Vol. 6, 1920 
ASTRONOMY: C. G. ABBOT 
93 
of the temperature of the world and its dependence on the balance of 
radiations of the sun and earth that it warrants careful attention. Some 
simple autographic means, theoretically satisfactory and instrumentally 
accurate, ought to be provided for estimating cloudiness, and this should 
be installed with the least possible delay at all first-rate meteorological 
stations in the world, and indeed as extensively as possible. The instru- 
ment should not be merely applicable to land conditions, but ought to 
be available for ships. It is hardly possible to overestimate the importance 
of this neglected branch of meteorology. rjOH?i: >u> 'v.^:.{h: 
3. Standard Instruments for Nocturnal Radiation. — As mentioned 
above. Dr. Angstrom has employed the pyrgeometer for an extensive and 
valuable research on nocturnal radiation and the determination of at- 
mospheric re-radiation at different temperatures and under various other 
meteorological conditions. There has also been devised at the Smithsonian 
Institution, by Messrs. Abbot and Aldrich, an instrument called the 
pyranometer, which is applicable for the same purpose. Neither of these 
instruments, however, is thoroughly satisfactory, for this reason: They 
both present blackened flat surfaces which radiate towards the atmosphere 
and space, and it is assumed in the theory of the instrument that the 
absorbing and radiating properties of the blackening upon these strips is 
known. Experiments, however, seem to show that the materials used 
for the blackening purposes become less and less satisfactory for the longer 
wave-lengths, so that for rays beyond 15 microns there is a strong chance 
that the instruments are very imperfect. In order to really cover the 
objection it is necessary to produce a nocturnal radiation instrument which 
employs the well-known hollow receiver principle of the "black body." 
This is a matter of some difficulty because the intensity of the radiation 
to be considered is small, and it must be emitted and received at all angles 
within a hemisphere. There are very formidable difficulties in the way 
of a satisfactory solution of this requirement, but in order to give really 
definite information as to the nocturnal radiation of the earth and at- 
mosphere they must be overcome. Associated with this problem is a 
definitive solution of the question of the constant of the fourth power 
radiation formula of Stefan. The measurements hitherto made by physi- 
cists on this constant, usually called c, are in considerable disaccord. The 
margin of possible error is, perhaps, not very large, but it cannot be allowed 
as certain that we know this constant within 2%. 
4. Statistical Studies. — As already stated, the sun is known to be a 
variable star. The effect of its variations on the terrestrial climate 
has lately been investigated to a considerable extent by Mr. H. H. Clayton^ 
of Buenos Aires. Investigations of a similar kind have been made also 
by Dr. Arctowski of New York and by Dr. Helland-Hansen and Dr. 
Nansen^ of Norway. These investigators have all reached the conclu- 
sion that the variations of the sun reported by the Smithsonian observers 
