238 



KNOWLEDGE. 



[OcTOBHl 1, 1896. 



often seen swirls of cloud ranued in parallel rows, each 

 swirl liite a single puff of smoke, the swirls in each row 

 growing in size and thus getting closer together until a 

 continuous bar was formed. 



With a veering wind the direction of rippling is changed, 

 and a barred may be changed into a dappled sky, the 

 existing clouds being farrowed and cut into diamond shape. 



An air billow and an eddy, or vortex, is formed when 

 wind blows over a sharp ridge — an instance of the rule that 

 an eddy is formed behind any sharp-edged body which 

 obstructs a current. In air, however, a single obstacle 

 does not seem to give rise to a train of waves, such as is 

 formed to leeward of a boulder in a shallow stream, but 

 only to one, conspicuous, billow. Air diffuses more rapidly 

 than water, and its inertia is much less, so that a gravi- 

 tation wave in air must, I apprehend, die out quickly. 

 The vortex in the lee of a mountain top exposed to wind 

 produces the banner cloud, a phenomenon very charac- 

 teristic of Alpine districts. Fig. 5 is from a sketch of the 

 banner cloud of the Eiger taken at Grindelwald, April 



blue sky. The intei'spaces are not regular as in the case 

 of a wave motion, but there is a rough proportionality 

 among the clouds. 



The raising of the sea by wind is a case of rippling at 

 the boundary of two lluida in relative motion. The differ- 

 ence of density, however, is so groat that mixing of the 

 two lluids cannot take place in the vortex in the usual 

 manner, but is achieved by spraying from the crest of the 

 water wave. In the be of each water billow there is an 

 eddy of air which, with a high sea, may become a source 

 of danger to ships by taking the wind out of the sails. 

 Above the water billows and the wind eddy the air is 

 doubtless itself in undulations, as Helmholtz pointed out ; 

 but the peculiar character of sea waves, which are con- 

 tinually dying out and creating new ones behind them, as 

 well as the complicating effects of their free run by 

 gravitation, probably prevents the air billows from attaining 

 any regularity. 



According to Plelmholtz, undulations of air, not neces- 

 sarily accompanied by cloud formation, are of ordinary 



Cloud Ripples. From a Photograph in Knowledge 



21st, 189G, at 2 p.m. The swirling motion of the air was 

 well shown in the convolutions of the cloud. Banner clouds 

 frequently have the shape of long streamers. A well-known 

 example in England is the " helm bar " often seen to the 

 ■west of Cross Fell when the wind is from the east. The 

 forms of cumulus cloud are the result of swirling motion 

 of the air. Overhead the cumulus appears nearly circular, 

 but when nearer the horizon it is seen to be an accumu- 

 lation of kidney-shaped masses. These are sure evidence 

 of vortices : an excellent example of this may be seen in 

 the kidney-shaped masses of black smoke near the mouth 

 of a steamer's funnel. This unsightly smoke also serves 

 to show the strong wind-eddy abaft the funnel. At a 

 certain position the smoke whirls violently towards the 

 funnel, in the direction of the ship's motion. 



I presume that the swirling motion of ascending currents 

 of air accounts for the circumstance that the morning's 

 sun does not spread a thin tilm of vapour over the sky, 

 but, instead, produces balls of cumulus separated by 



occurrence, and with high winds their dimensions are very 

 great, often some mdes in wave-length. Such waves, even 

 if originating at the height of a mile, would stir the air at 

 the earth's surface, and this has been suggested as a cause 

 of the gusts of wind, often accompanied by showers, which 

 occur at intervals in squally weather. 



The rippling of sand by wind has been alluded to. 

 Deserts have often been compared to a sea of sand, and 

 the likeness is real as well as apparent, for the sandhills 

 are billows raised by the wind. Being more enduring, 

 they grow to a greater size than ocean waves. In a strong 

 breeze a stream of sand flies from the crest of each dune, 

 just as the spray flies at sea. 



The wind also ripples snow, an effect which has been 

 shown by Mr. J. Wolff in a picture of "Ptarmigan in Winter," 

 No. 130 of the Prescott Hewett Gift, South Kensington. 

 I have even seen a gale of wind ripple the froth of the 

 breakers when the return of the wave has lefc the froth 

 nearly stranded on the shore. 



