The New York herald., 1851-08-29

the extremity Of IN arm on the nearer side, which
is 30 arranged a. to revovs around the axis Of the
plane OF Oscillation This ring rests upon marbles,
aid may be adjusted by movable friction wheels
place d outside the ring upon the inner wall Of the
monument With the aid Of these compass sights,
the rotation OF the plane of oscillation beco omesap
parent in less time than if the point at the bottom
OF the ball be observed, for the reason, that the
view confined to definite line, and also that the
distance OF the nearer sight from the centre is five
feet, while the radius Of the graduated arc on the
floor is rather less than three ano half feet The
sight, through which the observation is made, is
moved by small geered wheel upon a graduated
arc OF brass placed in the doorway; and such is tae
nicely OF the adjustment that the progress OF the
plane Of oscillation can be noticedin in single vibra-
ton OF the pendulum.
The mode Of starting the pendulum is that
adopted by Foucaut. The ball being dra wn to
the margin of the circle, and secured BY thread,
is permitted to come entirely to rest. Whom this
is attained, ~hich requires but minute or two,
the thread is burned and the pendulum launched.
An unanticpated difficulty in the movement of
the monument itself, and to which more particular
reference will be made in another communication,
for some time interfered with the progress OF expe-
riment The imposslbility Of using the graduation
on the floor, led to the adoption Of movable Sec
tion of sector of BRASS of three feet radius having
the degree divisions prolonged six inches toward
the cenre to pre- vent the essenDg OF the length OF
the sweep from interfering with the observation.
The position OF this plate with regard to the centre
could be determined at any mo nent, by bringing
the pendulum to rest, and measuring from the so
ascertained centre.
arna,ure in which the ball was slung, notwithstand
ing the extreme slowness OF the movement-an os-
ciiiation requiring less than l. l6 seconds--ntsr
feTed seriously with the experiment.
Sometimes the plane OF oscillation would sweep
through degree In four minutes, and sometimes 1t
required eleven A series Of experiments elimi-
mated the source of this difficulty, aud the mode Of
so managing the apparatus as to sec re trustworthy
"a aI hraa ors,, ka Al
slightly lenticuar form, which, it is obvious,
osollate uniformly in but two directions, to witt
10 the direction OF the axis Of the lens, and right
angles to it. In either of these obtained correct
results. AS admitting the least tendency to error
uniformly adopted the latter direction THAT is in
which the plane Of the ring coincded with the plane
of oscillation My observations lin ted to
single degree-and that the first This was neces
sary from the dmnution in the length OF the sweep
unless l had constructed movable floor. 1t had
this advantage-that the ellipticvl motion which,
after longer or shorter period, invariably sets n
was uniformly not appreciabe. | have occa-
siOrally watched the motion Of the pendulum
when it started under the fairest circumstances
considering the imperfection of my ball, through
period Of twelve mlnutes without detecting ellipti
cal motion, though frequently it came on in less
time. Professor Norton, of Brown Uni yersity, has
informed me that in experiment conducted by
Professor Caswell and himself, the elptical motion
was frequently not observable for half an hour.
The eillptical motion was, with very rare excep-
tions, in the direction opposite to the hands of a
watch. The greatest conjugate diameter Of the
elpse described has rarely exceeded half inch.
This maximum, which frequently was gained in less
than twenty minutes, continued for four hOurs-the
ball itself not coming to rest in less than five six
hours The length Of the arc of oscillation was
six feet, the whole diameter Of the well being but
seven feet. The time required for single oscilla-
tion vas l. 10th SECONDS, as already mentioned,
losing one second in PA oscillations.
In observation, l at 6rst had the aid Of an assis-
tant; who c)unted the oscillations while observed
the time required for the rttation of the plane of
cillation through single degree. 1t was possible
to observe only at one extreme of the oscillation.
The determination of the time by oscillations, by
seconds, if ether oscillation TOO maay OF too
few, gave a range OF error Of 324 seconds. IL so
happened that the length of the pen iul1m at the
Monument, and the theoretical time at that loca-
hty, were marvellously suited TO each other. The
time required, Q take the calculation by Professor
Caswe, for the State House, Boson,) ON. 56s. Oh.
Forty four oscillations at Sm. lilo seconds for
oscillation, give 5m 55s. D.
In my observations, a second or two would be los.
at the outset In carrying the eye from the burning
thread TO the watch face, and at the conclusion, In
glancing from the index coincident with the degree
division, to the second hand But when not vary.
ing more than three four seconds, 1t would be
just to correct the time by the oscillations since the
time required for an oscillation had been ascer-
tanned with the greatest precision.
i will here give "a few of the observations with
their average, AID with IL the time indicated by
the oscilatons-and that by theory.