VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 6l J 



meter, or within an evacuated glass ball, but would also electrify the glass on the 

 outside ; I immerged a piece of dry glass in a basin of mercury, and found that by 

 taking it out the mercury was electrified minus, and the glass electrified plus to 

 a considerable degree. 1 found also that amber, sealing-wax, and island crystal, 

 when taken out of mercury, were all electrified positively.* How does it then 

 appear that the electricty which was observed on rubbing the last mentioned sub- 

 stance after it was taken out of mercury surrounded by ice, was owing to cold, 

 and not to the friction between it and the mercury in taking it out ? Island 

 crystal when warm is a non-conductor, and all non-conductors may be excited 

 with proper rubbers. 



LXXTI. An Attempt to assign the Cause, tvhy the Sun and Moon appear to 

 the naked Eye larger ivhcn they are near the Horizon. By Mr. Samuel Dunn. 

 p. 462. 



The sun and the moon when they are in or near the horizon, appear to the 

 naked eye, so very large in comparison with their apparent magnitude, when they 

 are in the zenith, or somewhat elevated, that several learned men have been led 

 to inquire into the cause of this phenomenon, antl after endeavouring to find 

 certain reasons founded on the principles of physics, they have at last pronounced 

 this phenomenon as a mere optical illusion. 



The principal dissertations conducive to give any information on this subject, 

 or helping to throw any light on the same, have been those printed in the Trans- 

 actions of the Royal Society, the Academy of Sciences at Paris, the German Acts, 

 and Dr. Smith's Optics ; but as all these accounts had not given Mr. D. satis- 

 faction, curiosity induced him to inquire after the cause of this singular phenome- 

 non in a manner somewhat diflrerent from what others had done before, and by 

 such experiments and observations as have appeared pertinent. 



From the common appearance of the sun near the horizon, and other like 

 circumstances, Mr. D. first began to suspect that a sudden dip of the sun into the 

 horizontal vapours might some how be the cause of a sudden apparent change of 

 magnitude, though the horizontal vapours had been disallowed to be able to 

 produce any other than a refraction in a vertical direction ; and reducing things 

 to calculation, he found that, from the time when the sun is within a diameter or 

 two of the horizon, to the time when he is a semi- diameter below the horizon, the 

 sun's rays become passable through such a length of medium, reckoning in the 

 direction of the rays, that the total quantity of medium, (reckoning both depth 



* A small quantity of an amalgama, or mixture of mercury and tin, with a very little chalk or 

 whiting, being rubbed on the cushion of a globe, or on the oiled silk-rubber of a tube, will excite the 

 globe or tube to a great degree, with very little friction ; especially if the rubbers be made more 

 damp, or dry, as occasion may require. — Orig. 



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