Mr.W.G. Armstrong on the Electricity of Expanding Air. 133 



writing out an investigation with whose results we are fa- 

 miliar. 



I request you to insert in your February Number the fol- 

 lowing correction : 



Philosophical Magazine, January 1841, page 9, line 6, read 

 as follows : 



" Here a = 0, and the approximate investigation for r^ at 

 the bottom of page 3, does not apply. Remarking, however, 



27r 



that r, is now = 0, the expression for — - V /i"^ + rf is re- 

 duced to —A + : the term corresponding to cos {e^co%0) 

 is cos or 1 : and the term corresponding to - — / (cos e ^ 



cos 6) (from to 2 tt) is — x 2 tt = 1. The expression for 



the intensity therefore becomes 1 + E-^^." 



No alteration is required after line 7, and the results of the 

 investigation are in no way affected. 



I am, Gentlemen, yours, &c. 



Royal Observatory, Greenwich, G. B. AlRY. 



Jan. 20, 1841. 



XXVII. On the Electricity of expanding Jir, as connected 

 with the Electrical Phcenomena of Effluent Steam. By Wm. 

 Geo. Armstrong, Esq. 



To the Editors of the Philosophical Magazine a?id Journal. 

 Gentlemen, 



IN connexion with the experiments which I have recently 

 published on the electricity of effluent steam, it occurred to 

 me that it would be well to inquire whether similar effects to 

 those I have described, could be produced by compressing 

 common air in a receiver, and then suffering it to escape in 

 a jet, in the same manner as steam had been discharged from 

 a boiler in the experiments alluded to. With this view, there- 

 fore, I condensed about eight atmospheres into a strong vessel, 

 the capacity of which was nearly six quarts ; I then insulated 

 the vessel, and discharged the air through a glass tube which 

 I had previously inserted for the purpose. 



On the first trial I obtained no indications of electricity 

 whatever, but upon repeating the experiment on a subsequent 

 day, the insulated vessel became so highly electrified, when 



