404 Intelligence and Miscellaneous Articles, 



ON AN OPTICAL PHENOMENON. 

 BY PROFESSOR ROBERT BALL, ESQ. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 Perhaps some of your readers may have never noticed the produc- 

 tion of the phenomenon of mirage by a source of heat other than 

 that of the sun. It was, however, very vividly shown under the fol- 

 lowing circumstances, 



From the deck of a steamer I was watching the rising moon, 

 when it happened that the line of vision grazed the heated funnel. 

 Instantly the light from the moon appeared reflected from the black- 

 ened surface, in fact it was difficult to conceive that the effect was 

 not produced by a brilliant mirror. The moon was distant from the 

 side of the chimney by an angle of about twenty minutes when thus 

 seen. Attention once directed to the matter, the same phenomenon 

 was easily recognized by viewing other sources of light under the same 

 circumstances. Thus were observed a distant lighthouse, the planet 

 Jupiter, and on a subsequent occasion the rising sun. 



The appearance is explained by the well-known theory of the ordi- 

 nary mirage ; the necessity of the heated surface in this case was 

 easily verified by noticing that the same phenomenon was not pro- 

 duced by the mast. 



I am, Sir, 



Yours &c, 



Robert Ball. 

 Royal College of Science for Ireland, 

 April 4, 1868. 



ON AN APPARATUS FOR PROVING THAT THE ELECTRIC SPARK 

 DOES NOT PASS THROUGH AN ABSOLUTE VACUUM. BY MM. 

 ALVERGNIAT. 



The tubes for this experiment have two platinum wires at a dis- 

 tance of 2 millims. An almost absolute vacuum is produced in them 

 by means of the mercury air-pump which we have constructed. Half 

 an hour is sufficient for this purpose. The tube is then heated to dull 

 redness, either by means of charcoal or by means of a furnace for 

 organic analysis. The latter means renders it possible to raise the 

 heat slowly and gradually to redness without incurring the danger 

 of cracking the tube. When the tube begins to glow we continue 

 to exhaust, and allow the spark to pass until it ceases to do so. The 

 connexion of the tube with the lamp is then broken by sealing it. 



In a tube so arranged, notwithstanding the slight distance between 

 both platinum points, the passage of electricity completely stops. — 

 Comptes Rendus, vol. lxv. p. 963. 





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