278 



• KNOWLEDGE • 



[Sept. 22, 1882 



CLOUDS IX THE AIR. 

 Bt the Editor. 



(Continued from page 200.) 



IN our last we considercil only how a horizontal layer of 

 mistlaJen air would appear, showing how the line of 

 ii^ht towards the horizon would pass through a much 

 longer range of such a layer than a line towards the point 

 overhead. The etVect in making the sky near the liorizon 

 niiBty is great in that case, but there is a much more 

 -marked elFect in the ease of a sky over whicli cumulus 

 clouds are scattered. 



There is a picture familiar to my memory, but I cannot 

 at tJie moment "place" it (I think, however, it is in 

 " Kaemtz' Meteorology," a book I have not seen for many 

 years), in which the effect of foreshortening in bringing 

 rounded clouds apparently closer together is very simply 

 shown. 



Suppose AZ (Fig. 1 ) to be a layer of cumulus clouds at 



equal distances from each other, tlie cloud at Z being over- 

 head for the ol»server at O, OH his horizon. Then if 

 lines are drawn from O to touch the sides of the clouds 

 shown at Z. 1. l", .», 4, ."., C, Ac, we see how the space 



a continuous layer. This layer, though really formed by 

 the under surfaces of the horizontal layer of clouds, 

 I appears to the observer at to rise vertically, or nearly 

 I so, from the horizon. 



I There is a rather pretty way (which I have not yet seen 

 ' described) of showing liow the sky whitens, in such a case, 

 I towards the horizon, assuming tlie clouds to bo roughly 

 I spherical, and strewn in reality with a certain general 

 uniformity throughout the layer which they form. It 

 j depends on the geometrical property tliat if a sphere be 



placed on a plane, and a fixed point on the same side of 

 the plane is supposed to be luminous, the shadow of the 

 sphere is an ellipse of unvarying minor axis wherever the 

 sphere may be set on the plane. This can be proved 



enclosed by the two lines touching any cloud licars a con- analytically in a numV)er of pleasant ways; but there is 



stantly increasing proportion to the space left between two also a simple proof which I devised when 1 was an under- 



clouds, until at last — as at ^>, G, 7, 8 — this latter space dis- graduat<! at Cambridge (somewhat to the disgust of the 



appears altogether, and the clouds to the left of this spot, lecturer on plane co-ordinate geometry, who wanted an 



though really as far apart as those near Z, appear to form analytical proof) :— Suppose DE (Fig. 2) to be the plane 



