a Sky ra 
162 S Newcomb— Variability of the Earth's axial rotation. 
of the invariability of the sidereal day. In the paper referred 
to it was remarked that observations of the interior planets, 
occurred to me that eclipses of the first satellite of Jupiter 
might afford a yet better and more decisive test. he definitive 
ence. 
There is, however, one circumstance which rendered the sat- 
isfactory application of the test very easy. Granting that the | 
inequalities were really to be accounted for by changes in the 
earth’s rotation, the most extraordinary and sudden change of 
which we have knowledge occurred about 1860. The velocity 
of rotation, which for the ten or twenty years previous had been 
rather slower than the average, was then suddenly accelerated, 
so as to cause a subsequent gain of perhaps a second per annum, 
which continued at least till 1872. Collecting all the accessible 
about half as great as that indicated by the moon, and no 
reater than its possible error; so that the result did not in any 
Year. s. Year. os Year, 8. 
1850°5 + 2 1862°5 +11 1868-5 + 2 
1855°5 + 5 1864°5 +10 1870°5 0 
1860°5 +10 1866°5 +6 1872°5 — 2 
