Dust Figures produced by Electric Sparks. 735 



plate-glass were used. The effect is shown in the enlarged 

 photograph of fig. 18. It is clearly seen that the striae are 

 coarser in the constrictions where the disturbances are more 

 intense and vice versa. A cover-plate was used in producing 

 this figure. 



It is noteworthy that, in the early progress of the sparking, 

 the striae appeared at the far end and at the constriction 

 near it, although the intervening wider spaces were up 

 to then quite devoid of striations. -As sparking con- 

 tinued, the striations were observed to spread from these 

 constricted portions until the whole figure was striated. 

 Photographs of these early and intermediate stages were 

 taken, but the striations were too fine for easy reproduction. 



Fig. 19 shows the effect obtained in iron filings with the 

 same boundaries and cover-plate as for fig. 18. Here the 

 disturbance is so confined that the filings, in spite of their 

 high density, were just sufficiently mobile to allow of a figure 

 being formed. 



Successive Sparks. — Although the simultaneous passage of 

 incident and reflected waves over a given region gave striae 

 in a single direction only, it seemed desirable to try whether 

 crossing striae were possible by the successive passing of 

 sparks at different places. The effect obtained is shown 

 in the enlarged photograph reproduced in fig. 20. The 

 positions of the two sparks may be inferred from the blown 

 spaces near them and the curvature of the surrounding 

 striae. This figure is interesting since it shows that when 

 striae have been formed by one spark, a second spark may be 

 able to produce a second set of striations at an angle with the 

 first, yet without thereby obliterating them. For the t\* o 

 sets of intersecting striae are clearly visible on the print. 



Echelon Reflector. — A step-wise boundary built up of 

 rectangular plate glasses was used to form an echelon 

 to test the possibility of obtaining phenomena analogous 

 to that of the musical echo of a single sudden source. This, 

 however, failed to give any separate evidence of the reflected 

 waves. 



Creeping of Stria? in Tubes. — The transverse striae in tubes 

 were often observed to move bodily along them, usually from 

 the spark, but less frequently towards it. At first the effects 

 presented themselves fortuitously and could not be obtained 

 readily at will. They were afterwards exhibited more plainly 

 and consistently by mean* of bent tubes. One such tube 

 had two 120° bends, the first and third portions of the tube 

 being about parallel. In each of the three portions of this 

 tube the striae moved towards the bends. A second tube was 

 used with one right-angle bend. In this tube the stria? 



