THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. l6l 



well as on this continent. The assistant in charge of the magnetometer 

 noticed early in the forenoon of this day an unusual disturbance of the 

 needle, and was thoughtful enough to make his observations every few 

 minutes, and sometimes, at the height of the storm, every half minute, 

 instead of the usual hourly observations. The annexed diagram shows 

 this storm between n h 53 A. M., and I2 U 53 p. M. The vertical lines 

 denote minutes of mean solar time, and the horizontal spaces are each 5' 

 of arc. To compare the disturbance during this hour with the usual 

 diurnal variation, as represented on page 14, it must be noted that if the 

 former were represented on the same scale as the latter, it would appear 

 to be nearly twenty-five times greater than here shown. The figures 

 at the right and left margin are reckoned from a zero below the limits of 

 this diagram, and which was the normal pointing when the needle was 

 undisturbed. It will be noticed that, at n h 53 m A. M., the north end of 

 the needle had moved eastward of its normal position 2 05'. This 

 movement commenced about 10 o'clock, and went on with some irregu- 

 larity up to the time when our diagram commences. From this time the 

 eastward movement was more rapid, particularly after 1 2 M., from 1 2 h to 

 1 2 h oi m , increasing i 25'. At i2 h O7 m the greatest eastward deviation 

 was reached, which was 5 25' east of the normal pointing. After this 

 the westward motion was quite rapid, with considerable disturbance, how- 

 ever, and with two very marked and sudden fluctuations to the east, in 

 which it reached, within 30', its maximum deviation. The most violent 

 disturbances of this storm occurred within the hour here represented. 

 After I2 h 53 m p. M., the needle became gradually more quiet, and by 3 

 p. M. had returned nearly or quite to its usual position. In the evening, 

 during the remarkable display of aurora, though somewhat disturbed, 

 the fluctuations were by no means so great as during the day. We can- 

 not, of course, positively affirm that this aurora was not present during 

 the day, but it seems more than probable that the needle is more affected 

 by the approach than by the presence of an aurora, particularly of one 

 like this, extending over the whole heavens. It would of course be 

 impossible to use the needle in surveying at such a time, as by the sudden 

 changes in the direction of the magnetic force it would be kept constantly 

 oscillating. It must not be understood that during this hour the nee- 

 dle moved steadily back and forth as shown in the diagram, but it was 

 VOL. i. 23 



