The sun. [volume] (New York [N.Y.]) 1833-1916, 1899-02-19

PLANTATION referred the phenomena TO tho ro,.
ton of the earth, and he considered thai, !,
Afforded n proof OF the Copernican systor
The theory OF tide-ReneratlnR force, which
afforded the firm basis on which all subseou.n,
researches have been made. Is due to Newton
who expounded it in his Principal" in les7'
In 1888 the Paris Academy Of Sciences orerod
the theory OF the tides as the subject fora rrI1o
or four essays which received prizes the AN.
thors Of three adopted not only the theory a,
Rravltation. but also Newton's theory of tide.
generating force to its fullest extent. Nothinc
OF i'm portance was added to our KNOWLEDGE
until the great French mathematician, Lapleco
took up tho subject in IVA. He 1t was who
for the first time. recognized the difficulty q,
the problem and showed that the earth's roto.
ton is an essential feature in the conditions
The actual treatment Of the tidal problem ia
in truth. due to Laplace. although the mode q,
presenting the theory has come to differ con.
siderably from his.
subsequently to Laplace the most importap,
workers In this field have been Sir John Iuh.
book. ST.; Whewell. Airy. and Lord Kelvin.
The work OF Lubbock and Whewell is chiefly
remarkable for the coordination and anah al,
OF enormous masses OF data at various ports.
and the construction OF trustworthy tide tables
Airy contributed an important review OF thu
whole tidal theory. He also studied profoundly
the theory OF waves In canals, and considered
the effects OF FRACTIONAL resistances on tina
progress OF tidal and other waves. Lord Kelvin
initiated a new and striking method OF consid.
ering tidal oscillations. His method possessss
a close analogy with that examined by our
author when discussing the Irregularities in
the motions OF the moon and planets. His
merit consists In the clear conception that tina
plan of procedure which has been so successful
in the one case would be applicable to the other
Tho difference between the laws of the moons
motion and those of tidal oscillations is, how.
ever, so considerable that there is scarcely anv
superficial resemblance between the two meth.
DDS. Nevertheless. tho so-called harmonic
analysis " Of the tides is daily growing in favor
in the eyes of men of science. and Is likely 10
supersede all the older explanations. 1t is eS.
pounded by Prof. Darwin in tho eleventh
chapter of this book.
In the first sixteen chapters the author refer,
but once to the influence OF the sun upon tbs
tides. On page 2S7. however. he acknowledges
that this influence is Of considerable impor.
tance. being large enough to cause the conspic.
uous phenomena of spring and neap tides.
While the moon is retarding the earth's roia.
ton the sun is doing so also. But the solar
tides react only on the earth's motion round
the sun. leaving the moons motion round the
earth unaffected. 1t might. perhaps, be OK.
pected that parallel changes in the earth's orbit
1890 t''NNN'ANA = W CNtl. INV lIuJ
the earth might be traced to an origin close to
the sun. The earth's mass is less than
l-3OO.OOO part of the suns. and the reactiva
effect on the earth's orbit round the sun Is al.
together negligible. Prof. Darwin deems It
improbable. in fact. that the earth's year is,
from this cause at any rate. longer by more
than a few seconds than it was at the very
birth OF the solar system.
Although. however, the solar tides cannot
have had nDv DprraptibIa inproAoo ,noa Al.
movement in its orbit. they will in
time have affected the rotation OF the earth TO
a considerable extent. Let us imagine our.
Selves says Prof. Darwin. TRANSPORTED to
the indefinite future. when the moons orbital
period and the earth's diurnal period shall
both be prolonged to ~ffty-flve OF our present
days. The lunar tides in the earth will then he
hnchanging. Just as the earth's tides in the
moon are now fixed; but the earth will ba
with reference to the sun, and if there
are still oceans on the earth her rotation will
be subject to retardation in consequence Of
the solar tidal friction. The day will then he.
longer than the month, while the moon
Will. at first. continue to revolve round the
earth In hfty-fve days. Lunar tides will now he
again generated. but. as the motion of the
earth will be slow relatively to the moon,
me oscillations will also be slow. and subject to
little friction. That friction. such as it Is. how
aver. will act in opposition to the solar tides.
ana the carth's rotation will. to some slight ex.
tent. be assisted by the moon. The moon her
self will slowly approach the earth. moving
with a shorter period, and. must. ultimately,
all back into the earth. We know that there
ire neither oceans nor atmosphere on tho
noon, but. If there were such. the moon would
nave been subject to solar tidal friction. and
would now be rotating more slowly than shs
1t must be remembered that. in
Is71. Prot. Asaph Paul discovered in the sys-
en- of the planet Mars a case Of the kind Of
notion which Prof. Darwin foresees as tho
uture fate Of the moon and earth. for he found
hat the planet was attended by two satellites.
he nearer Of which has q month shorter than
he planets day.