ACTINOMETRIC MEASUREMENTS ys) 
To obtain measurements for computing the solar 
constant Abbot has constructed a special Angstrém 
pyrheliometer in which the edge effect is reduced to 
practically zero. However, a detailed survey of the 
factors influencing measurements with this type of 
instrument leads one to conclude that errors of the 
order of 0.3 per cent can hardly be avoided completely. 
The silver-disk pyrheliometer has been extensively 
used, especially in connection with Abbot’s earlier work 
on the solar constant. Courvoisier has subjected it to a 
very thorough theoretical examination, with due regard 
to the physical constants of the instrument. Some minor 
corrections must be applied to Abbot’s theory on which 
the method of measurement is founded. These correc- 
tions may affect the absolute values obtained. What 
interests us here especially are the variable errors aris- 
ing from the variation of different physical and me- 
teorological factors. Strictly speaking, Abbot’s theory 
holds true only when the mstrument is maintained at 
semiconstant temperature durmg the periodical tem- 
perature variation to which it is subjected when alter- 
nately exposed to and screened from solar radiation. 
This puts a rather high demand on the precautions to 
be taken during exposure. 
Further effects arise from the unavoidable fluctua- 
tions of the temperature of the surrounding air, which 
in turn affect the temperature of the cylindrical wooden 
cover by which the silver disk is protected. Courvoisier 
concludes that errors up to 0.3 per cent are possible on 
that account. Mérikofer [7] estimates the average error 
from various causes to be about 0.5 to 1.0 per cent. 
Among the instruments rather widely used, the 
Michelson actinometer is especially convenient, since 
_ it is rapidly read and does not require auxiliary instru- 
ments. Its accuracy is less than that of the Angstrém 
or the silver-disk instruments, the average error prob- 
ably amounting to +1.0—-1.5 per cent. The Michelson 
actinometer should be checked at regular intervals 
since it is sometimes subject to deterioration. As the 
accuracy of the instrument seems sufficient for general 
meteorological purposes, it would be highly desirable 
that mstruments of a similar construction should be 
built in greater number and at moderate cost. 
Small differences exist between the indications of all 
actinometers in practical use for measuring direct solar 
- radiation. They arise from the fact that the instruments 
receive not only direct solar radiation but also diffuse 
radiation from the sky in the immediate neighborhood 
of the sun, within certain aperture angles which vary 
with the instrument. Thus the silver-disk pyrheliometer 
has an aperture of 6° (formerly 10.5°), while the Ang- 
strém compensation pyrheliometer has a rectangular 
aperture with plane angles of 24° and 6° (more recently 
10° and 3°), respectively. The Angstrém instrument 
consequently receives in general somewhat more scat- 
tered light, and the difference between the readings of 
the Angstrém (A) and the silver-disk (S) instruments 
can be approximately expressed by 
4 A—S=—A+ f(g, 8B, a, m), (7) 
where A is the difference which is to be expected at the 
limit of the atmosphere where no diffuse radiation is 
added to the direct radiation, and f (q, 8, a, m) is a 
function of the quantities qg, etc. (notation as given 
earlier). The function f (q, 8, a, m) is equal to zero when 
q equals unity and 8 vanishes. According to experience 
recently acquired, this function seems to vary, at 
stations near sea level, between about 0.01 cal em 
min for low values of 6 to about 0.02 cal em)? mins" 
for high values of 6. The difference S — A therefore 
seems to vary between about 1.5 and 4.0 per cent, 
where the upper and lower limits correspond to extreme 
conditions (Angstr6m—uncorrected for edge effect; sil- 
ver-disk—Smithsonian scale, 1934). 
With due regard to recent investigations, we may, 
chiefly according to Mérikofer [7], give the following sur- 
vey on the status of absolute actinometry. Setting 
the indication of the Angstrém pyrheliometer equal to 
100 per cent, we have: 
a Instrument Per cent 
Angstrom |((umcorre cued) ps ee a 100.0 
Angstr6m (corrected for edge effect)....... 101.3 + 0.5 
Smithsonian scales Ol Suan eee seen 103.5 
Solar Radiation Commissions (preliminary re- 
Sallis) UGE (GSB)... ce onccocceussasconee 101.0 
Guild, Physical Laboratory, 1934.......... 101.3 
Abbot (Smithsonian scale, 1934)........... 101.2 
Actinometric Equipment at Meteorological Stations. 
It would be futile to attempt actinometric measure- 
ments, even at selected meteorological stations, for 
the purpose of computing the solar constant, and still 
more so for determining its possible variations. The 
required elaborateness and quality of the actinometric 
equipment are such that measurements for this purpose 
must be reserved for observatories with very high 
standards, at locations carefully selected with regard 
to atmospheric conditions. 
On the other hand, the chief problems which actino- 
metric observations from a meteorological network 
might help to solve are those which we have briefly 
discussed in the first section of this article. Thus, the 
measurements should (1) furnish data for an evaluation 
of the heat exchange at the surface of the earth and 
enable us to treat, on this basis, the fundamental prob- 
lem of the energy transport withm the atmosphere; 
(2) provide the means for a general optical analysis of 
absorption and scattering in the atmosphere; and (3) 
furnish the basis for correlation studies of the relation- 
ship between the radiation processes at the earth’s 
surface and a number of biological phenomena such 
as the growth of plants, crop conditions, the health of 
man, and the incidence and spread of certain diseases. 
It is clear from what has been said above that the 
accuracy required of our actinometric equipment for 
these purposes is much less than that for the determina- 
tion of the solar constant. The accuracy and case of 
operation of our present instruments are undoubtedly 
satisfactory for these general purposes. 
The details of the organization must be based on a 
realization of the main problems, on experience or 
knowledge, otherwise gained, concerning actinometric 
instruments and their qualities, and finally on some 
acquaintance with meteorological observations in gen- 
