42 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
Reference was then made to the startling announcement by Otto 
Struve, in 1851, that a careful comparison of the earlier with the 
later measurements showed that during the two hundred years of 
observation the rings had been widening, and the inner edge steadily 
approaching the body of the planet.* Considering the necessarily 
vast antiquity of the Saturnian system, such a change during the 
brief interval of human existence seems @ priori almost infinitely 
improbable. The hypothesis of some that a meteoric ring has been 
drawn in by Saturn’s attraction, within comparatively recent ages, 
seems entirely negatived by the circular symmetry of the system. 
It is not surprising, therefore, that Struve’s inference has been re- 
ceived with an almost universal incredulity by the astronomical 
world. Robert Main, of the Greenwich Observatory, from a dis- 
cussion of his own measurements taken in the winter of 1852-3, 
and in 1854, disputed the accuracy of Struve’s measures ; and con- 
cluded that “no change has taken place in the system since the 
time of Huyghens.”+ And Prof. F. Kaiser, in a paper on “The 
Hypothesis of Otto Struve respecting the gradual increase of 
Saturn’s Ring,” etc., arrives at the same conclusion, and believes 
“there exists no reason whatever for supposing that the compound 
ring of Saturn is gradually increasing in breadth.” { 
There seems to be little doubt of some unintentional exaggeration 
in Struve’s tabulated results, which range from 4”.6:6”.5 for the 
ratio of ring breadth to space between ring and ball, in the time of 
Huyghens, 1657, to 7”.4:3”.7 for the ratio of breadth to space, by 
his own observation in 1851. Nevertheless it is a noteworthy fact 
that all the early drawings of Saturn made in the seventeenth cen- 
tury (many of which are figured by Huyghens in his Systema Sat- 
urnium, 1659) plainly exhibit the width of the ring as sensibly 
less than the dark space within; while all modern observers would 
agree that the bright ring is now wider than the dark space, in 
about the ratio of 8:2; or were we to take the average of the esti- 
* Recueil des Mémoires présentés [etc.] par les Astronomes de Poulkoya. 4to. 
St. Petersburg, 1853. Vol. I, pp. 349-385. ‘Sur les Dimensions des Anneaux 
de Saturne.” (Memoir read before Acad. Sci.) A brief abstract Of the memoir 
is given in the Monthly Notices, R. A. S., November 12, 1852. Vol. XIII, pp. 
22-24. \ 
+ Monthly Notices, R. A. S., December 14, 1855. ~ Vol. XVI, pp. 30-36. 
t Mem. Acad. Sci., Amsterdam, 1858. A translation of the memoir is given 
in the Monthly Notices, R. A. S., January 11, 1856. Vol. XVI, pp. 66-72. 
