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XLIII. Abridgment of certain Papers written on the appa- 

 rent Magnitude of the horizontal Moon, and published at 

 sundry Times*. To which are added new Experiments to 

 prove the Truth of the Author's Theory, and to exhibit a 

 clear Representation of the Phcenomenon on optical Prin- 

 ciples. By Ez. Walkeb, Esq. 



To Mr. Tilloch. 



5 J j5^ Lynn, March 1 1 , 1 80G. 



When the full mcon is just rising above a clear horizon, 

 .she is an object that pleases every eye. But at the same 

 time that she pleases the eye of the philosopher, she em- 

 barrasses his reasonina; in attempting to assign the cause 

 why htr apparent magnitude is greater near the horizon than 

 at higher e'evation's. 



When men in former ages began to reason on this phse- 

 nomenon, they imagined that the angle subtended by the 

 moon was really increased, by a refraction of her rays, in 

 passing through the vapours contained in the air near the 

 earth's surface. But as soon as it became known that she 

 subtended the least angle at the eye when her apparent mag- 

 nitude was greatest, philosophers said that the eye was im- 

 posed upon by the long series of objects interposed between 

 the eye and the extremity of the sensible horizon. And 

 after it was discovered that the same phaenomenon was ob- 

 served at sea, where no land objects could be seen, they 

 blamed the clouds for deceiving them ; and when the clouds 

 flew away, the spirit of inquiry flew after them, to seek for 

 information in the apparent concavity ot the sky. 



Such were the erroneous opinions maintained -by men of 

 the greatest celebrity, both in antient and in modern times* 

 To mention any more of them would be useless labour : a 

 single quotation from Dr. Snnth's Optics will, I presume, 

 be quite sufficient to show how little was known of this 

 Jnatter at the time when he wrote. 



After the professor had finished his explanation of this 



♦ In Mr. Nicholson's Journal. 



phDenomenon> 



