NATURE FOE ITS OWN SAKE 



unchanged as ever. It never seems to move, 

 it never seems to shift ; and yet again, it is 

 far from being an unvarying appearance. Sir 

 Isaac Newton discovered years ago, from the 

 twinkling of the stars and the shaking of shad- 

 ows cast by high towers, that "the air is in a 

 perpetual tremor/' Down close to the ground 

 on a hot day we can see, in little, this tremor of 

 the air as the heat currents rise from the earth ; 

 and the mixture and intermixture of hot and cold 

 currents in the upper air, the blowing of winds, 

 and the drift of clouds must shake and disturb 

 the lower layers of the blue, though this dis- 

 turbance is not often noticed by us. At times 

 I have seen, or fancied I have seen, in studying 

 the clear sky, what might be called waves mov- 

 ing across it. The motion did not seem to be 

 that of ringed waves, such as one sees when a 

 stone is thrown into a pond, but of deep undu- 

 lations of varying blue succeeding each other 

 slowly like the heave and roll of a glassy sea. 

 Only on very hot days has this effect been ap- 

 parent ; and I would not be certain that it is an 

 actual fact, for the eye after long gazing at 

 light and color is liable to become confused and 

 see falsely. Still, I have seen the appearance a 

 number of times, and I believe it to be reality 



