205 



The Committee on Major Jas. D. Graham's paper, entitled 

 " Observations for the Magnetic Dip, made at several positions, 

 chiefly on the South-Western and North-Eastern frontiers of 

 the United States; and of the Magnetic Declination at two 

 positions on the river Sabine, in 1840, by Major Jas. D. Gra- 

 ham," reported an abstract made by the author, and recom- 

 mended its publication in the Transactions, which was ordered 

 accordingly. 



The observations of the magnetic dip and declination, from the 

 29th of January to the 5th of June, contained in this paper, were 

 made while Major Graham 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 32° of north latitude. The observations for 

 the dip, subsequent to that period, were made while he 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 demar- 

 cation of the boundary line between the United States and Texas, 

 that could be applied to the determination of the decHnation, consisted 

 of the surveyor's compasses furnished for the survey, and the needle 

 of a theodoHte. 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 on with the poles direct and 

 then again with the poles reversed. In every instance the errors of 

 the instrument 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 read- 

 ings 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 sepa- 

 rate results was adopted for the correct declination at each station. 



The observations for the magnetic dip were made with an instru- 

 ment constructed by Troughton and Simms, of London, in the year 

 1838. It was obtained for the commission for the Texan boundary 

 survey, from Messrs. William Bond and Son, of Boston, for whom it 

 was made. The whole instrument is of brass. The azimuth circle 

 is divided by the aid of a vernier to read to minutes. The vertical or 



VOL. IV. 2 D 



