332 F. A. Ferret— T/te Flashing Arcs: 



crater and that the edge alone is visible and therefore appears 

 from any point of view as an are. The movement of this 

 visible portion will therefore be at right angles to the line of 

 sight and the arrival of the sound — from the crater radially to 

 the observer — must be compared with the arrival of the arc at 

 some point equidistant from the crater but at right angles to 

 this radius. As the arcs rapidly fade into invisibility with 

 increasing distance fron their source the difficulty of accurate 

 measurement will seem to be very great. 



But if we attack the problem by the way of exclusion we 

 shall find, I believe, that the velocity — even assigning to it 

 values having a considerable margin above or below that of 

 sound — can be made to harmonize with no other mode of 

 motion. Actual mass movement, i. <?., motion of translation of 

 any material, solid, liquid or gaseous, is negatived by the flash- 

 ing of the arcs amid the relatively slowly rising and perfectly 

 undisturbed volutes of vapor and detritus as well as by their 

 direction of propagation, which is outward and downward as 

 well as upward. On the other hand, any attempt at explanation 

 on the basis of Hertzian or electromagnetic effects, due possi- 

 bly to stress relief, etc. must also be excluded on the speed basis, 

 which now becomes much too low. 



After all is said, is there any real difficulty in accounting 

 for the flashing arcs on the basis of the proposed hypothesis? 

 Sound is propagated in air by compressional-rarefactional waves 

 projected normally. The conditions for the production of the 

 arcs are sudden explosions on a large scale. Given these in 

 sufficiency, may we not imagine that in the resulting aerial 

 condensational-rarefactional wave — or sheet of superposed 

 waves — the refractive and reflective indices will be so altered 

 as to form in bright daylight a zone visible by contrast, espec- 

 ially when viewed longitudinally, i. e., through the edges of the 

 transparent sphere ? We are familiar with the visibility by 

 contrast of the refrangibility of hot and cold air, and it would 

 seem that mechanically engendered compressional-rarefactional 

 aerial waves should be visible in the same manner if sufficiently 

 accentuated. It is a question of the degree of condensation 

 and rarefaction and this evidently depends directly on the 

 power and inversely on the time factor. The proposition will 

 be more truly scientific and possibly more acceptable if stated 

 thus : An explosion propagates normal condensational-rare- 

 factional waves in air — these are perceived by the ear as sound 

 and may also be visible by unequal refraction when sufficiently 

 powerful. 



The writer publishes this brief reference to a phenomenon 

 which he has twice been privileged to witness with the feeling — 

 so common in cases of this nature — that he may be stating that 



