Final Remarks on Lunar Olservalions. 219 



clear, fine, and healthy, though extremely hot from April to 

 October ; and the water of the Euphrates is held in almost as 

 high estimation at the present day by the Arabs, as that of the 

 Choaspes (the modern Karoon) was regarded by the imperial 

 lords of Ecbatana in ancient times." 



XXXIl. Final Remarks, ly Mr. Edward Riddle, on 

 finding the Longitude by Lunar Observations, &c. 



To Mr. Tilloch, 



Sir, — Having now seen both Mr. Meikle's reply and his 

 continuation of it, I take this opportunity of making a iew and 

 final observations on what he has advanced in his defence. I 

 shall take his letters in tlie order in which they are pubhshed. 

 First then, for his letter of November 2d : I had observed that he 

 misunderstood the term parallax, and in answer to the accusa- 

 tion he has taken the trouble of proving that he does so. His 

 exposition of Playfair's definition would certainly have produced 

 a smile from the venerable philosopher. I defined parallax to be 

 *' the angular distance between the places of the centre of a ce- 

 lestial body, as seen at the surface and at the centre of the earth ;" 

 or, which amounts to the same thing, "the angle under which the 

 earth's seniidiameter would appear if viewed from the centre of 

 the celestial object." Mr. M. asserts that this is a" counterfeit 

 definition ;" and that it is " invented to serve a particular pur- 

 pose." It is almost needless to quote authorities on such a sub- 

 ject ; but the following extract from Rees's Cyclopcedia, article 

 Parallax, will show that my definition was neither counter- 

 feit, nor invented to serve a particular purpose : " Parallax is 

 used for the angle made in the centre of the star by two right 

 lines drawn, the one from the centre, the other from the surface 

 of the earth." It is in this sense that all astronomers under- 

 stand the term. The following extract from Playfair, too, will 

 .show how far his notions of parallax agree with those which 

 Mr. M. ascribes to him. " The parallax affects the position of 

 a body only by depressing it in the direction of a vertical plane." 

 — Outlines of Nat. Phil. vol. ii. art. 79. This is the "common 

 popular doctrine," which Mr.M. endeavoured to prove was not 

 true ; yet he asserts that Playfair understood parallax in the same 

 sense that he does. 



In the example of correcting the moon's altitude, Mr. Melkle 

 says, J have concealed " a great deal of needless labour." On 

 this assertion it is sudicicnt to observe, that I concealed no labour 



whatever, 



