On Capl. Ross's Reply to Capt. Saline. 251 



It apjiears from what I have said in the foregoing letter, that 

 if Captain S. applied the contracted semidiametev to the observed 

 altitude, and then the refraction, Sec. corresponding to the alti- 

 tude of the centre so found, both the true and apparent altitudes 

 of the centre would be obtained ; the true altitude in fact would 

 be the same as that obtained in the waj' in which Captain Ross 

 says he has been accustomed to apply the corrections. And it 

 appears also, that even if the semidiameters which Captain S. ap- 

 plied were not corrected for the effect of refraction, the circum- 

 stance was of little or no importance in lunars, if the corrections 

 ■were taken out for the approximate altitude of the centre which 

 the semidiameters so applied would produce. In problems in- 

 deed where the true altitude ovbj was wanted, the deduction 

 from the semidiameter ought to have been made ; and if Cap- 

 tain S. did make it, Captain Ross's objections are unfounded; 

 as the method was then only in appearance different from his 

 own. — If Captain S. did not in such cases make the requisite de- 

 duction, his results would be in theory slightly incorrect. But 

 if Captain Ross has been accustomed to consider the correction 

 for parallax and refraction corresponding to the altitude of the 

 limb, as the deviation in altitude which ought to be used in the 

 computation for clearing the distance, I need not here tell those 

 who may honour the preceding letter with a perusal, that, consi- 

 dered even in the most unfavourable light. Captain S.'s method 

 of correcting the altitudes must produce results much nearer the 

 truth than that of Captain Ross. No considerable mistake in 

 the longitude can have arisen from the way in which Captain S. 

 is said to have applied the corrections. Rut if I understand 

 Captain Ross rightly, this cannot be said for the method of ap- 

 plying them in the forms which have been furnished for the pre- 

 sent expedition, or for the similar method which he says he has 

 been accustomed to pursue. 



But before forming any conclusion on the subject, it appears 

 desirable that more precise information should be afforded than 

 that which is contained in the above extract from Captain Ross's 

 pamphlet. 



If both the forms which were used by Captain Ross and Cap- 

 tain Sabine in the late voyage were laid before the public ; and 

 if the work, at length, of an observation as it was taken, and 

 worked, i>y eacli gentleman, during the voyage ; and further, if 

 the printed forms which were found on board the Alexander, and 

 those which have been furnished for the new expedition, were 

 given, — we should determine with more certainty the merits or 

 defects of either method, than we are likely to do from any ac- 

 coiuit which wc mav have of them otherwise. 



If 



