BY A. H. S. LUCAS. 553 



V)y a rapid motion of propagation, or drift. But with a more 

 moderate velocity of the upper layers, the undulations will be on 

 a smaller scale, and the advance correspondingly slower. In a 

 storm, under the influence of a gale of wind water-waves assume 

 their largest dimensions and advance with a greatl}' increased 

 velocity. When a breeze springs up on a calm day, or as the 

 wind lulls or dies down after a more violent disturbance of the 

 atmospheric equilibrium, smaller, even relatively tiny, wavelets are 

 produced in the water, and these advance with a greatly diminished 

 velocity. It is these lesser wavelets which contribute to the 

 formation of ripple-mai-k, and similai-ly, we conclude, it is the 

 lesser atmospheric waves which contribute to the formation of 

 alto-cumulus clouds. 



The wave-motion becomes visible under certain conditions. 

 When an air stratum is thrown into waves, the pressure of the 

 air in the ridges of the waves will become less, while the 

 pressure of the air in the troughs of the waves will become greater, 

 than the previous mean pressure of the stratum. The air in the 

 crests will then expand, and its temperature will consequently 

 fall. If an undulating stratum be so nearly saturated with water- 

 ^apour that a slight reduction of temperature will be sufficient to 

 cause partial condensation, the parallel lines of cloud so formed 

 will reveal to the eye the actual position of the crests of the air- 

 waves. 



If, as must happen only rarely, the horizontal layers of different 

 densities are uniformly thick and similar over a wide ai^ea, and 

 the horizontally directed wave-motion is not interfered with by 

 uprising vertical currents over a vast stretch of sky, we shall have 

 such a grouping of the clouds as was seen in our typical and 

 magnificently developed Mackerel Sky of April 20th. 



The genei'al idea of this explanation of the phenomenon occurred 

 to me whilst admiring this remarkable spectacle, and I mentioned 

 it to others who were with me at the time, including Dr. Harris, 

 the Head Master of the King's School, and Mr. Robin, M.A., of 

 Newington College. About a month after, I observed from a 

 short notice in " Nature " of an important paper published in the 

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