OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 339 
i p [Mean solartime|North end of Needle|South end of Needle] Mean of both ends tached Wesne 
Needle. Morked ond iametsek m3 ntvedis| “of observation. reads SEE gic of needle. Siearencinati: ss a 
of Needle, H. M, Deg.|Mins.gTenths, Deg.|Mins.§*Tenths- |Dee- ‘|Mins. .Tenths. (Fahrenheit.) |Observer. 
1 s E E |4,45r.0./60.35.0 [60.450  |60. 40.0 J.D.@. 
: a WwW WwW 5 . 40 60 . 05.0 60 . 00.0 . 02.5 
te ide WwW E 6. 20 61 . 28.0 61 . 23.0 61 . 26.5 
ze My E W 6.23 61 . 50.0 62 . 00.0 OL. 80.0 
Needle No. 1, with marked end north, Bives 6 6 et ee ee + 4 BLO 021.5 
Needle No. 1, with marked end south, gives AGs garinaiweta 61 00.75 
Magnetic dip, by sixteen observations on needle No. 1, em 
A very light south-west wind; weather clear, and the atmosphere very dry, all day. 
May 29th, at Gaines’s Ferry. 
2 Ss 1D f= | 6. 30P.m.[65 . 15.0 55 . 30.0 65.235 ane 
“ «“ WwW W |6.35 |65.10.0 65 . 00.0 65 . 05.0 
w | E |6.40 (55.950  |65.18.0 —[85.. 26.0 
“ “ E W |6.47 [64.520 — (65. 15.0 65. 03.5 j 
Qe 1A, N W | W |%. 05a.m|56.40.0 [56 . 50.0 56. 45.0 
“ «“ E E |7.12 |65. 55.0 65 . 35.0 65 . 45.0 
“ “ E W |7.20 /56. 50.0 56 . 40.0 56 . 45.0 
« “ Ww E |7.25 |66. 00.0 66.25.0 (66. 12.6 
Needle No. 2, gives, with marked end north, gC ubh us da ke a ee 
With marked end south, es ee ee eee y 
Dip, by sixteen observations on needle No. 2, | ty os a hee, OO ee 
Dip, by sixteen observations on needle No.1, . . « . ww as 15, Sh. 01'S 
* Tt will be remarked that the difference between the readings of needle No. 2 with the marked side east, and 
those which it gives with the marked side west, is very great, amounting to an average of nearly ten degrees in 
the last observations made upon it. ‘The needle has assumed this character since I parted with it at our camp 
near the mouth of the river Sabine, on the 17th of March last. 
Lieutenant Lee, who has had charge of the instrument since that time until my arrival at Gaines’s Ferry, on 
the 2'7th of May, states, that while observing upon it at Gaines’s Ferry, in the early part of April last, a violent 
storm, accompanied with thunder and lightning, interrupted his observations. 
He left the instrument in the observing tent with this needle suspended, as is usual, for observation, and 
retired to his own teft until the storm should subside. During his absence from it, a very severe shock of light- 
ning occurred, which was sensibly felt by several persons in the neighbouring tents. After the storm was over, 
he returned to the instrument, and found that the needle had been thrown from its position by the shock of light- 
ning, although the tent and the other parts of the instrument remained apparently uninjured. 
Ever after this, the needle showed similar discrepant results in its direct and reversed positions on its axis, 
although the mean result by it did not appear to differ much more than formerly from that shown by needle No. 1. 
Needle No. 2 had, after the accident, evidently a warp in it, which had not before been perceived. This may 
have arisen from the effect of the lightning upon the fibres of the steel. ‘The axis may also have possibly been 
thrown somewhat out of its former adjustment. I have thought that under all circumstances, it would be just to 
give, in the last observations and always hereafter, twice the weight to the results of needle No. 1 which are 
given to those of needle No. 2, and in this way to state the dip as deduced from both needles. 
Accordingly, by the last observations,— 
The result by needle No. 1, = 61° 01'.6 % 2 = 122° 08.2 
The result by needle No. 2, = 60° 477.9 x 1 = 60° 47'.9 
The sum, = 182° 51'.1 
which, divided by 3, gives 60° 57’ for the dip deduced for Gaines’s Ferry, on the 28th and 29th of May, 1840. 
VOL, IX.-—89 
