404 Intelligence and Miscellaneous Articles. 



B. Rogers, by direct measurement, as equal to that of from 10,000 to 

 13,000 standard candles. 



If we consider the electromotive force of a Bunsen cell equal 

 to the evolution of one cubic centimetre of mixed oxygen and hydro- 

 gen gases per minute, in a circuit of which the total resistance is 

 equal to that of 4400 feet of a round wire ^"th of an inch in dia- 

 meter, made from electrotype copper, and if we assume also the 

 internal resistance of such a cell to be equal to 15 feet of such wire 

 (these are about the average measurements), then the maximum 

 available electrical energy which these 250 cells would evolve would 

 be 



(gg£ = l x (50X4400)' 

 2 2>< , 50x15 



Since about 614 of these units of electrical energy are the equiva- 

 lent of one unit of mechanical energy, we find = 131,000 



614 



foot-lbs. as the mechanical energy equivalent to the light developed. 

 Dividing this by the estimated amount of light, we get 



131000 1Q . 131000 lft , , .» 



— -^- = 13-1, or =10-1 foot-lbs. 



10000 13000 



per minute as the mechanical equivalent of a candle-light, a remark- 

 ably close agreement with the results of Professor Thomsen. — Silli- 

 man's American Journal for March 1866. 



ON THE COMPOSITION OF FORCES. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, Belfast, April 12th, 1866. 



Since the publication of the April Number of the Philosophical 

 Magazine, in which you did me the honour to insert my communi- 

 cation " On the Composition of Forces," I have perceived that the 

 demonstration of (4) proposed in the last paragraph of the note at 

 foot of page 251 is inconclusive, as Prop. II. is not applicable to the 

 case of the two pairs of forces referred to in that paragraph of the 

 note ; I therefore beg leave to withdraw that paragraph. But the 

 proof of the same subsection (4) of Prop. IV., given above in 

 the text, page 250, is unexceptionable and not much longer; and 

 therefore this method of completing Laplace's proof is, I conceive, 

 still perfect. 



I have the honour to be, 



Your obedient Servant, 



John Stevelly. 



