SCIENTIFIC SUMMARY. 
359 
indicated by the observations, namely, 10"‘3. The agreement was so close, 
that Laplace did not consider it necessary to push his calculations any 
further. Subsequently other astronomers did push the calculation further, 
taking into account a number of small factors omitted by Laplace, and found 
10"’63 for the theoretical value, and 10"*8 for the observed value. In the 
year 1853, a remarkable paper was communicated to the Royal Society by 
Prof. J. C. Adams, and in this memoir it was clearly shown that these later 
astronomers had taken into account only a portion of the terms omitted by 
Laplace, and had entirely overlooked a most important class of these ad- 
ditional terms. Taking all these into consideration, Prof. Adams showed 
that the real value of this acceleration produced by the variation in the 
eccentricity of the terrestrial orbit, was only 6"'20. For some time Prof. 
Adams’ paper did not excite the attention it deserved ; but during the period 
1858 to 1863, Prof. Adams’ investigation was warmly discussed by all the 
leading masters of the lunar theory. In the end, the correctness of Prof. 
Adams’ results were unanimously admitted by all, and independent proofs 
were furnished by, amongst others, M. Delaunay and Prof. Cayley. For the 
last fifteen years, therefore, it has been generally admitted by astronomers 
that Prof. Adams had conclusively established the correctness of his views. 
It is true that the researches of Prof. Hansen seemed to show that the ob- 
servations indicated an actual acceleration of over 12"‘2, but it was known 
that the whole of this acceleration did not necessarily proceed from the dimi- 
nution in the eccentricity of the Earth’s orbit. 
. Although Sir George Airy made no reference to Prof. Adams in his 
paper, it was obvious that the paper could only be regarded as a direct at- 
tack on Prof. Adams’ views ; for if Sir George Airy was correct, then Prof. 
Adams was as certainly wrong. Naturally, therefore, the publication of Sir 
George Airy’s paper was waited for with impatience by astronomers, who 
were curious to see the grounds on which Sir George Airy had been led to 
impugn the correctness of the unanimous opinion of all the greatest authori- 
ties on the lunar theory. The paper was published in the beginning of May, 
and must have been read with astonishment by astronomers. The im- 
portance of this question is far too great to permit of any doubt being allowed 
to remain as to the truth, so that, although Sir George Airy disclaimed any 
wish to raise a controversy on the matter, it was absolutely indispensable 
that the real truth should be at once made known. Everyone looked, there- 
fore, to Prof. Adams for a reply. This Prof. Adams lost no time in sending 
in to the Royal Astronomical Society, where it was read at the May meeting 
and published in the middle of June. In this reply, Prof. Adams points out 
that Sir George Airy has misunderstood the nature of the results which he 
had arrived at in his investigation ; and that, instead of the value deduced by 
Sir George Airy being the complete value of the secular acceleration as 
he supposed, it was only a portion of the whole. Prof. Adams further 
showed that Sir George Airy had neglected as insensible, or else omitted as 
unimportant, all the very quantities which had been shown by Delaunay, 
Cayley, and himself to have so important an influence on the value of the 
secular acceleration ; and that, in fact, Sir George Airy had, without sus- 
pecting it, practically arrived in a roundabout manner at the same approxi- 
mate result as was obtained by Laplace nearly a century earlier, — a result long 
