GENERAL MEETING. 43 
mates of the last century, it would probably not vary far from 
5”.25:5”.75; while the general average for the present century 
would probably be about 6”.5:4”.5. There seems, therefore, to be 
a real difference, not accounted for by inferiority of earlier instru- 
ments and estimates, nor by the existing uncertainties of modern 
measurements. The question will probably be definitely settled in 
less than a century. Meanwhile there is a need of some explana- 
tion of the apparently systematic and progressive divergence first 
pointed out by Struve; and we naturally ask, What indications 
are afforded by theory ? 
The elder Herschel, in 1789, (at the Saturnian equinox, when the 
edge of the ring was presented to view,) from supposed observation 
of protuberances moving on the line, believed that he had detected 
a rotation, whose period he estimated at 10h. 32m. 15s., for the 
outer edge of the ring.* The correctness of this interpretation was 
controverted by Schroeter, from observations at Lilienthal, on the 
next passage of Saturn’s equatorial node in 1803; as it was after- 
ward questioned by Prof. G. P. Bond, of Harvard Observatory, 
from observations in 1848.+ It is scarcely doubtful that Herschel’s 
period was derived from an entire misconception of the nature of 
the ring—which he firmly held to be solid—and that it possesses no 
scientific value whatever. A. Secchi, from certain recurrent irreg- 
ularities of phase observed at Rome in 1854, 1855, and 1856, in- 
ferred a rotation period of 14h. 23m. This is doubtless a nearer 
approximation (for the outer edge of the ring) than Herschel’s es- 
timate. It is not probable, however, that the period of any portion 
of the ring will be determined by observation. 
Accepting the meteoric theory of the rings as now established, 
we may by Kepler’s law compute with confidence the period of 
rotation of any part of the ring; and we thus find— 
From the period of the inner Satellite (AZ@mas)_._. 22h. 373m.— 
The period/of ‘otter edge ofiring -L 2 JL kee sisi} obs | eet) bene 
os dividing. stripe js s oe eee eer eDED. 20), 
ge inner edge of bright ring _-_-_-___ Tog sk 20 
oe inner edge of dusky ring _--_-_--.-- 5h. 45 m. 
Mean period of ring (supposed solid) about _--_-_ Ioh. 50 m. 
‘The period of the planet Saturn is . > ‘ 10h. 14m. 
* Phil. Trans. Roy. Soc. 1790: Vol. LXXX, p. 479; and 1792: Vol. 
LXXXII, p. 6. 
j Gould’s Astronomical Journal. 1850. Vol. I, pp. 20, 21. 
