Mr. Ivory on Atmospheric Refraction. 177 



I considered that if this could be accomplished, more time 

 would be afforded for the fluid and gunpowder to be in contact, 

 and the latter, in consequence, more likely to be ignited." But 

 this is not the case ; the electric fluid does not come at all into 

 contact with the gunpowder when it is not ignited by the ex- 

 plosive discharge. For this substance is a non-conductor of 

 electricity, and the scattering and dispersing of it is not caused 

 by any direct impetus or action of the electric fluid itself on 

 the gunpowder, but by the rapid expansion and displacement 

 of the air, which is driven in every direction with considerable 

 force by the electric fluid from the point whence it explodes. 

 This is, I believe, the true solution of the difficulty which has 

 been so long experienced of inflaming loose or slightly con- 

 fined gunpowder by the explosive discharge; and it is con- 

 firmed by the circumstance, that if a portion of any quantity of 

 gunpowder be bruised and mixed with the rest, and the whole 

 be well and closely confined so as to exclude the air as much 

 as possible, an explosive charge of moderate strength on its 

 transmission through the circuit will be found to inflame the 



o 



gunpowder. 



I have pleasure in perceiving that Mr. Sturgeon has realized 

 an experiment which I had long contemplated, but had not 

 time and opportunity to execute, — the discharge of guns by 

 electricity ; and I hope he will be successful in his attempt to 

 render the decomposition of water by the same agent more 

 easily practicable. I am, sir, 



Your obliged servant, 



Hereford, Aug. 7, 1826. . Thomas Howldy. 



XXV. Atmospheric Refraction at very low Temperatures and 

 Altitudes. By J. Ivory, Esq. A.M. F.R.S.* 



r T , HE 42nd Number of the Quarterly Journal of Science, 

 **- published in July last, contains a great number of refrac- 

 tions observed at small altitudes and very low temperatures. 

 Such observations are of great value, and throw considerable 

 light upon some important questions relating to the tables in the 

 hands of astronomers. We may inquire ; 1 st, Whether any 

 table hitherto published represents the refractions within 2° 

 or l°f of the horizon with tolerable regularity and certainty. 

 2dly, Whether the tables, which, at altitudes above 2° or 

 1°-|, are known to approach near the truth at the usual tem- 

 peratures, continue to preserve the same degree of accuracy, 



* Communicated by the Author. 

 Vol. 68. No. 341. Sept. 1826. Z or 



