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suspicion. The moon’s advance was slight, it is true,but to the astronomer it was as real as though it hadtaken place under his very eyes. The theory of gra-vitation seemed to give no account of this accelerationof the moon’s motion. At length, however, Laplacewas led to turn his attention to the variation of theearth’s eccentricity as a probable cause of the pecu-liarity. His calculation of the effects due to this vari-ation accorded very closely with the observed amountof the acceleration. Yet, although this agreementmight have appeared convincing, and although a por-tion of the acceleration is undoubtedly due to thecause in question, the inquiries of Professor Adams(confirmed by the researches of Delaunay and others,and now universally admitted) show that in realityonly half the observed acceleration can be explainedby the change in the earth’s orbital eccentricity.
But it is to be noted that the variation itself isexceedingly small, as is also the discrepancy betweenobservation and theory. We have seen that theannual equation causes the moon to be displaced byabout one-third of its diameter in opposite directionsin spring and autumn, the actual range of this oscil-latory variation being therefore equal to about two-
noster motum medium Lunse, cum motu diurno terrse collatumpaulatim accelerari primus omnium quod sciam deprehendit.’ ”
I have given an account of the subject in the QuarterlyJournal of Science for October, 1866, in an essay entitled “Prof.
Adams’s Kecent Discoveries,’’and a more popular account appears
in my Light Science for Leisure Hours, in a paper called “ Our ChiefTimepiece losing Time.”