accordingly, holds them _against_ the whirling earth, which revolves
like a shaft in a fixed collar, slowly losing motion and gaining heat,
eventually dissipated through space.[957] This must go on (so far as we
can see) until the periods of the earth's rotation and of the moon's
revolution coincide. Nay, the process will be continued--should our
oceans survive so long--by the feebler tide-raising power of the sun,
ceasing only when day and night cease to alternate, when one side of our
planet is plunged in perpetual darkness and the other seared by
unchanging light.

Here, then, we have the secret of the moon's turning always the same
face towards the earth. It is that in primeval times, when the moon was
liquid or plastic, an earth-raised tidal wave rapidly and forcibly
reduced her rotation to its present exact agreement with her period of
revolution. This was divined by Kant[958] nearly a century before the
necessity for such a mode of action presented itself to any other
thinker. In a weekly paper published at Koenigsberg in 1754, the modern
doctrine of "tidal friction" was clearly outlined by him, both as
regards its effects actually in progress on the rotation of the earth,
and as regards its effects already consummated on the rotation of the
moon--the whole forming a preliminary attempt at what he called a
"natural history" of the heavens. His sagacious suggestion, however,
remained entirely unnoticed until revived--it would seem
independently--by Julius Robert Mayer in 1848;[959] while similar, and
probably original, conclusions were reached by William Ferrel of
Allensville, Kentucky, in 1858.[960]

Delaunay was not then the inventor or discoverer of tidal friction; he
merely displayed it as an effective cause of change. He showed reason
for believing that its action in checking the earth's rotation, far from
being, as Ferrel had supposed, completely neutralised by the contraction
of the globe through cooling, was a fact to be reckoned with in
computing the movements, as well as in speculating on the history, of
the heavenly bodies. The outstanding acceleration of the moon was thus
at once explained. It was explained as apparent only--the reflection of
a real lengthening, by one second in 100,000 years, of the day. But on
this point the last word has not yet been spoken.

Professor Newcomb undertook in 1870 the onerous task of investigating
the errors of Hansen's Lunar Tables as compared with observations pr