The Abbeville banner. [volume] (Abbeville, S.C.) 1847-1869, 1859-02-17
THE PROCESSION OF THE EoUIN0xEs

All rnEVHooloN Uf IdE EaUlNVAES.
NJ. Poinsot, tho distinguished member of
the Institute, and celebrated for the dscoy-
cry of the theory of couples in mechanics,
has recently solved an important problem,
which has baffled the ingenuity of mathe
maticians since the time Of Newton, 1t is
we'll known that the intersections of the
terrestrial equator with the ecliptic, called
the equinoxes, never occur twice at the
same point, but that every year they appear
to reccde about 50:lS seconds. This retro
grade movement is called the prcccssior of
the cquinoxes. To explain this motion hv
the theory Of universal gravitation became
an important problem at the time when
Newton made that great discovery lie
himself attempted an explanation of it, but
the imperfect state of astronomy at that
period deprived him of the advantage of
certain data which were indispensable
to arrive at tho desired solution. In
I'LL, Bradley, by series of l,borious
observations, patiently continued for the
space Of twenty years, at length asccr
tanned that the carlll's axis had nc
culiar swinging motion, of which no as
tronomcr before him had even suspected
the existence; and this motion lie called
mutation. lie also proved that this nuta
ton, Which causes the axis Of the earth to
describe in space sort of ilutcd conc lnv
ing a base OF IN clIiplical form, but mod
He'd by certain minute undula:ions, about
l4oO III number, was closely connected
with the motion of procession. D'Al
cmbcrt now took up the problem, and in
l749 published a solution of it, which in
deed accounts for the above motions in a
general way, but is far from sufficient To
explain the cause Of those singular undu
Iations which the pole of tho earth dc
scribes. Tho dclicacy of the question will
be easily understood from the fact, that
within the compass Of the earth the axis
appears TO have no motion at all, and that
the phenomenon of mutation is only per
cept'ble at an immeasurable distance in
the heavens.
We now come to NJ. Poinsot's admira
blc solution, founded on his we'll known
theory Of couples. Ncglectins all useless
considerations that had embarrassed his
predecessors, he proves by mathematical
calculations, which, considering the diff
Guilty of the problem, have tho merit Of ex
traordinary simplicity, that, by the law of
gravitation, the ear,h's axis must describe
an oscillation Of l.Os seconds in virtue of
tho attraction Of the sun, and 100 seconds
in virtue of that of the moon, or about Is
seconds i'll all, in the course of nine years
and three months, after which a similar os
ciIlation takes place in the contrary di,cc
Lion. This quantity of l8 seconds all bu.
exactly comedies with the results of obser
vation; Of his determination of tho pre-
cession is equally exact, since he finds it
10 be 50.4 seconds. 1t must be borne in
mind that observation always has great
advantage over calculation in astronomy,
since it gives facts as they are, while ia
calculation it is often necessary to reject
certain small quantities which arc in the
way Of integration. NJ. Poinsot proves a
singular fact, viz: that the procession would
be the same if the earth, instead of being
a solid spheroid, were hollow, or if it's mass
or volume were changed, provided its mo-
mentum of inertia remain the same. Thus
all D'Alembert's complicated spcculations
touching the influence of the sea, the na
ture OF tho straw Of tho earth, &C.. turn
out to be useless. NJ Poinsot demon
strates various other curious theorcms
connected with the subject, but which
are too abstruse TO DO mentioned here
Paris Paper.