42 On the Formation of Hippie-mark, [Nov. 22, 



Fia. 13. 



It seems probable that what is called a mackerel sky is an evidence 

 of a closely similar mode of motion ; and this agrees with M. de Can- 

 dolle's suggestion that cirrus is aerial ripple-mark. The layer of 

 transition between two currents of fluid is dynamically unstable, but if 

 a series of vortices be interpolated, so as to form friction rollers as it 

 were, it probably becomes stable. It is likely that in air a mode of 

 motion would be set up by friction, which in frictionless fluid would 

 be stable. If one of the currents of air be colder than the other, a 

 precipitation of vapour will be caused in the vortices, and their shapes 

 will be rendered evident by clouds.* 



The direction of striation and velocity of translation of mackerel 

 clouds require consideration according to this theory. If the velocity 

 of the upper current be u, and of the lower current be —u, the 

 interposed vortices have a velocity zero, and have their axes perpen- 

 dicular to the velocity u. Hence if relatively to the earth the rect- 

 angular components of the upper current are u + w, v, and of the 

 lower are — u + w, v, the component velocities of the vortices are w, v, 

 and their axes are parallel to the component v. 



Therefore the striations should be parallel to that direction in which 

 the two currents have equal components, and the component velocity 

 of the clouds parallel to the striations should be equal that of either 

 current in the same direction. The resultant velocity of the clouds is 

 clearly equal to a half of that of the two currents, and the component 

 velocity of the striations perpendicular to themselves is the mean of 

 the components of the two currents in the same direction. 



[If one of the currents veers the axes of the vortices are shifted. 



* I had suggested that the centrifugal force of the vortical rotation must 

 pro luce rarefaction, fall of temperature, and precipitation of vapour. I have to 

 thank Professor Stokes for pointing out that the fall of temperature would neces- 

 sarily be so small as only to cause precipitation if the air were almost completely 

 saturated. I think the fall of temperature at the centre of the vortex might be 

 between a hundredth and a fiftieth of a degree Centigrade. [Sir W. Thomson tells 

 me tbat he had given this vortical explanation before the B. A. in 1876. The 

 volume, however, gives no abstract. He explains the formation of cloud as due to 

 the upward motion in the vortices, and consequent rarefaction.— Jan. 4, 1884.] 



