J4-6 ASTRONOMICAL OBSERVATIONS 



These mensurations may have a farther use, be- 

 etles ascertaining the adjustment of the quadrant. 

 If the eye could determine, with perfect accuracy, 

 the contact cf the limbs, the mean between the two 

 measurements of the sun's diameter would be ex- 

 actly equal to his apparent diameter, as determined 

 by calculation, and given in the Ephemerh ; but. 

 from the imperfection of our organs, it happens 

 that the limbs will sometimes appear to be in contact, 

 when a little space remains between them 5 at others, 

 when they overlap one another: in the former case, 

 the diameter^will appear greater; in the latter, less 

 than the truth. But it is probable that, at nearly 

 the same period of time, the state of the eye, or of 

 the sensorium, by which we judge of this contact, 

 is, in the same person, nearly the same. Of this 

 1 have made some trials, and found, that, when the 

 sun's diameter, by my mensuration, differed from 

 that in the Ephemeris , on repeating the mensurations, 

 at short intervals, the difference remained nearly the 

 same. Therefore, if we observe the sun's altitude, a 

 little time before or after measuring his diameter, 

 the contact of the limbs will, probably, appear to 

 take place in the same real situation of those limbs 

 as when we measured the sun's diameter. But here, 

 the effect of too open or too close observation will 

 be reversed ;. the former making the altitude appear 

 less ; the latter, greater than the truth. These mea- 

 suremenrs then may be applied a$ corrections of the 

 observed altitude. Thus, if the diameter of the sun 

 has appeared too great, add the quantity of its ex~ 

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