exhibit beautiful octahedral markings so delicate as to be invisible to the naked eye, and somewhat like those of the Tazewell, Claiborne Co., meteorite, though not more than one-tenth the thickness.

The following analysis was kindly furnished by Mr. J. Edward Whitfield, of the United States Geological Survey, through the courtesy of Prof. F. W. Clarke.

It most closely resembles the Tazewell, Claiborne and Bear Creek, Col., meteorites in composition. I herewith take pleasure in thanking Mr. Norman Spang for his kindness in allowing me to secure the iron and facts of finding, Mr. J. Edward Whitfield and Prof. F. W. Clarke for the analysis.

2. Meteoric Iron from Laramie County, Wyoming. The Laramie County mass of meteoric iront was found by Mr. Edward J. Sweet, in the latter part of January, 1887, while he was prospecting in the Silver Crown District, almost in the center of town 14, range 70, between the middle and south fork of Crow Creek, Laramie County, Wyoming Territory, about 21 miles west of Cheyenne, in long. 105° 20' west of Greenwich and north lat. 41° 10'. When found it was half buried in decomposed granite and earth. After being a ten days' wonder among the miners at the camp it was sent to Dr. Wilbur C. Knight, of Cheyenne, Wyoming, through whom it came into my possession.

In shape this mass somewhat resembles an anvil (see fig. 3, two-thirds natural size). It weighs 25.61 lbs., 363 oz. Troy (11.616 kilos.); and is 17.5cm high, 14cm thick at the center and 19cm at the widest point. The entire surface is still covered with the original crust of magnetic oxide of iron, which has been slightly acted upon by the atmospheric agencies. No trace of chloride of iron was perceived. There is no exudation although it has been in my possession some months. The * Original Researches, 1884, p. 439. †This Journal, II, vol. xix, p. 153.

+ Exhibited and described at the New York Academy of Sciences, Dec. 5, 1887.

surface is irregularly pitted, the largest of the pits being 3cm by 2cm and very deep for their size. No troilite was observed either in the cutting or in the pitting. This iron is one of the Braunite group of Meunier.

When etched with dilute nitric acid this iron does not show the Widmanstätten figures, but under the glass the markings are seen to be similar to the Braunau Hauptmanndorff iron described by Tschermak* and Huntington,† (see figure 4). This beautiful structure is broken only by the thin layers of schreibersite, which divide a surface 25mm square into over twentyfive irregular crystalline parts.

The specific gravity is 7.630. The following analysis was kindly made by Mr. Howard L. McIlvain:

It approaches more closely to the Rowton, Charlotte and Jewel Hill meteorites in composition.

I take pleasure in thanking Dr. Wilbur C. Knight for assistance in securing the iron, and Mr. Howard L. McIlvain for the analysis.

ART. XXX.-Experiments on the Effect of Magnetic Force on the Equipotential Lines of an Electric Current; by E. H. HALL, Assistant Professor of Physics in Harvard College. [Continued from the August number of this Journal, p. 146.]

THE next to the last column in this table (August number) shows that the intensity of magnetic induction in the thin cross remains throughout very nearly the same as that of the magnetizing field, but the intensity of magnetic induction in the thick cross, about 14 times as great as that of the magnetizing field when this is weakest, falls to a value about 1 times as great as that of the magnetizing field when this is strongest. The intensity thus attained is about 22000 c. g. s. units, a notably high one.T

* Sitzungsber. Akad. Wiss., Wien, lxx, Abth. i., p. 449.

Proc. Amer. Acad. Arts and Sciences, May 12, 1886, p. 478.

Trans. Royal Soc., Feb. 9, 1882. § This Journal, III, vol. x, p. 349.

Ibid., II, xxxix, p. 24.

There may be a constant error of several per cent in the values of magnetic induction given in this article, as the area of the little test-coil used between the poles of the magnet is difficult to measure accurately.

It remains to give an account of experiments with other metals in which the effect of shape of cross-section has been in some measure tested. With these experiments will be described certain others closely related to them.

Cobalt.-Wide and narrow: Two crosses of cobalt were cut from a rolled bar of this metal furnished by the courtesy of Mr. Joseph Wharton of Philadelphia. In each cross a line which had been transverse to the original bar became the direction of the main current. The approximate dimensions were as follows:

These crosses were compared in a magnetic field of intensity about 10700. The visible effect was large and the test was easily made. The result was

R. P. of No. 2
R. P. of No. 1

1.12.

