138 REPORT — 1891. 



condensers are substitated the spark becomes thicker and brighter, and 

 its minor irregularities frequently disappear. 



Next remove the condensers from the machine, and connect their 

 inner coatings with the prime conductors, while the outer coatings are 

 imperfectly insulated, as, for instance, by placing them on a wooden 

 table. If the jars are near each other, as each spark passes between 

 the discharging knobs another will pass between the outer coatings. 



Gradually increase the distance between the jars. The spark be- 

 tween the outer coatings will become more irregular as it grows longer, 

 and at a certain distance it will suddenly cease. At this moment the 

 discharge between the knobs entirely alters its character. If the strik- 

 ing distance is short, the form assumed is that of a bright pink band, 

 generally brighter at its margins than elsewhere, and showing a beauti- 

 ful fluted structure. Its duration is short, but it is nevertheless easy to 

 see that it is a really intermittent. 



Again increase the striking distance step by step. The discharge is 

 still intermittent, but thin, brilliant white sparks make their appearance. 

 At first the pink discharge can be recognised passing obliquely between 

 these bright sparks, but as the distance increases the pink light disap- 

 pears, and. the discharge becomes a rapid volley of bright sparks. 



The photographs from No. 1 E to No. 9 E show these phenomena. 



Again, if the discharging knobs are placed some distance from the 

 machiue, so that the field due to their charge is but little afiected by the 

 movements of the machine or operator, it may often be noticed that with 

 ordinary bright sparks their form is repeatedly the same. No. 10 E shows 

 a series of sparks taken under such conditions at intervals of about one 

 second. 



Now, it is probable that all these forms of discharge have their 

 analogues in lightning. The bright sparks with small condensers are 

 the counterpart of the commoner type of lightning. Those from the 

 large Leyden jars and between the outer coatings correspond to more 

 powerful flashes, the latter being the 'impulsive discharge' described by 

 Professor O. Lodge. The volleys of bright sparks are also the type of many 

 observed multiple flashes. Thei'e remain only the pink discharges, and 

 surely these are the counterpart of the flashes which yield photographs 

 like No. 4 B. 



Moreover there seems to be no prima, facie absurdity in supposing 

 that a short series of flashes may occur during a brief time along parallel 

 paths. Such a phenomenon is conceivably explicable — 



(a) by an identity of conditions over the whole area traversed by the 

 flashes ; 



(b) by the movemeut of the chai'ged cloud causing the conditions 

 which held in one place at a given moment to hold a short distance 

 away at another ; 



(c) by the movement of the air sweeping along the disturbance 

 caused by the first spark, so that a path of least resistance resulting 

 from that disturbance occupies difl"ereut positions. Your committee 

 would draw attention to the similarity between the appeai'ance of the 

 bright pink discharge and that through rarefied air. Some of the dis- 

 charges, Nos. 3 E to 7 E, look as if the passage of the bright sparks caused 

 a partial vacuum between them, and the pink sparks then struck through 

 this lessened resistance along the paths of the bright sparks and across 

 the low resisting interval between them, the slope of these transverse 



