92 
AStkONOMY: C. G: ABBOT 
Proc. N. a. S. 
- :> DHE ,3i9£le: Investigations Required .jiioxo -.(^iu^jcluiq 
1. Ozone and Radiation. — Very little is known concerning the influence 
of ozone on the earth's radiation. It is certain that a conspicuous band of 
this substance occurs at the wave-length of about 10 microns, where water 
vapor is almost completely transparent. It is a little hard to see how the 
investigation of the influence of ozone can be best undertaken. The 
region of 10 microns is very difficult to work with in the solar spectrum. 
AU terrestrial surroundings give out waves of this wave-length. It is 
as if the investigator of the visible spectrum were troubled with stray 
light from every direction upon his photographic plates or other means 
of observation. Furthermore, and still more serious, the more intense 
rays of the solar spectrum, coming from regions of much shorter wave- 
length, are continually encountered as stray light in the spectrum, even 
hundreds of fold greater in intensity than the rays of the spectrum at 
the region in question. Rock salt must be employed for the optical train 
and its hygroscopic character is a serious difliculty. Finally the measure- 
ments ought to be quantitative in terms of energy. Something can be 
done to avoid stray light by reflecting the solar beam by means of several 
silvered ground glass mirrors. In this way the shorter wave-lengths are 
scattered, while the longer wave-lengths desired are reflected by virtue 
of the greatness of their wave-length, compared with the roughness of 
the mirrors. Yet the prospects of getting good bolographic representa- 
tions of this region of spectrum day after day are not good. Perhaps 
the quantity of ozone existing in the air could be determined daily by 
measurements made in the ultra-violet spectrum by the aid of photography 
and these results could be compared with measurements of the nocturnal 
radiation made after the method of Angstrom. Allowance being made for 
variations of temperature and water vapor, the influence of variations 
of the ozone might then appear. At all events, determination of the effects 
of ozone on the terrestrial radiation is a desirable investigation though so 
extremely difficult. 
2. Cloud Observations. — The effect of clouds on terrestrial temperature 
is extremely important. Recent measurements by Mr. Aldrich of the 
Smithsonian Institution, made from a military balloon, show that a smooth 
layer of clouds reflects about 78% of the total direct and indirect solar 
radiation incident upon it from the sun and sky. Investigations of the 
quantity and distribution of the cloudiness of the surface of the earth 
have been published by Tisserenc du Bort, and Arrhenius. They de- 
pend, however, on very imperfect material. Cloud observations are 
much more fragmentary than temperature observations, and besides 
are not based upon standardized instruments in general, even if there be 
such, but are based on observers' estimates of the proportion of the sky 
obscured. The matter is so fundamental to the proper understanding 