The question arose whether any large part of the superiority shown by the narrow cross could be attributed to a greater heating of this cross by the direct current. Experiment convinced me that this was not the case.

The R. P. of No. 1 appeared from a rough test to be about times as great as that of a very thin cross of cobalt taken from the same bar, and to be described hereafter. This discrepancy is hardly significant in view of the roughness of the comparison and the uncertainty as to the thickness of the thin

nest cross.

Thin cross; field of varying strength: A part of the cobalt bar already mentioned was heated and hammered to reduce its thickness. The reducing process was continued by grinding, etc., until finally a cross was obtained, the dimensions of which were approximately as follows:

The value given for the thickness is a rough estimation of the average thickness between the arms. It may be wrong by 10 or 15 per cent in either excess or defect.

This piece of metal was, like most others that I have used, imbedded in a cement of bees-wax and rosin on a glass plate. It was placed between the poles of the magnet in a narrow tank through which water flowed to control the temperature of the metal. The character and results of the examination are set forth in the following table, where M is the intensity of the magnetic field in absolute units:

The main current through the cross was always about 0·1 absolute unit.

The two determinations of M on Aug. 7 were not entirely independent, they being made from three sets of observations, of which the second set was used with the first to give the first value of M and again with the third to give the second value of M. The one test at a low temperature, 2.9°, was probably less reliable than the others.

The fall of R. P. with fall of temperature appears to be about per cent per 1° C.

The increase in the R. P. column in fields ranging from 793 to 5243 is perhaps purely accidental. The small but decided fall in going from the field 5243 to the field 9186 undoubtedly shows a true decline. The significance of this decline should not be overlooked. The intensity of magnetic induction through this very thin piece of metal must in all cases have been almost exactly as great as that of the magnetizing field. Hence the decline in the R. P. as the intensity of the field increases means a failure of the transverse effect in the cobalt to keep pace with the intensity of magnetic induction through the

same.

In this Journal for February, 1885, I gave as the R. P. of cobalt 2460×10-6,* with a fall of nearly 1 per cent for a fall of 1° C. The specimen that gave these values was very different in character from that used in the experiments of this article. It was cast cobalt and was quite brittle. It probably contained little, if any, iron, but was known to contain nickel. The specimen used in these later experiments is known to contain some iron and some carbon. I do not know whether it contains nickel. It is harder than most kinds of iron, but not so hard as tempered steel. It is not brittle. Treatment which would temper or anneal steel produced no marked effect upon it.

The impurities of the first specimen probably diminished its R. P. It is not known what effect the iron and carbon

* This is the corrected value. See explanation in this Journal for August, 1888.

have in the second. In view of the great difference between the R. P. of soft iron and that of tempered steel, the latter being, in some cases at least, about four times as great as the former, the difference in the behavior of these two specimens of cobalt should not, perhaps, excite surprise.

An experiment was made to determine how much of the effect produced upon the equipotential lines in this more recently used cross by a magnetic field of about 9000 would survive the removal of the cross from the field. It appeared that this permanent effect was rather less than 1 per cent of the temporary effect and in the same direction.

Nickel.-Thin Cross; field of varying strength: The specimen of nickel that gave the quantitative results relating to this metal which I have published in previous papers, was a piece of commercial nickel plating stripped from the surface upon which it had been electrolytically deposited. I had now at hand a thin sheet of rolled nickel quite different in character from this specimen, much softer and more pliable. Little is known concerning the purity of the first specimen. second, which like the cobalt bar already mentioned was given me by Mr. Wharton, is probably much the purer of the two, although it shows a trace of iron. The thickness is about 0.3mm From this sheet a cross was made, of which the dimensions were approximately as follows:

The

The direct current through the cross was about 0·1 absolute unit.

It appears that the R. P. of this specimen of nickel is about two-thirds or three-fourths as great as that of the piece previously used, but the uncertainty which exists regarding the thickness of each prevents any accurate conclusion upon this point. The diminution of R. P. when M increases from moderate values to higher values is very marked in each. It is somewhat greater in the later specimen than in the earlier, in which there was, according to the experiments of 1881, a fall from about 12500 with M=3600 to about 8400 with M-9100. This diminution, like that noted with the thin cobalt and in certain cases with iron, indicates a failure of the transverse