October 1, 1896.] 



KNOWLEDGE, 



237 



the next. In this stage the vortices would seem to be in 

 the act of degrading the ripple mark, but they are not 

 either of them at their maximum of intensity, and the 

 time during which this holds good is exceedingly short 

 compared with the whole semi-period of oscillation." The 

 form given in the figure (Stage IV.) of the ripple mark in 

 the mean position of the crest is not exactly the true 

 shape, which is flatter in the troughs and sharper on the 

 ridge, with hollowed sides and a knife edge at the crest. 

 The form at either end of the swing, a sloping back and a 

 steep lee face, is like that of the rippling produced by wind 

 when blowing over loose sand ; the form is homologous 

 with that of water waves raised by a wind which keeps 

 constant in direction. On the other hand, I have frequently 

 noticed the symmetrical ripple-mark form of the waves in a 

 cross sea : for instance, when sailing round headlands. 

 With an off-shore wind two sets of waves grow in the bays 

 on either side, and, travelling seawards, meet and pass 

 through each other just off the headland. The momentary 

 combinations of the opposing billows give the steep, 

 symmetrical waves. They make what sailors call a " nasty 

 lop " off' headlands, even when there are no tidal currents 

 or rocky ledges to increase the turbulence of the water. 



Rippling is generally produced at the surface of two 

 fluids of different densities which are in relative motion. 

 A curious example is that of tar and water. If water be 

 poured upon the tar, and the vessel be rocked, the surface 

 of the tar is quickly rippled, and the sticky crests of the 

 ridges are jerked backwards and forwards in clumsy imita- 

 tion of the dance of the sand in the seashore ripples. 

 More important, however, is the rippling which occurs 

 between layers of air of different density when the upper 

 and the lower layer have different motions. The visible 

 evidence of such rippling is the formation of beautiful 

 parallel bars of cloud, in which, as Mr. Kmkin wrote long 



Fig. 3.— Foinvition of Cloud Ripples. 



since, " the vapour .... falls into ripples like sand." 

 (" Modern Painters," Vol. V., Part VII., chap, i.) 



The condition at the flat surface of two fluids of different 

 densities and capable of mixing, is unstable when the fluids 

 are in relative motion, the form of the surface being liable 

 to undergo great change in a sudden and perhaps tumul- 

 tuous manner. The two surfaces become corrugated, and 

 between them are interpolated vortices, which act as friction 

 roUers, enabling one surface to glide smoothly over the 

 other. In these vortices the two fluids mix (sir Fig. 3). 



Now the mixing of two airs of different temperature is a 

 well-known method of producing condensation of vapour 

 with formation of a cloud. Probably, therefore, the parallel 

 bars of cloud mark the position of the vortices in air 

 rippling — not, as some have supposed, of the crests of the 

 waves of the lower layer of air (Fig. 3). Once the atten- 

 tion has been drawn to these bars one is struck by the 

 frequency of their occurrence. They occur chielly in the 

 upper regions of the cloud world. The wave-length is 

 remarkably constant in any one group of bars, but the 



scale of the pattern varies greatly. A distance of one 

 degree to two degrees of arc from bar to bar is a common 

 size for the smaller and better defined ripples, which, for a 

 distance of thirty thousand feet from the observer, corre- 

 sponds to a wave-length of two hundred and fifty to five 

 hundred feet ; but I have seen the larger sort of bars with 

 a wave-length of probably half a mile or more. 



The best defined cloud bars have a sharp edge at one 

 side where the cloud looks thickest, so that the vortex is 

 probably not usually symmetrical, as represented in Fig. 8. 

 I have often noticed the densest part of the bars on the 

 weather side, i.e., the direction from which the clouds 

 drift ; but on some occasions it seemed as if the contrary 





Fig. 4. — Cloud Ripples, as seen from Crrindelwald. 



were the case. Sometimes the bars form in ca-lo purn, a 

 beautiful phenomenon which I have seen on several occa- 

 sions during the present year. As one bar after another 

 appears, one learns to look in front of the lengthening group 

 to catch the first beginnings of the next forming bar, fixing 

 one's attention on the exact spot in the blue sky where its 

 appearance is due. At other times the ripple bars are 

 formed when a wind blows upon a cloud ; this I have 

 frequently noticed to occur in the fleecy clouds which form 

 upon the tops of high mountains soon after the gathering 

 power of the sun has begun to draw up moisture from the 

 ice and snow. 



Fig. 4 is from a sketch made at Grindelwald, April 17th, 

 1896, 7.15 A M. The sky was quite clear at sunrise, then 

 the snow slopes and glaciers began to "smoke," fleecy 



FiQ. 5. — Banner Cloud above the Eiger, from Grindelwald. 



masses collected round the summit of the mountains, and 

 these were then blown into ripples. 



The perfect bar-like form with straight edges does not 

 give one the impression of a swirling motion, but I have 



