50 
Sidereal Astronomy. 
[January, 
round their common centre of gravity under the ac 5 tion of 
the same dynamic laws which govern our solar system.” 
This, then, is a great faCt, of which in all probability 
Newton himself had never thought. 
The total number of the systems observed by W. Herschel 
is 812. His general catalogue was re-edited in 1867, by 
his son. 
These researches were followed, a short time after the 
death of William Herschel, by observations made by his 
son, Sir John Herschel, and by his friend James South. 
They observed together in England, and afterwards sepa- 
rately, James South having come to Paris, and having 
there fixed his observatory. The first catalogue of these 
two astronomers contained 360 stars ; the second, of South 
alone, contained 838. These observations were published 
in the “ Philosophical Transactions ” for the year 1826. 
William Struve afterwards undertook, in Russia, his 
long and laborious revision of the heavens, for the verifica- 
tion and discovery of all the double stars. His work is 
composed principally of three folio volumes, written in 
Latin : — 
1. “ Catalogus Stellarum Duplicium et Multiplicium 
Dorpati DedeCtarum. Anno 1827 editus.” 
2. “ Mensurae Micrometricae Stellarum Duplicium. Anno 
1837 editae, cui accedit additamentum,” 
3. “ Positiones Mediae Stellarum Fixarum Duplicium et 
Multiplicium. St. Petersbourg, 1852.” 
In making this general review of the heavens, in carrying 
on his investigations with the stars of the first eight magni- 
tudes, and even on the most brilliant of the ninth, which 
are comprised between the North Pole and 15 0 south of the 
Equator, William Struve observed and catalogued (in- 
cluding Herschel’s stars) 3057 double stars. 
The celebrated Russian astronomer divided these stars 
into eight classes, in the order of angular distance, from 
o" to 32". This division is, without doubt, very useful, but 
it has a defective side, which will cause it to be inevitably 
abandoned. All classification ought to be made on an inva- 
riable base. Nothing is more variable than the angular 
distance of the components of a double star. Sometimes 
for the same binary system this distance is less than a 
second, sometimes it gets beyond a second, sometimes 
again it gets beyond two or more. Also a star placed in 
one year in one class will be found in another year 
belonging to another class. It will be necessary to found 
this arrangement on mean distances ; but these distances 
