34 
form the basis, as it were, of the cumulous clouds, which are 
the upward terminations of ascending currents. Now, as in 
May the height of this stratum is, according to Mr. Cros- 
thwaite’s observations, greater than in any other month, 
there will be a larger mass of air which can successively 
come in contact with the surface and furnish ozone. But 
we shall only have the full benefit of this ozone when an 
active process of mixture is going on. At night the air in 
contact with the earth is cooler than that above, and there- 
fore tends to lie in a stagnant layer which can often he 
detected by the mist or smoke which it contains. This 
layer will be rapidly exhausted of ozone, and will be filled 
with the organic exhalations from the earth. Hence arises 
the comparatively unhealthy character, and in some climates 
the poisonous nature, of night air, and it may constantly be 
observed that a moderate wind may be blowing above our 
heads, as shown by the motions of the clouds or the wind 
felt on a mountain top, without breaking up the stagnant 
layer on the surface. This is one reason why the air is 
calmer at night than in the day. But during high winds 
the gusts penetrate to the surface and prevent any stagna- 
tion, so that, as I apprehend, during stormy weather the 
deficiency of ozone in the evening would not be observed. 
In the day time, on the contrary, the sun’s heat occasions 
a perpetual circulation or convection in the lowest mass of 
air up to the level of the cumuli. Every portion of the air 
is thus successively brought to the surface and organic sub- 
stances are carried off and oxidated. 
During my own observations on ozone I felt strongly 
the imperfections of the method of measurement alluded to 
by Mr. Baxendell, and I thoroughly agree with him that the 
mysterious variations of ozone will not be understood until 
not only the quantity of air brought into contact with the 
paper be measured or regulated, but the varying source and 
magnitude of supply be considered. 
