ARTICLE XII. 
Observations of the Magnetic Dip, made at several positions, chiefly on the south-western 
and north-eastern frontiers of the United States, and of the Magnetic Dechnation at 
imo positions on the River Sabine; in 1840. By Major James D. Graham, of the 
United States’ corps of Topographical Engineers. Read 16th August, 1844. 
Tur observations of the magnetic dip and declination, from the 29th of January to the 
5th of June, 1840, contained in this paper, were made while I was attached to the joint 
commission for the demarcation of that portion of the boundary, between the United 
States and Texas, included between the mouth of the river Sabine and its intersection 
with the thirty-second degree of north latitude. 
The observations for the dip, subsequent to that period, were made while I was serving 
as a commissioner, on the part of the United States, for the survey and exploration of the 
territory then in dispute with the government of Great Britain, upon our north-eastern 
frontier. . The only apparatus in possession of the Commission for the demarcation of 
the boundary between the United States and Texas, that could be applied to the deter- 
mination of the declination or horizontal variation of the magnetic needle, consisted of 
the surveyor’s compasses furnished for the survey, and the needle of a theodolite. 
Although a good variation transit would have been preferred for this purpose, yet the 
want of such an instrument was, in a great measure, compensated for, by the use of three 
different needles instead of one. ‘These were observed upon, with the poles direct, and 
then again, with the poles reversed. In every instance, the errors of the instruments 
were compensated, as far as practicable, by noting the readings of both ends of the 
needles, and by reversing the direction of the vanes of the compasses, and then making 
another set of readings in the same manner. When the theodolite needle was observed 
upon, both ends were read before, and then again, after a reversal, by a horizontal 
motion of the azimuth plate; and a mean of all the separate results was adopted, for the 
correct declination, at each station. 
The observations for the magnetic dip were made with an instrument constructed by 
Troughton & Simms, of London, in the year 1838. It was obtained for the Commission 
for the Texan boundary survey, from Messrs. William Bond & Son, of Boston, for 
