Saturn and Its System

to make as complete as possible. The explanations of astronomical terms extend only to terms actually used in the pre

sent work.

In the body of the work I have adopted for the sun's equatorial horizontal solar parallax (the earth at her mean distance) Professor Hansen's determination, namely, 8"-9159. More than ten years have elapsed since M. Hansen first pointed out that the solar parallax (8"-5776) deduced by Encke from the transits of Venus in 1761 and 1769, requires to be increased, to correspond with the observed extent of the moon's parallactic inequality. The above value, M Hansen's later calculation, corresponds very closely with the results obtained by Leverrier and others. In the tables of Appendix II., two values are given (corresponding to the above values of the solar parallax) of every element whose determination depends upon the sun's distance.

I have endeavoured to make the engravings represent as accurately as possible what they are meant to illustrate. This is to be understood of all the figures throughout the work, unless it is expressly stated in the text that general principles only are illustrated (as in the first six figures of Plate X.); or that a part of any figure is purposely exaggerated (as the rings in fig. 1, Plate VIII.)

In the figures of Plate I., the outlines of the planet and rings, and of their shadows, have been determined from calculations founded on the dimensions of the planet and rings adopted in Tables III. and IV.; such details have then been introduced as have been noted by the best observers. The slope of the figures is, to a certain extent, a matter of indifference, since it varies with the hour of observation; but to preserve uniformity I have adopted the following rule :The horizontal line through the centre of the disc in the

figures of Plates I., VII., IX., and XIII. represents the planet's heliocentric path, the direction of the planet's motion being from left to right in the figures of Plate I. (which are supposed to be seen through an inverting telescope), and from right to left in all the other figures.

The star-maps in Plates II. and III. are on the gnomonic projection.* The stars in Plate III. have been taken (with a correction for precession) from the maps of the Society for the Diffusion of Useful Knowledge. The positions of the stars in Plate II. have been corrected for the precession of the equinoxes, but not for the proper motions, since we have no means of learning whether the proper motions of the stars are constant for long periods. The path of Saturn in Plate III. has been taken from the 'Nautical Almanacs' for the years 1859-1866; the path in Plate II. has been calculated for an undisturbed elliptic orbit, corrected from Saturn's present orbit for variations in the position of the nodal line and perihelion: thus the minor irregularities in the path on Plate III. (chiefly due to the disturbing attractions of Jupiter) are wanting in Plate II. The figures of the constellations have been slightly altered from the Society's

maps.

* The gnomonic projection seems the natural mode of projection for star-maps, since the eye of an observer viewing the celestial sphere actually occupies the 'point of sight' of the projection-that is the centre of the sphere. The object to be sought in all star-maps, is that the greatest portion possible of the celestial sphere should be visible at a single view with as little distortion as possible. In the Society's maps, the celestial sphere is projected from the centre upon the circumscribing cube; thus forming six maps considerably distorted near the angles. I am preparing a series of star-maps on a plan which appears to offer greater advantages. The celestial sphere is projected from the centre upon the circumscribing dodecahedron; thus forming twelve pentagonal maps much less distorted near the angles than the Society's maps: and further, by presenting the six maps of each hemisphere in a single plate in their proper relative positions—that is, as five pentagons upon the five sides of the polar pentagonal map-the relative positions of the northern or of the southern constellations are seen at a glance. A slight addition to the outer maps brings the whole length of the equator into each sextuple map.

The orbit of Nysa in fig. 3, Plate VI., has been derived from the elements given in Nichol's 'Cyclopædia of the Physical Sciences,' and Mitchel's 'Popular Astronomy' (both published in 1860). I think it little probable that these elements are even approximately correct. The direction of the planetary motions is indicated in this figure, in fig. 1, Plate VIII., and in fig. 7, Plate X., by the Zodiacal signs outside the orbit of Saturn; that is, the motions are supposed in these figures to take place in a direction contrary to that in which the hands of a watch move.

The dimensions of the satellites Mimas, Enceladus, and Hyperion, in fig. 1, Plate VII., are slightly exaggerated. These satellites would scarcely be visible on the scale of that figure.

The figures of Plate XIV. are derived from woodcuts in Layard's Nineveh and Babylon' and Nineveh and its Remains.'

Collingwood Villas, Stoke, Devon:

May, 1865.

CONTENTS.

CHAPTER I.

DISCOVERY OF SATURN-THE SIMPLER ELEMENTS OF HIS ORBIT.

From the Interval in which he returns to the apparent Place he had occupied

when in Opposition

From his Synodical Period

From the Interval in which he apparently completes the Circle of the Zodiac
Saturn's Distance determined from his Sidereal Period and his Motion between
his Stations