Astronomical and Nautical Collections, 415 



When the example which Mr. B. has considered is com- 

 puted according to the principles now laid down, no dis- 

 cordance will be found between the results of the two 

 methods. (See the annexed calculation.) Mr. B. introduces 

 refraction into his computations, but this is anunnecessary 

 complication of the problem ; for, as refraction is supposed 

 to operate equally upon two celestial bodies in apparent 

 contact, their relative position is not thereby affected. 



Upon the whole, it is hoped that the objection stated to 

 the method in question has been shown not to be well 

 founded. I believe that it will further be admitted, that 

 the method of computing the parallaxes for the star's place 

 is more convenient than the other. 



Du Sejour, in the work already referred to, (vol. i., p. 270,) 

 has shown that the supposition of the portion of the moon's 

 orbit described during an eclipse or occultation being a 

 straight line causes only an insensible error. An error of 

 greater magnitude may arise from the horary motion being 

 supposed to be uniform, while [it is generally variable ; but 

 this may be corrected by making an allowance for the varia- 

 tion of the horary motion. — Quarterly/ Journal, vol. xx., 

 p. 328. 



Mr. B.'s remarks on Dr. Young's method seem to be cor- 

 rect. It is to be regretted that the difficulty of obtaining 

 the difference of the apparent altitudes, with the requisite 

 accuracy, should prevent the success of a method which is so 

 very simple. The small correction which he proposes was 

 under consideration when the method was first published, 

 and it was remarked that it " may be altogether omitted 

 without inconvenience." Quarterly Journal, vol. xv., p. 360. 

 Besides, if the utmost exactness were attempted, it would be 

 necessary to take into account the spheroidal figure of the 

 earth, which besides occasioning an alteration in the parallax 

 in altitude, gives to the moon a parallax in azimuth. 



Leopold-Place y Edinburgh, Thos. Henderson. 



April 11, 1828. 



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