as necessary for continuing the
Circle Observations with the existing instruments, whilst the new
instrument was under erection. When the new Transit is completely
mounted, the old Transit Instrument may be removed, and the Transit
Room will be free for any other purpose. I propose to take it as
Private Room for the Astronomer Royal.--On May 12th I made my first
proposal of the Reflex Zenith Tube. The principle of it is as follows:
Let the micrometer be placed close to the object-glass, the frame of
the micrometer being firmly connected with the object-glass cell, and
a reflecting eye-piece being used with no material tube passing over
the object-glass: and let a basin of quicksilver be placed below the
object-glass, but in no mechanical connection with it, at a distance
equal to half the focal length of the object-glass. Such an
instrument would at least be free from all uncertainties of twist of
plumb-line, viscosity of water, attachment of upper plumb-line
microscope, attachment of lower plumb-line microscope, and the
observations connected with them: and might be expected, as a result
of this extreme simplicity, to give accurate results.--A considerable
error was discovered in the graduation of Troughton's Circle,
amounting in one part to six seconds, which is referred to as follows:
'This instance has strongly confirmed me in an opinion which I have
long held--that no independent division is comparable in general
accuracy to engine-division,--where the fundamental divisions of the
engine have been made by Troughton's method, and where in any case the
determination by the astronomer of errors of a few divisions will
suffice, in consequence of the uniformity of law of error, to give the
errors of the intermediate divisions.'--The method of observing with
the Altazimuth is carefully described, and the effect of it, in
increasing the number of observations of the Moon, is thus given for
the thirteen lunations between 1847, May 15, and 1848, May 30. 'Number
of days of complete observations with the Meridional Instruments, 111;
number of days of complete observations with Altitude and Azimuth
Instrument, 203. The results of the observations appear very good;
perhaps a little, and but a little, inferior to those of the
Meridional Instruments. I consider that the object for which this
instrument was erected is successfully attained.'--Being satisfied
with the general efficiency of the system arranged by Mr Brooke for
our p