The Press.
London, England, 1856-08-16 00:00:00

THE BRITISH ASSOCIATION.

SEVERAL papers upon suhj.•ets of general interest
have been read during the week. Among them one
by Mr. SYMONS on

LUNAR MOTION.

No mathematicians either misunderstood the motion of
the moon, which had been just correctly and lucidly ex-
plained 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 360 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
revolutions, 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 attached to the centre of her orbit. He wished
so to define it. He was glad that Professor Whewell
had distinctly stated that her movement might he thus
correctly described ; though he differed from him in pre-
ferring the mathematical mode of describing it, as one of
"rotation." Some astronomers, such as Woodhonse,
Vince, and others, had added, that the fact that the moon
rotated on her axis in the same time as she revolved in her
orbit was "an extraordinary coincidence." This syn-
chronic movement was, on the contrary, a physical neces-
sity; and he was inclined to think that by using such an
expression the old astronomers really mistook the moon's
movement, and that modem ones had inadvertently copied

THE PRESS.

these descriptions, knowing better, just as Newton had
cklmilated the height of tides in lunar seas, of which
modern astronomers denied the existence.

Mr. Symons then showed the three different modes
in which the moon, or any sphere, might turn round
a distant centre, by means of a simple instrument,
by which the first gave the motion popularly under-
stood by astronomical descriptions of one orbital re-
volution, 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, Ler simple motion of revolution. The two
former were both produced by wheels turning on
their own axes ; the second having a retrograde mo-
tion, neutralising the direct revolution of the disc on
which it was carried round; the third or lunar mo
tion was alone produced without any wheel or axis,
the ball being fixed firmly to the revolving disc; the
latter alone presented the same face to the centre of
orbit, turning the opposite face successively once
round space. With respect to the term " rotation "
and its mathematical application to this lunar revo•
lotion, Mr. Symons laid special stress on the tendency
it would have to increase the ambiguity and want of
precision in the terminology of practical mechanics.
It was, he affirmed, a modern change in the ordinary
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 axis
included) forming concentric rings round the distant
centre of her orbit. This was a distinct species of
revolution, whether considered geometrically,
dynamically, or mathematically ; and it required a
distinct name, lie cited thee passages from Bar-
low's Mathematical Dictionary, to establish this as
the old definition of rotation

Rotation.—The motion of the different parts of a solid
body about an axis, called the axis of rotation, being thus
distinguished from the progressive motion of a body about
some distant point or centre ; thus the diurnal motion of
the earth is a motion of rotation, but its annual motion
one of revolntion When a solid body turns
. . .

round an axis, retaining its shape and dimensions unaltered,
every particle is actually describing a circle round this axis,
which axis passes through the centre of the circle and is
perpendicular to its plane.
De Poinsot's definition, now adopted by astronomers,
made any motion one of rotation, in which a section
of the revolving body does not preserve its parallelism
during its revolution, whether round its own axis or
round a distant round. Professor Whewell was of
opinion that this was more convenient, as otherwise
the moon's relation to the earth might be deemed
" casual and arbitrary," which Mr. Symons denied ;
he also controverted the relevancy to this argument
of any reference such as the professor had introduced
to the moon's undulating orbit round the sun; her
rotation or non•rotation had no reference to space,
and could he properly determined by her own
internal motion and that of her different parts in re-
lation to each other and to the centre of her orbit,
nor could any ambiguity arise in so doing, or in re-
stricting the use of the term rotation to its original
meaning, which would simplify the great truths of
astronomy, distinguishing different motions, and
render them intelligible to the whole people, espe-
cially in books for popular instruction. This was his
sole object in mooting the question. He felt he had
been unfairly attacked for so doing, and, though lie
had been far from seeking to provoke controversy,
he had been compelled to put his back to the wall
and fight it out.
The Association on Saturday made an excursion
to Cirencester, and inspected the Roman remains of
domestic articles discovered in 1850. In the even•
ing Colonel Sir Henry RAWLINSON delivered his
lecture npon the recent di,coveries in Assyria and
Babylonia, and on the results of