THE AMERICAN JOURNAL OF SCIENCE AND ARTS. [SECOND SERIES.] ART. XXI.-Results of Magnetical Observations made at Eastport, Maine, between 1860 and 1864, for the United States Coast Survey; communicated by A. D. BACHE, Supdt., under authority from the Treasury Department. THESE observations were made in connexion with the general system of magnetical determinations on the coast, and with the special object of ascertaining the law of the secular change in the easternmost coast region of the United States. At the Eastport station, selected for that purpose, the plan of work required the observation of the magnetic declination, dip and horizontal intensity, during three days near the middle of each month. It is worthy of remark, as a result of these observations, continued for over four years, that the most important magnetic features of a locality may be developed by a system of observations requiring comparatively but a small sacrifice of time, such as is in the power of many persons to make, who are engaged in other pursuits. The instruments used were a theodolite magnetometer and a dip circle, both constructed by Mr. Wm. Wurdemann, according to designs by Mr. J. E. Hilgard. The principal novelty in the theodolite magnetometer, besides many details affording greater convenience in making the adjustments than heretofore had, consists in having the collimator magnets so light as to be readily supported by a single silk fiber, the torsion resistance of which is extremely small, and susceptible of a stable adjustment. The magnets are 3 inches AM. JOUR. SCI.-SECOND SERIES, VOL. XLII, No. 125.-SEPT., 1866. |