Mr. W. Spottiswoode on Stratified Discharges. 537 



times compressed and at others dilated, the phenomena always 

 maintained their characteristic features. 



The condition of the striae here described, whether flowing or sta- 

 tionary, may be comprised under the general term " steady;" and 

 when there is no motion in either direction, they may be speci- 

 fically denominated as " stationary." 



Two questions here presented themselves : — First, what are the 

 conditions necessary for the production of steady striae ? Secondly, 

 what are the conditions and circumstances of the advance and 

 retreat, in other words, of the flow of steady striae ? 



With a view of ascertaining the nature of the distinction between 

 the ordinary and the steady striae, careful observations were made 

 with the revolving mirror. It having been noticed that when 

 the wheel break moved slowly ordinary or irregular striae were 

 produced, and that when it moved rapidly steady striae resulted, it 

 seemed probable that the latter effect might be due to the short time 

 of contact, and to the consequent absence of many of the features 

 described in Part II. of these researches. This is, in fact, identical 

 with the suggestion there made, that the fluttering appearance was 

 due to the unequal duration of the striae themselves, and to the 

 irregular positions of the points at which they were renewed at 

 successive discharges of the coil. And such, in fact, proved to be 

 the case ; for as the speed of the wheel was increased the dura- 

 tion of the discharges diminished ; the image as seen in the mirror 

 became narrower and simpler in its configuration, until, when the 

 steady effect was produced, each discharge showed only a single 

 column of striae of a width proportional to the apparent width 

 of the slit. The proper motion, implied by the inclination of 

 the individual striae to the vertical, was still perceptible, and was 

 directed, as usual, towards the negative pole. 



From a comparison of the number of striae as seen by the eye 

 with those seen in the revolving mirror, it was found that the striae 

 so formed were of the kind called " simple " in former communi- 

 cations . And the phenomena of the flow may therefore be considered 

 to be due to the different positions taken up by the striae in successive 

 discharges. If in each discharge the striae occupy positions in 

 advance of those occupied in a previous discharge, the column 

 will appear to advance ; if the reverse be the case, they will appear 

 to recede. If the positions remain unchanged, the column will 

 appear stationary. 



The following consequence of this explanation of the flow will 

 readily occur to the reader, viz. that the rapidity of the flow will 

 increase with the extent of advance made by the striae in each 

 successive discharge, until that advance amounts to half the distance 

 between two contiguous striae. Before this is attained the flow will 

 have become too rapid to be followed by the unassisted eye, and can 

 only be observed by the aid of the mirror. When this rate of 

 advance has been exceeded, the flow will appear to be reversed. If 

 the rate of advance still continues to increase, the rapidity in the 

 reverse direction will appear to decrease until the advance amounts 



