1869.] Lieut. Rokeby on Magnetic Observations. 



397 



in a state of high chemical tension, will, by their tendency to develope those 

 vibrations, either determine the explosion of that substance, or at any rate 

 greatly aid the disturbing effect of mechanical force suddenly applied, 

 while, in the instance of another explosion, which developes vibratory im- 

 pulses of different character, the mechanical force applied through its 

 agency has to operate with little or no aid, greater force, or a more power- 

 ful detonation, being therefore required in the latter instance to accom- 

 plish the same result. 



Instances of the apparently simultaneous explosion of numerous distinct 

 and even somewhat widely separated masses of explosive substances (such 

 as simultaneous explosions in several distinct buildings at powder-mills) do 

 not unfrequently occur, in which the generation of a disruptive impulse 

 by the first or initiative explosion, which is communicated with extreme 

 rapidity to contiguous masses of the same nature, appears much more 

 likely to be the operating cause, than that such simultaneous explosions 

 should be brought about by the direct operation of heat and mechanical 

 force. 



A practical examination has been instituted into the influence which the 

 explosion of gun-cotton through the agency of a detonation, exercises upon 

 the nature of its metamorphosis, upon the character and effects of its 

 explosion, and upon the uses to which gun-cotton is susceptible of application. 



* 



III. u Results of Magnetical Observations made at Ascension Island, 

 Latitude 7° 55' 20" South, Longitude 14° 25' 30" West, from 

 July 1863 to March 1866." By Lieut. Rokeby, R.M. Re- 

 duced by G. M. Whipple, Magnetical Assistant at the Kew 

 Observatory. Communicated by B. Stewart, LL.D. Received 

 March 11, 1869. 



On leaving England for Ascension Island in ITay 1862, Lieut. Rokeby 

 was supplied by General Sabine with the following instruments for the 

 purpose of making observations of magnetical variation and intensity, 

 viz. : — 



A portable declinometer and unifilar for absolute observations of declina- 

 tion and horizontal intensity, a Barrow's dip -circle (No. 24), a differential 

 declinometer, and a differential bifilar. 



The differential declinometer and the bifilar were erected at George Town, 

 Ascension, in August 1862, and bihorary observations commenced; but in 

 consequence of instability in the supports of the instruments, caused pro- 

 bably by the shifting of the volcanic cinders which formed the ground at 

 the observing-station, the observations made exhibit frequent discrepancies. 

 The whole of the bifilar observations, and all the differential declinometer 

 observations prior to June 1864, have therefore been omitted from the 

 present discussion. 



