--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.
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