ARTICLE X. 



Additional Observations of the Magnetic Dip in the United States. Bij Elias 

 Loomis, Professor of Mathematics and Natural Philosophy in Western Re- 

 serve College. Read October 14, 1839. 



The following observations were made with the same instrument, and the 

 mode of observing adopted was the same as described in a former paper. In 

 needle No. 1, 1 have continued to find the magnetic axis to coincide very nearly 

 with the geometrical. The inclination has never exceeded a few minutes. In 

 needle No. 2, on the contrary, this inclination has seldom been less than one 

 degree, and, in a few instances, has exceeded two degrees. Yet, although the 

 magnetic axis of this needle never coincides with the geometrical, it does not 

 maintain a constant position with respect to it. If the reading of the needle is 

 at one time too great, when its polarity is reversed, in the same position of the 

 instrument and needle, the reading is invariably too small, showing that the 

 magnetic pole has passed to the other side of the geometrical. In other words, 

 the magnetic axis, instead of revolving 180° from a reversal of the poles, re- 

 volves only 175° or 178°. That this anomaly is not to be ascribed to the mode 

 of magnetising employed is, I think, evident from the uniformity of the effect, 

 and from the fact that in needle No. 1, though magnetised in the same way, 

 nothing similar is observed. I can only ascribe it to some peculiarity in the 

 material or temper of the needle. In the transportation of the instrument from 

 place to place by public conveyances, the level v/as, in several instances, slightly 

 deranged. This was always verified by reversal, previous to commencing the 

 observations, and the level readjusted when necessary. It is believed that the 

 instrument has sustained no injury from transportation, and that it is suscep- 

 tible of as great accuracy now as formerly. The observations made at Hudson, 



VII. — 2 A 



