2.-REPORT OF LIEUTENANT THOMAS N. BAILEY, CORPS OF ENGINEERS.

WABASHA, MINN., August 14, 1876. MAJOR: I have the honor to submit the following report of magnetic observations, made under your direction by me at Detroit, Mich., and Wabasha, Minn.

The instruments used were Würdemann theodolite-magnetometer No. 3, Barrow dip-circle No. 26, Gambey sextant No. 1, and Bond mean time chronometer No. 201. The magnets employed were new ones, sufficiently light to admit of suspension, without risk of breakage, by a single fiber of unspun silk.

The values of one division of the magnets were determined by observations upon different parts of the scales. No irregularities of divisions were detected. The temperature co-efficient of the long magnet was deduced from deflections at about 32° and 100° F.

The value of the co-efficient P depending upon the distribution of magnetism was found so small that it was neglected.

The moment of inertia of the long magnet and stirrup was obtained from loaded and unloaded oscillations; the load being a brass ring.

The instrumental corrections of the dip circle were neglected because of their small value.

Previous to selecting a station, observations were made to detect local attraction, bearings being taken with the magnetometer from the ends of two radii at right angles to each other and from 300 to 600 feet long.

At each station two results for azimuth were obtained, and two results for declination, each of the latter being the mean of the morning and afternoon elongations; torsion was carefully watched; in each case the torsion-bar was left suspended overnight before observing. As a check upon large errors, declination was also observed with a compass-needle.

At least two independent determinations of horizontal force were made: by oscillations and right-angled deflections at two distances.

Dip was observed on two days. Each time both needles were used with their poles direct and reverse; there being generally 160 readings with microscopes for each day's determination.

Care was taken during observations to remove to a safe distance all objects which might affect the magnets.

The approximate latitude of the station at Wabasha, by circum-meridian altitudes of the sun, is 44° 18'.

Descriptious of stations and other details will be found in my office-report and note

books.

It was intended that other points should be occupied during the summer, but while at Wabasha I received orders changing my station to West Point, N. Y. Tables of results are appended.