by Lunar Observations. 243 



of the centre E above the sensible horizon AF. If the refrac- 

 tion E A C corresponding to F A E be deducted from that angle, 

 the remainder will be the angle C A F, the true altitude of the 

 centre F above the sensible horizon; and if the parallax A C G 

 corresponding to C A F be added to it, the sum will be H G C, the 

 true altitude of the centre above the rational horizon G H. 



This is the principle of the method which is generally adopted 

 in correcting the altitudes ; and it obviously gives correctly the 

 true and apparent altitudes of the centre of the luminary. There* 

 fore by the methods on which Mr. M. animadverts with so much 

 severity, the true and apparent altitudes of the centres of the ob- 

 jects will be correctly obtained ; and the apparent distance of 

 the centres will also i)e obtained correctly to within a very small 

 fraction of a second. The distance may then, from these data, 

 be cleared from the effects of parallax and refraction, by the 

 usual formula ; and from whatever points the apparent distance 

 is estimated, the result of the computation will be the true di- 

 stance of the same. 



Now the object of the computation for clearing the distance, 

 13 to deduce, from observations made upon the surface of the 

 earth, the angular distance of the objects observed as they would 

 be seen at the centre of it. By comparing the distances so de- 

 duced with those previously computed for a known meridian, 

 the time at that meridian is found. And as those previously 

 computed distances are the distances of the centres of the ob- 

 jects, it is obvious that the distances compared with them ought 

 to be the distances of the same points. 



Mr. Meikle proposes as an improvement, that the apparent di- 

 stance should be estimated not from the centre, but from an ex- 

 centric and variable point ! 



What I have already said will enable us to appreciate the value 

 of this extraordinary proposal, the absurdity of which is only to 

 be equalled by the air of self-gratulation with which it is deli- 

 vered, and the contemptuous allusion which the author of it 

 makes to " the old-established habit" of estimating the apparent 

 distance from the centre. It would indeed appear that he is not 

 aware that the central distance of the objects is what is required. 



I have now, I trust, sufficiently vindicated the deduction from 

 the semidiamcters applied to the distance from the charge of a 

 tendency to produce error. I have also shown that, in the cor- 

 rection of the altitudes, the deduction from the vertical semidia- 

 meters ought to be applied to obtain correctly the altitudes of 

 the points of which it is ^he object of the problem to compute 

 the distance. In the princijjles, therefore, which direct the com- 

 mon application of the corrections in question, there is no error. 



Q 2 Those 



