days 

 after the moon is north of the equator, attains its highest point about 

 two days after the moon is farthest north, and afterwards it descends 

 till the moon is again near the equator ; thus forming a figure like 

 a portion of an ellipse with its vertex about one day after the moon 

 is farthest north, the major axis making an angle of about — 30° 

 with the magnetic meridian. It will be remarked that so far this 

 motion is quite similar to that for the sun's position in declination, 

 with the exception of the axis of the figure being on the opposite 

 side of the magnetic meridian ; when we trace the figure farther, the 

 analogy still subsists ; — as the moon proceeds south of the equator 

 the north end of the needle again ascends till the moon is farthest 

 south, thereafter descending, and, in crossing its previously ascending 

 path, a loop is formed lying partially out of the principal figure, as 

 in the case of the annual motion. 



