OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 331 
and were always sent in a direction east or west (magnetic) 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 exhi- 
bited, after the accident by lightning, to needle No. 2, described hereafter, will serve 
as a striking illustration of the importance of the several reversals, of the face of the 
instrument, of the face of the needles, by turning their axes end for end, and of the 
poles. Previous to that accident, the indications by needle No. 2, were quite as uniform 
throughout its various positions, as those by needle No. 1. After the accident, needle 
No. 2 always exhibited a variation in its extreme indications, arising from change of 
position, amounting to ten, and, sometimes, to nearly twelve 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 reversals 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. In order to show the characters of the two needles in the various positions necessary 
for a course of observations, both before and after the accident to No. 2, all the readings 
are given for two sets of observations by each needle, both prior to and subsequent to 
that period; namely, for the stations at the light-house of the south-west pass of the 
Mississippi, and the mouth of the River Sabine, where observations were made before 
that accident, and for Gaines’ Ferry, in Texas, and West Point, New York, where 
observations were made after the accident. It was at this last-mentioned position that 
needle No. 2 was observed upon, for the last time, on the 24th of August, 1840. 
Its place was afterwards substituted by two other needles, marked No. 3 and 4. They 
were made expressly for me, by Mr. J. N. Baur, of New York, and are of the acute 
lozenge shape, very similar, in form, to No. 2. 
In order to show the characters of the two new needles, it has been thought important 
to give a full set of observations by each of them. These are exhibited in the first 
observations that were made with them, immediately after they were magnetized ; 
namely, for the station 4578 feet directly north from the monument erected to designate 
the source of the river St. Croix, under the fifth article of the treaty of 1794. They will 
be found under the dates of October 18th and 20th, 1840. 
The needles were always charged as strongly as practicable, and generally to satura- 
tion, both when the poles were direct and also when reversed. This is an important 
object in observing the dip, for the stronger the needles are charged with magnetism, the 
less is the effect of any want of perfect balance, or any imperceptible dust or other parti- 
cles that may adhere to them, to deflect them from the true angle of dip. 
The localities occupied by the instrument, at the several stations, have been minutely 
described, in order that they may be easily found by future observers, and that an exact 
comparison of results may be made, and the change of dip, with the lapse of time, be 
the more accurately ascertained. 
J. D. GRAHAM, Maj. Top. Engineers, 
VOL. IX.-——-87 
