512 Adams Recent Astronomical Discovery. | Oct., 
been hitherto recognized by astronomers as the “one constant 
element,’ may be found to be no exception to the system of 
perpetual variation and fluctuation ruling elsewhere throughout the 
universe. It is not indeed precisely in this aspect that the great 
discovery we are now abcut to examine has been presented by 
its author. But this result, if, as appears probable, it should be 
established as the legitimate consequence of Adams’ discovery, 
would undoubtedly be its most important fruit, and in the words 
of the late President of the Royal Astronomical Society, would 
deserve “to be ranked amongst the most important contributions 
to astronomical physics.” 
A lunar theory having been formed and its accuracy tested by 
careful observation of the lunar motions for a considerable length of 
time, it became practicable not only to predict the moon’s motions 
and position at distant future epochs, but to determine with equal 
accuracy the motions and position of the moon at long past epochs. 
Now, in general, it would be a matter of great difficulty to ascertain 
whether these last-mentioned determinations coincided with the real 
motions of the moon in past times; for the observations of ancient 
astronomers were neither effected nor recorded with the accuracy 
requisite for such a comparison. But in eclipses of the sun and 
moon we have phenomena so marked and strikig as to be almost 
independent (so to speak) of observational inaccuracy save of the 
erossest kind. Accordingly very early in the history of gravitation, 
only six years indeed after the publication of the ‘ Principia,’ Halley 
pointed out the valuable results which would accrue from an exami- 
nation of ancient eclipses recorded by Ptolemy and by the Arabian 
astronomers. He expressed also the opinion that the result of such 
a comparison would be to show that the moon was now moving 
with a greater angular velocity round the earth than she had moved 
with at the epoch of the earliest recorded observations. Dunthorne, 
in 1749, communicated to the Royal Society a paper in which, after 
comparing the observed and computed times of several lunar and 
solar eclipses, he expressed the opinion that the acceleration may be 
expressed by 10” #, where ¢ denotes the number of centuries before 
or after the fixed epoch from which the reckoning is supposed to 
commence. Mayer, in his lunar tables published in 1753, made 
the acceleration 6”: 7 in a century ; but in 1770 he raised his estimate 
to 9”. Lalande, Bouvard, and Burg arrived at results confirming 
those deduced by Dunthorne and Mayer. 
The explanation of this peculiarity of lunar motion was not 
found to be an easy matter. Kuler and Lagrange in turn attacked 
the problem without success: the former, in 1772, expressing as 
the result of his analysis that the secular inequality is not caused 
by the forces of gravitation; the latter, in 1774, concluding that 
the best solution of the difficulty is to deny the existence of the 
