which would certainly have rendered distinctly visible a line of light one twentieth of a second in breadth. Since the linear value of 1" at Saturn's mean distance is about 4228 miles, it would

follow that the thickness is less than 210. Sir J. Herschel admits, however, that it may possibly be so great as 250 miles.

The thickness, is, therefore, certainly less than the 100th part of the extreme breadth of the two rings, and according to the scale on which the fig. 72 is drawn, it would be represented by the thickness of a leaf of the volume now before the reader.

433. Conditions under which the ring becomes invisible from the earth.-The rings of Saturn viewed from the earth may become invisible, either because the parts presented to the eye are not illuminated by the sun, or, being illuminated, have dimensions too small to subtend a sensible visual angle.

In every position assumed by the planet in its orbital motion, one side or the other of the rings is illuminated with more or less intensity, except at the Saturnian equinoxes, when, the plane of the

ring passing through the sun, its edge alone is illuminated. Owing to the extreme thinness of the plate of matter composing the rings, they cease in this case to be visible, except by feeble and uncertain indications observed with high magnifying powers. It has been inferred by Sir John Herschel, from observations made with telescopes of great power, that the major limit of their possible thickness is 250 miles. The visual angle which this thickness would subtend at the distance of Saturn in opposition, is o"064. The visual angle would, therefore, be less than the fifteenth part of a second.

The rings, therefore, disappear from this cause at Saturn's equinoxes, which occur at intervals of 14 years.

When the dark side of the rings is exposed to the earth, it is evident that the sun and earth must be on opposite sides of the plane of the rings, and therefore that plane must have such a position that its direction would pass between the sun and the earth. This can only happen within a certain limited distance of the planet's equinoxes.

The disappearance of the rings of Saturn was well witnessed at the Saturnian equinox in 1848. The northern surface of the ring had then been visible for nearly fifteen years. The motions of the planet and the earth brought the plane of the ring to that position on the 22nd of April in which, its edge being presented to the earth, it became invisible, the sun being still north of the plane. On the 3rd of September the sun, passing through the plane of the ring, illuminated its southern surface, and, the earth being on the same side, the ring was visible. On the 12th, the earth again passing through the plane of the ring, its northern surface was exposed to the observer, which was invisible, the sun being on the southern side. The ring continued thus to be invisible until the 18th of January, 1849, when, the earth once more passing through the plane of the ring, the southern surface illuminated by the sun came into view. This side of the ring continued to be exposed to both the earth and the sun until 1861-2, the epoch of the last equinox, when a like succession of appearances and disappearances took place,-the sun and earth eventually passing to the northern side, on which they will continue for a like interval.

434. Schmidt's observations and drawings of Saturn with the ring seen edgeways.-At the Saturnian equinox which took place in 1848, a series of observations was made at Bonn, the results of which have demonstrated the existence of great inequalities of surface on the rings, having the character of mountains of considerable elevation. The observations were made and published, accompanied by seventeen drawings of the appearance

of the planet, its belts, and ring, by M. Julius Schmidt, of the Bonn Observatory.

We have selected from these drawings four, which are given in Plate XX.

On the 26th of June, the planet presented an appearance, fig. 1, closely resembling that of Jupiter, except that a dark streak was seen along its equator, produced by the shadow of the ring, the earth being then a little above the common plane of the ring and the sun. A few feeble streaks, of a greyish colour, were visible on each hemisphere, which however disappeared towards the poles. A very feeble star was seen at the western extremity of the ring, which was supposed to be one of the nearer satellites. The ring exhibited the appearance of a broken line of light projecting from each side of the planet's disk.

After this day the shadow across the planet disappeared, but was again faintly seen on the 25th of July.

The ring continued to be invisible until the 3rd of September, when a very slight indication of it was seen, but on the next night it became distinctly visible with an interruption in two places, as represented in fig. 2. The bright equatorial belt was divided into two unequal parts by the ring, the northern portion being the narrower. Three small satellites were seen on the prolongation of the direction of the ring.

On the 5th, the ring was symmetrically broken on both sides,

fig. 3.

On the 7th, the western side was divided into three parts.

On the 11th, the ring and planet presented the appearance represented in fig. 4.

The broken and changing appearances of the ring on this occasion can only be explained by the admission of great inequalities of surface rendering some parts of the ring so thick as to be visible, and others so thin as to be invisible, when presented edgeways to the observer.

435. Observations of Herschel. These observations of Schmidt are corroborative of those made at a much earlier epoch by Sir W. Herschel, who discovered the existence of appearances on the surface of the rings indicating mountainous inequalities.

436. Supposed multiplicity of rings. Some observations made at Rome and elsewhere gave grounds for the conjecture, that the outer ring instead of being double, is quintuple, and that instead of having a single division, there are four. It was even affirmed with some confidence, that the ring was septuple, and consisted of seven concentric rings suspended in the same plane,

• Astronomische Nachrichten, Vol. xxviii. No. 650.