ZIGZAG MOTION OF THE ELECTRIC SPARK. 227 



one is Art. II, signed J. Phoenix, concerning the zig-zag 

 motion of the electric spark ; and the other Art. XI, by 

 Dr. Maycock on the production of electrical excitement by 

 friction, which is the continuation of another in your No. 

 131. These papers concerning electricity have strongly ex- 

 cited my attention, as you may suppose from my papers on 

 the same subject in your Journal. But for the present I 

 shall confine myself to the paper signed J. Pkcenix, on the 

 zig-zag motion of the electric spark. 



The author says (p. 243), "that this subject seems to Cause of the 



■ "have been withheld entirely from public discussion." But 2i ,?»g n)o ; iotl 

 , , . . , J f . ,. , . , ot the spark, 



he immediately mentions the true explanation or this phe- 

 nomenon in the following manner. " The only account I 

 ** have heard in lectures was, that by its own rapidity of 

 " motion it condensates the air to such a degree, that it has 

 " to move, as it were, from a solid to a less dense medium ; 

 " which seems to me impossible.' 1 I shall first consider this 

 rejected explanation with respect, not only to its possibility , 

 but to its sufficiency. 



The electric fluid moves with a great velocity, as we may Capable of 



judge by the sight: and it is such, that we cannot estimate " mp ess,n g 

 •f a J ; » ' • ■ „ the air before 



it, comparatively to that of light ; but it is much denser, as we it, so as to be 



see by the hole that a strong spark produces in a card which re P e ^ led side- 

 is opposed to its course ; it may therefore occasion a sufficient 

 compression in the air before it, so that at last it is repulsed 

 sidewise. 



We have an example of the repulsion in the air itself Example of 

 The instrument called anemometer shows the velocity of the tlus jn the a,r * 

 wind, because the air in motion, finding in it an obstacle, is 

 condensed against it, and thus presses it forward ; bu* if it 

 finds less resistance on one side, it escapes and presses the 

 obstacle sidewise. The immediate pressure of air is shown 

 in the ingenious anemometer of Dr. Lind, described in the 

 65th vol. of the Philos. Transactions, p. 363. This instru- 

 ment consists of a glass siphon, having quicksilver in both 

 its branches, open at their extremities, one of which is bent 

 forward at a right angle. When the siphon is held upright, 

 and the opening of the bent branch is turned towards the 

 wind, the quicksilver is depressed in it, and ascends in the 

 other ? in proportion to the velocity of the current of air. 



As 



