Page:Proceedings of the Royal Society of London Vol 1.djvu/224

Rh largest; the equatorial diameter the next; and the polar diameter the smallest.

June 1.—Two measures of the latitude of the greatest curvature were taken, by setting the ﬁxed thread of the micrometer to the direction of the ring. The mean of these was 43° 20′.

June 2nd.—The two planets were viewed alternately, with powers of 300, of 200, and of 160; and even with the lowest of these, the difference in the ﬁgure of the planets was very evident.

The telescopic appearance of Saturn was then compared with a figure drawn from the measures Dr. Herschel had taken, combined with the proportion between the equatorial and polar diameters determined in the year 1789. From these a corrected figure was formed, of which an exact copy is given. The dimensions of it, in proportional parts, are,—

These observations, Dr. Herschel thinks, may tend, in some measure, to ascertain the quantity of matter in the ring and its solidity; they also afford a new instance of the effect of gravitation on the figure of planets; for in the present case the opposite influence of two centripetal, and two centrifugal forces, must be considered.

Mr. Lane having observed that hardened iron is not so readily attracted by the magnet as soft iron, was proceeding to make some experiments on the subject, when he was led, by Mr. Hatchett's paper on Magnetical Pyrites, &c., to examine what magnetical properties iron possessed when free from inflammable matter. For this purpose he procured some of the precipitate sold at Apothecaries' Hall under the name of Ferrum precipitatum, and which is prepared by adding purified kali to a solution of sulphate of iron. This precipitate, the author says, has no magnetic particles; nor, when exposed to a clear red heat, does it acquire any, except when smoke or flame have access to it. The solar heat, when concentrated to the degree at which glass melts, does not render this oxide magnetic, provided it be protected by glass from the dust floating in the air; if not so protected, many of the particles become magnetic.

Mr. Lane then rubbed various portions of the oxide, in a glass mortar, with different combustible substances, namely, coal, sulphur, charcoal, camphor, ether, alcohol, &c., but found the oxide was not thereby rendered magnetic, without the assistance of a degree of heat equal to that of melting lead; with that degree, however, it became magnetic. Hydrogen, when aided by a red heat, had the same effect. Charcoal and cinders, well burnt, were found to require a longer continuance of the heat, to have their full effect on