TRANSACTIONS 



OF 



THE AMERICAN PHILOSOPHICAL SOCIETY. 



ARTICLE I. 



Observations to determine the Magnetic Dip at various places in Ohio and Michi- 

 gan. Bij Elias Loomis, Professor of Mathematics and Natural Philosophy 

 in Western Reserve College. In a letter to Sears C. Walker, Esq., M. A. P. S. 

 Read June 2lst, 1839. 



The instrument employed for the following observations was made by Gam- 

 bey, for Western Reserve College. The vertical circle upon which the dip is 

 read is graduated to ten minutes, which I am accustomed to divide, by esti- 

 mation, to single minutes, by the aid of two microscopes attached to the glass 

 case which covers the instrument. This circle is made of copper, and plated 

 with silver. The horizontal circle is graduated to half degrees, and reads by a 

 vernier to single minutes. The axis of the needle rests upon agate supports, 

 and is centred by two copper y's. A sensitive level is attached to the instru- 

 ment, which rests upon three adjusting screws. The two needles which 

 accompany the instrument are each of them nine inches and six tenths in 

 length. Their breadth, in the middle, is a half inch, and they terminate at 

 each extremity in a sharp point. They have, throughout, a uniform thickness 

 of about the fortieth of an inch. 



Vll. A 



