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meters, as taken with the okl wire micrometer, made in the 

 best manner, by Mr. Dolland; with his divided object-glass 

 micrometer ; and with a ten-inch reflecting sextant, exe- 

 cuted, in a very capital stile indeed, by Messrs. Troughton. 

 Before I proceed to the detail of the observations, it 

 may be proper to premise a short account of the nature 

 and adjustuicnts of the several instruments, that were the 

 subjects of this experiment. The wire micrometer, as its 

 name denotes, measures intervals, by the separation of two 

 moveable wires: these wires should perfectly coincide, when 

 the index of the scale marks or zero: and the quantity of 

 the separation of the wires, made by the turning of the 

 screw which effects it, is denoted by revolutions, and parts 

 of revolutions, of the index, over a graduated circle, at- 

 tached to the micrometer's screw; which, in this instrument, 

 consists of fifty sub-divisions. There are several ways of 

 ascertaining the values of these revolutions and sub-divi- 

 sions, in arcs of a great circle in the heavens. The method, 

 which I adopted was this: the microscope being fitted to 

 an achromatic telescope, on an equatorial stand, I carefully 

 separated the wires by fifteen exact revolutions ; and then 

 turning round the whole system, till a fixed wire, at right 

 angles to the measuring wires, was in a plane parallel to the 

 equator, I measured, by the sydereal clock, the time the 

 sun's limb, and various fixed stars took, to run along the 

 fixed wire, from centre to centre of the measuring wires. This 

 trial was very frequently and repeatedly made; and the 

 stars and sun's limb, being all reduced to the equator, the 



• general 



