nr 
vw 
NATURE FOR ITS OWN SAKE 
Sky waves. 
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 ina 
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 
T 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 isan 
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 
