BY A. II. S. LUCAS. 555 



einem Male mit Furchen durchzielit, wahrend die Wolken selber 

 nur langsam weiter schreiten. 



" Diese moraentane Entstehen paralleler Reihen von Wolken 

 findet sain vollkommenes Analogon in dem Vorgange, der eintritt, 

 wenn ein plotzlich einfallender Wind eine glatte Wasserflache 

 ti'ifft imd dadurch in einem Augenblicke die Flache mit vielen 

 hunderten von Wellen bedeckt." 



This regular and simultaneous starting into view of the rows 

 of clouds seems again to agree better with condensation by expan- 

 sion in the crests than with mixture of the masses. In the 

 particular exhibition of this form of wave cloud of April '20th 

 the actual formation of the clouds was not witnessed. Mr. 

 Russell noticed them early in the day far away in the W.8.W., 

 when being seen end on as it were, the clouds seemed to form a 

 band of narrow stratus, which on coming nearer resolved itself 

 into Wiilst-Cumulus. 



As points in favour of the wave theoiy of formation of these 

 clouds, may be added — 



(1) That the general trend of the cloud bands was at right 



angles to the direction of the air in the upper stratum. 



(2) That the drift of the clouds was very slow. It took 



nearly the whole of the day for them to pass over 

 Sydney. 



We have no measurements of the actual height at which the 

 clouds passed, but it must have been considerable. 



Mr. Ralph Abercromby in his book "Weather," 1887, calls the 

 Mackerel Sky " one of the rarest skies." Certainly the display 

 of April 20th was on the most unusual scale. The photographs 

 published with von Bezold's paper, and in the " Specola Vaticana," 

 show very much less extensive banding than do those taken by 

 Mr. Russell on this occasion. While the phenomenon of wave- 

 clouds may be seen almost every day on a small scale, it must be 

 only exceptionally that a wide horizontal stretch of air, even at a 

 great height, is absolutely free from disturbance by uprising- 

 currents, and particularly if this be over a diversified land surface. 



