The Sun.
London, England, 1856-08-14 00:00:00

LUNAR MOTION.

At the meeting of the mathematical section (A) of the
British Association at Cheltenham, on August 9, Mr.
SYmoNa read a paper on lunar motion, of which this is an
abstract :—No mathematicians ever misunderstood the
motion of the moon, which had been just correctly and
lucidly explained by Professor Whewell ; nor were they
likely to be misled by the modern astronomical use of the
term " rotation" as applied to it, or to fancy that any point
in the moon's circumference turned more than SGO degrees
in making her orbital revolution. But it was far other-
wise with the bulk of the people, who, being told in all
astronomical books that the moon rotated once on her
own axis, in addition to her revolution once in her
orbit, could derive no other impression than that she spun
round like a pegtop, which, together with her turning
round in her orbit, would cause her to make two entire re-
volutions, turning different faces successively to the earth ;
whereas, in order to keep the same face towards it, it was
necessary that she should so turn as if she were rigidly at-
tached to the centre of her orbit. He wished so to define it.
He was.glad that Prof. Whewell had distinctly statedthat
her movement might be thus correctly described; though
he differed from him in preferring the mathematical mode
of describing it, as one of "rotation." Some astronomers,
such as Woodhouse, Vince, and others, had added, thatthe
fact that the moon rotated on her axis in the same time as
she revolved in heror bit was "an extraordinary coincidence."
This synehronic movement was, on the contrary, it physical
necessity ; and he was inclined to think that by using
such an expression the old astronomers really mistook
moon's movement, mild that modern ones had ill-
ative) ten tly copied these descriptions, knowing batter,
just as Newton had calculated-the height of tides in lunar
seas, of which modem astronomers denied the existence.
Mr:Symons showed the three different modes in which the
moon, or any spheres might turn round a distant centre,
by means of a simple instrument, by which he first gave
the motion popularly understood by astronomical descrip-
tions of one orbital rcvo'uVon, plus one axial rotation,
whereby a point of the moon's circumference would point
twice round space ; secondly, that by which she preserves
her parallelism, pointing always one way in space • and,
thirdly, her simple motion of revolution. The two former
were both produced by wheels turning on their own axis ;
the second having a retrograde motion, neutralising the
direct revolution of the disc on which it was carried round;
the third or lunar motion was alone produce i without any
wheel or axis, the ball being fixed firmly to the revolving
disc ; the latter alone presented the same face to the cen-
t .e of orbit, turning the opposite face successively once
round space. It had been attempted to show the separate
rotation of the moon round her axis by librations, which
Mr. Symons contended could at the utmost prove ro-
tation only to the exact extent of the minute zone disclosed
by the Monition, which in some degree, be submitted,
might be accounted for by the change in the angle of pa-
rallax caused by her varying distances from the earth in
her elliptical orbit. He ridiculed the attempt to show the
previous motion of a body ceasing to revolve in space by
sticking a spindle. through a ball fixed to a revolving rod,
clearly changing the resultant of the forces- by the mecha-
nical axis, au •i giving scope to the tangential momentum
without any counteracting centripetal force. Mr. Symons
then reverted to the misimpression and confusion wising
from the mathematical application of the term rotation to
this lunar kind of revolution. He laid especial stress on the
tendency it would have to increase the ambiguity and want
of precis'on in the terminology of practical mechanics. A.
learned judge, in a recent action on a patent, had com-
plained that there were not more and exacter definitions of
different kinds of motion; and, were the mathematical defi-
nition of rotation to be generally introd need, it would greatly
increase the ambiguity of patentt, and litigation there-
from. It was, he affirmed, a modern change in the ordi-
nary definition, which was formerly confined to revolving
bodies, in which"any points formed concentric rings round
a centre within them. No point in the moon did this ; all
points in her (her own centre or nods included) forming
coucentric rings round the distant centre of her orbit. This
was a distinct species of revolution, whether considered
goutnetiiCally, dynamically, or mathematically ; and it re-
quired a oistinct name. lie cited these passages from Bar-
law's Iklathe loath:al Dictionary, to establish this as the old
definition of rJta.,io i :
" Rotation —The mot i in of the different parts of a solid
body about an axis, called the axis of rotation, being thus
distingaished from the progressive motion of a body about
&Me distant point or centre ; thus the diurnal motion of
the earth is a motion of rotation, but its annual motion one
of revolution. . . When a solid body turns round an
axis, retaining its shape and dimensions unaltered, every
part cle is actually describing a circle round this axis,
which axis passes through the centre of the circle and is
perpendicular to its plane."
De Poiusot's definition, now adopted byastronomers, natide
any motion one of rotation, in which a section of the re.

A erirrospondent, writing to tho Cologne Gazette from
Neidenburg, under date of the 6th inst., warns Polish
exiles that most of them have not rightly understood the
amnesty lately decreed by Alexander IL Each case is
decided upon in Poland after the emigrant's return, bpp
until the decision has been formed he himself is provi-
sionally detained in prison.