116 



1795. 

 June 19. The measures, with the diiFerent micrometers, were taken 

 with the greatest care; and a mean of internal and 

 external contacts, of the sun's limb to the micrometer 

 wires, was used as the measure of the sun's disk by 

 tlie wire micrometer. 



S. D . KJt diimcter of the fuu. ia the NujUc4l Almanack. 



D. O. O. Mic..> u, «^.6 < ^^, > ,^, 



* Wire Micr'. • • V 15'. 47", 505 ^ 5 



The sextant, on June 25"^, shewed, from a careful set of mea- 

 sures, the apogeal semidiameter of the sun, 15'. 44'', 



On attending to the difference of the sun's apogeal semi- 

 diameters, as shewn by the divided object-glass micrometer, 

 and the wire micrometer, I had recourse to some former 

 astronomical records on this subject. By referring to De 

 la Lande's Astronomy, article 1387, I find, that, in the 

 year 1758, De la Caille observed the apogeal semidiameter 

 to be 15'. 47",2; and that De la Lande, in 1760, made 

 it 15'. 45",25. 



These two measures happen to con-espond so exactly 

 with mine, as made with the different micrometers,^ that it 

 may be a matter of some consequence, to enquire, what kin<i„ 



of 



* This measure comes nearer to the calculated apogeal semidiameter of the 

 iun than the former; but as, at the making of these observations, the state of 

 the air caused the sun's limb to undulate, perhaps the divided object-glass 

 micrometer, having a much greater magnifying power, than was used with the 

 ■^■ire micrometer, its observations may have been rendered more uncertain. 



