©4 - ON COMPARATIVE 



shewing, at one view, a ^sort of harmony of micrometers. 

 My own opinion, on this subject, entirely coinciding with that 

 of the learned friend, who made this proposal, I set about 

 making comparative observations of the measures of the sun's 

 diameters, as taken with the old wire micrometer, made in the 

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

 micrometer, and whh a ten-inch reflecting sextant, executed, 

 in a very capital style indeed, by Messrs. Troughton. 



Pescription Before I proceed to the detail of the observations, it may be 



and acconnt. or ^ ' ■' 



the wire mi- proper to premise a short account of the nature and adjust- 

 cromeier. ments of the several instruments, that were the siibjects 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 revoluiions, of the 

 index, over a graduated circle, attached to the micrometer- 

 screwj which, in this instrument, consists of fifty sub-divi- 

 sions. There are several ways of ascertaining the values of 

 these revolutions and sub-divisions, in arcs of a great cirde in 

 the heavens. The method which I adopted was 'this : the 

 microscope being fitted to an achromatic telescope, on an equa* 

 torial 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 

 its scale ^eter- plane parallel to the equator, I measured, by the sydereal 

 mined. clock, the time the sun's limb, and various fixed stars took, to 



run along the fixed wire, from centre to centre of the mea- 

 suring wires. This trial was very frequently and repeatedly 

 made; and the stars and sun's limb, being all reduced -to the 

 equator, the general result gave 121", J, for the equatorial in- 

 terval of the fifteen revolutions. This interval, reduced to 

 space, made each revolution of the figured head =^ to 2' and 

 1'' of nneasure ; and, of course, each of the fifty sub-divisions 

 = to2'',42 nearly of an arch of the equator. In making the 

 sub,'^equent measures of the sun's diameter, or that of any 

 other celestial arc, the measure was always finished, by mov- 

 ing the wire in the direction in which the fifteen revolutions 

 were originally made. The advantage of this micrometer is 



