/ MICROMETEK MEASURES. gj^ 



Observations 

 „ -^ Mean apogeal semi- Semidiameters of G> of the sxm's 



o. Ut, 0. aiumeter of thesun. a»given in Nau. Aim. diameter. 



D.O.G.Micr.7 15'. 45V26 J ly^e^'-^g 5 15' 47'Ao 

 * Wire Micr. § 15'. 4.7",£05 I ^^ ' ^^ '^^ V ' ^^ '^' 



The sextant, on June 25th, shewed, from a carcfal set of 



measures, the apoffeal semidiameter of the sun, 15'. 4-i'". ^ . , 



^ ,-1 1-rt' r 1 , 1 • Deieritunaiioa 



On attending to the ditterence ot the sun s apogcal semi- ^^ ^^g jjyQ.j 



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



and the wire micrometer, I had recourse to some former as- °^^ ^^' 



tronomical records on this subject. By referring to De la 



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



175S, De la Cailie 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 correspond so exactly with 

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

 be a matter of some consequence, to inquire, what kind 

 of micrometers they used to deduce their respective semi- 

 diameters. 



It is unnecessary to extend these observations any farther. 

 I shall, therefore, only add to this paper, that it will appear, by ^ 



comparing the divided object-glass micrometer's measures of 

 the sun's diameters, of Decembers I5j 1794, and of June 19, 

 1795, that the difference of the perigeai and apogeal diameters 

 of the sun was found to be 65", 14-. De la Lande found this 

 difference 64!', S. but he calls it, in round numbers, 65". 



Note. Where no notice is taken of the time of observa- 

 tions, it is to be understood they were taken very near to noon, 

 and as soon after each other, as micrometers could be changed. 



The originals of these observations, and several others, are 



* This measure comes nearer to the calculated apogeal semi- 

 diameter of the sun 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 wire 

 micrometer, its obscrvafions may have been rendered more uc- 

 certain» 



