206 



dip circle, is 8 inches in diameter, divided on a silver-washed surface, 

 to read to 15 nainutes of arc. But, by the aid of a magnifying lens, 

 the readings may easily be estimated to the nearest two minutes. 



In making the observations, the following method was always pur- 

 sued, viz: Both ends of the needle were read with the face of the dip 

 circle to the east, and also to the west, and with the face or marked 

 side of the needle twice to the east and twice to the west ; that is, once 

 each way, by a change of direction of the face of the instrument, and 

 once each way again, by a reversal of the needle on its axis. A 

 mean of all the readings was then taken and noted. The poles of the 

 needle were then changed, and the same course repeated, and a mean 

 of all the readings noted. A mean of these two results was taken for 

 the dip indicated by the needle. The same operation was then per- 

 formed with the other needle, and a mean of the final results of the 

 two needles was adopted as the magnetic dip for the station. 



Care was always taken, in selecting positions for observations, both 

 for the dip and the declination, to have them beyond the reach of in- 

 fluence upon the needles arising from the iron in buildings and from 

 any other local cause. The magnets, all iron or steel belonging to 

 the camp, were always kept remote from the instruments during the 

 observations, and were always sent in a direction east or west (mag- 

 netic) of the instrument, in order thus to neutralize all influence upon 

 the needles. 



The close coincidence in the results of the two dipping needles, 

 which was still exhibited after the accident by lightning to needle 

 No. 2, described in the memoir, will serve as a striking illustration of 

 the importance of the several reversals of the face of the instrument, 

 of the faces of the needle, by turning their axes end for end, and of 

 the poles. Previous to that accident, the indications of needle No. 2, 

 were quite as uniform throughout its various positions as those of 

 needle No. 1. After the accident, needle No. 2, always exhibited a 

 variation in its extreme indications, arising from change of position, 

 amounting to 10 and sometimes to nearly 12 degrees. Still the mean 

 of all the observations made in the manner described seems to have 

 agreed as well, or nearly as well, as before the accident, with the dip 

 indicated by needle No. 1, which always remained in good order. 



The absolute error of needle No. 2, seems to have been constant, 

 or very nearly so, but by the several i-eversals of position, and of the 

 poles, this error was rendered as often positive in its character, as it 

 was negative, and was thus neutralized, or very nearly so. 



Needle No 8 was observed upon for the last time, at West Point, 



