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• Thursday, February 4, 2010 - At Sierra Leone, nineteen out of
seventy-two...
| At Sierra Leone, nineteen out of
seventy-two have the same accordance; and of the moon
culminating stars, p409, twelve out of twenty-four are equally
exact With larger instruments, and in great observatories, this
is not always the case
Captain Kater has given, in the Philosophical Transactions, 1819,
p427, a series of transits, with a three and a half foot
transit, in which about one-eleventh part of them only have this
degree of accuracy; and it should be observed that not merely the
instrument, but the stars selected, have, in this instance, an
advantage over Captain Sabine's
The transit of MBessel is five feet in length, made by
Frauenhofer, and the magnifying power employed is 182; yet, out
of some observations of his in January, 1826, only one-eleventh
have this degree of accordance In thirty-three of the Greenwich
observations of January, 1828, fifteen have this agreement, or
five-elevenths; but this is with a ten-feet transit Now in none
of these instances do the times agree within a tenth of a second
between all the wires; but I have accounted those as agreeing in
all the wires in which there is not more than four-tenths of a
second between the greatest and least
This superior accuracy of the small instrument requires some
explanation One which has been suggested is, that Captain
Sabine employs a chronometer to observe transits with; and that
since it beats five times in two seconds, each beat will give
four-tenths of a second; and this being the smallest quantity
registered, the agreement becomes more probable than if tenths
were the smallest quantities noticed In general, the larger the
lowest unity employed the greater will be the apparent agreement
amongst the differences Thus, if, in the transit of stars near
the pole, the times of passing the wires were only registered to
the nearest minute, the intervals would almost certainly be
equal There is another circumstance, about which there is some
difficulty It is understood that the same instrument,--the
thirty-inch transit, was employed by Lieutenant Foster; and it
has not been stated that the wires were changed, although this
has most probably been the case Now, in the transits which the
later observer has given, he has found it necessary to correct
for a considerable inequality between the first and second wires
(See Phil If an erroneous impression has gone
abroad on this subject, it is doing a service to science to
insure its correction, by drawing attention to it
Should these observations be confirmed by other observers, it
would seem to follow that the use of a chronometer renders a
transit more exact, and therefore that it ought to be used in
observatories
Among the instruments employed by Captain Sabine, was a repeating
circle of six inches diameter, made by order of the Board of
Longitude, for the express purpose of ascertaining how far
repeating instruments might be diminished in size:--a most
important subject, on which the Board seem to have entertained a
very commendable degree of anxiety
The following extract from the "Pendulum Experiments" is
important:
"The repeating circle was made by the direction, and at the
expense of the Board of Longitude, for the purpose of
exemplifying the principle of repetition when applied to a circle
of so small a diameter as six inches, carrying a telescope of
seven inches focal length, and one inch aperture; and of
practically ascertaining the degree of accuracy which might be
retained, whilst the portability of the instrument should be
increased, by a reduction in the size to half the amount which had
been previously regarded by the most eminent artists as the
extreme limit of diminution to which repeating circles, designed
for astronomical purposes, ought to be carried
"The practical value of the six-inch repeating circle may be
estimated, by comparing the differences of the partial results
from the mean at each station, with the correspondence of any
similar collection of observations made with a circle, on the
original construction, and of large dimensions; such, for
instance, as the latitudes of the stations of the French are,
recorded in the Base du Systeme Metrique: when, if due allowance
be made for the extensive experience and great skill of the
distinguished persons who conducted the French observations, the
comparison will scarcely appear to the disadvantage of the
smaller circle, even if extended generally through all the
stations of the present volume; but if it be particularly
directed to Maranham and Spitzbergen,--at which stations the
partial results were more numerous than elsewhere, and obtained
with especial regard to every circumstance by which their
accuracy might be affected, the performance of the six-inch
circle will appear fully equal to that of circles of the larger
dimension The comparison with the two stations, at which a more
than usual attention was bestowed, is the more appropriate,
because it was essential to the purposes for which the latitudes
of the French stations were required, that the observations
should always be conducted with the utmost possible regard to
accuracy
"It would appear, therefore, that in a repeating circle of six
inches, the disadvantages of a smaller image enabling a less
precise contact or bisection, and of an arch of less radius
admitting of a less minute subdivision, may be compensated by the
principle of repetition
Captain Sabine has pointed out Maranham and Spitzbergen as places
most favourable to the comparison Let us take the former of
these places, and compare the observations made there with the
small repeating instrument of six inches diameter, with those
made by the French astronomers at Formentera, with a repeating
circle of forty-one centi-metres, or about sixteen inches in
diameter, made by Fortin It is singular that this instrument
was directed, by the French Board of Longitude, to be made
expressly for this survey, and the French astronomers paid
particular attention to it, from the circumstance of some doubts
having been entertained respecting the value of the principle of
repetition
The following series of observations were made with the two
instruments[I have chosen the inferior meridian altitude of
Polaris, merely because the number of sets of observations are
rather fewer The difference between the extremes of the
altitude of Polaris, deduced from sets taken above the pole by
the same observers, amounts to seven seconds and a half
Latitude deduced from Polaris, with a repeating circle, 16 inches
diameter-BASE DU SYSTEME METRIQUE, tom
Number of Latitude Names of Observers
deg
64 38 39 55 Biot
100 54 Arago
10 56 Biot
88 56 Biot
120 56 Arago
84 54 Biot
100 shop 56 |
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