THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 



less change every hour of the twenty-four ; but, ordinarily, the change is 

 much less during the night than in the day-time. The extent of this 

 variation is not the same in all places, nor on different days in the same 

 place. It is greater in summer than in winter, and on clear days than in 

 cloudy weather. At Hanover it is about 15' in winter, and perhaps 20' 

 in summer. The morning deviation eastward from the direction during 

 the night is usually about one third of the whole variation, or one half of 

 the westward deviation from the same direction at 2 o'clock p. M. We 

 give herewith a few curves showing the diurnal variation at Hanover, in 

 January, 1872, from which a better idea can be obtained than from any 

 verbal explanation. Of these curves we shall see that no two are pre- 

 cisely alike; and, if we should examine the curves for each day of the 

 year, we should find the same variety that is observed in the weather of 

 different days. In these diagrams, each curve has upon it the date at 

 which it was observed. The vertical divisions indicate minutes of arc, 

 and are numbered for Jan. i on the left hand, for Jan. 2 on the right, and 

 so on, alternating for each day. Thus, while the zero line of each curve 

 represents the same direction of the needle, a different line in- each case 

 is used for zero, to prevent confusion by the curves intersecting and 

 blending together. A tendency of the curve upward indicates motion of 

 the north end of the needle eastward, or decrease of declination, and 

 downward indicates increase of declination. To determine, therefore, 

 the relative pointings of the needle at the same time on any two of these 

 days, compare each with its own zero. For example, at 2 o'clock p. M., on 

 Jan. i, the pointing was 6'. 5 ; Jan. 2, it was 6'; Jan. 3, 2'; Jan. 5, 

 2 '.4 ; Jan. 6, 2 '.5, &c. January 9, it will be observed, was a day of con- 

 siderable disturbance, and, at 2 p. M., the pointing was -y-?'^, being from 

 10' to 14' farther east than on previous days; and the average pointing 

 on that day and for several succeeding days was about 10' or 12' east- 

 ward of the usual direction of the needle. Of this we shall say more in 

 speaking of magnetic storms. 



4. The annual variation of the Declination. Besides the changes in 

 declination already mentioned, there is an annual variation produced by 

 the changing seasons of the year. This is perhaps so small as to be of 

 little practical importance in the ordinary use of the needle, but it should 

 not be omitted in a full discussion of the subject. Observations have 



