THE CENTENARY OF THE DISCOVERY OF URANUS. 
217 
Some observations made in 1872-3 witb the great six-foot 
reflector of Lord Rosse, may here be briefly referred to. A 
number of measures, both of position and distance, of Oberon 
and Titania , were made,* and a few of Umbriel and Ariel , but 
‘the shortness of the time available (40 minutes) each night 
for the observation of the planet with the six-foot instrument, 
the atmospheric disturbance, so often a source of annoyance in 
using so large an aperture, and other unfavourable circum- 
stances, tended to affect the value of the observations, and to 
make the two inner satellites rarely within detection/ On 
Feb. 10, 1872, Lord Rosse notes that all four satellites were 
seen on the same side of the planet. On Jan. 16, 1873, when 
definition was good, no traces of any markings were seen. 
Diameter of Uranus = 5 // *29. Power 414 was usually employed, 
though at times the inner satellites could be more satisfactorily 
seen with 625. 
It may be mentioned, as an interesting point, that some fifty 
years after the first discovery of Uranus by Herschel, it was 
accidentally re-discovered by his son, Sir John Herschel, who 
recognized it by its disk, and had no idea as to the identity of 
the object, until an ephemeris was referred to. Sir John men- 
tions the fact as follows, in a letter to Admiral Smyth, written 
in 1830, August 8: ‘I have just completed two 20-feet 
reflectors, and have got some interesting observations of the 
satellites of Uranus. The first sweep I made with my new 
mirror I re-discovered this planet by its disk, having blundered 
upon it by the merest accident for 19 Capricorni’ 
In commenting upon the centenary of an important scien- 
tific discovery we are naturally attracted to inquire what progress 
has been made in the same field during the comparatively short 
interval of one hundred years which has elapsed since it occurred. 
We have called it a short interval because it cannot be considered 
otherwise from an astronomical or geological point of view, 
though, as far as human life is concerned, it can only be regarded 
as a very lengthy period, including several generations within 
its limits. Since Herschel in 1781 discovered Uranus, Astronomy 
has progressed with great rapidity, so that it would be impos- 
sible to enumerate in a brief memoir the many additional dis- 
coveries which have resulted from assiduous observation. A 
century ago only five planets were known (excluding the 
Earth), now we are acquainted with about 230 of these bodies ; 
and one of these, found in 1846, is a large planet, whose orbit 
lies exterior to that of Uranus. In fact, the state of astro- 
nomical knowledge a century ago has undergone wonderful 
changes. It has been rendered far more complete and compre- 
hensive by the diligence of its adherents and by the unweary- 
* Monthly Notices R. A. S., March, 1875. 
