54 RADIATION 
eral and with what can reasonably be demanded from 
the observers. On the basis of such considerations the 
following general pattern for the actmometric stations 
within a meteorological network is recommended. The 
proposed organization is based partly on the author’s 
own experience, but is, to a very large extent, the result 
of discussions with colleagues. 
Central Actinometric Observatory (CAO). It seems de- 
sirable that at least one such observatory should be 
established within every large country. Among its main 
objectives would be scientific investigations of actino- 
metric problems of a geophysical character, or those 
with geophysical applications. A plan for such mvesti- 
gations will not be discussed here, since it must be 
based chiefly on the personal initiative and scientific 
ideas of the observatory personnel. Among the prob- 
lems deserving general attention are especially the dis- 
tribution of ozone and its determination by actino- 
metric methods and also special imvestigations of 
ultraviolet radiation. The former problem is at present 
being vigorously pursued by G. M. B. Dobson at Ox- 
ford, and it seems probable that in the near future his 
investigations will result in the construction of appara- 
tus which can be generally accepted for ozone measure- 
ments. The work at the Bureau of Standards by W. 
Coblentz and later by R. Stair and his collaborators 
will probably lead to similar results with respect to the 
measurement of ultraviolet radiation. 
First we shall discuss the relation which should exist 
between the activities of the central actinometric ob- 
servatory and the actinometric stations of the mete- 
orological network, as they will be described below. The 
CAO must form a center for calibration, standardiza- 
tion, and checking of secondary instruments, and for 
this purpose such a central observatory should be 
equipped with absolute mstruments. The actinom- 
eter perhaps most widely used for measuring direct 
solar radiation, the Michelson actinometer, is a second- 
ary instrument and, because of a certain delicacy in its 
construction, it seems desirable that it be compared 
at regular intervals with an absolute mstrument or at 
least with other instruments of greater stability. The 
silver-disk pyrheliometer of the Smithsonian Institu- 
tion is also a secondary instrument. Its construction is 
much less delicate than that of the Michelson actinom- 
eter, and in several instances the mstrument has 
retained its calibration unchanged during rather long 
periods. However, as a precaution against error, if the 
instrument is to be used daily, it should be compared 
from time to time with an absolute mstrument or a 
secondary standard, preferably at least once a year. 
The Angstr6m compensation pyrheliometer is strictly 
an absolute instrument. Its ‘‘constant,’’ which is ap- 
plied to the direct reading in order to yield the cor- 
responding radiation values, can be determined through 
simple physical measurements of resistance, dimensions, 
etc., on the instrument. But in practice such measure- 
ments are very seldom made, and they can hardly be 
carried out without rather elaborate physical equip- 
ment. Therefore, the mstrument is actually used at 
most stations simply as a secondary instrument relying 
upon the correctness of a standardization carried out 
prior to delivery of the mstrument. It is clear, especially 
since the construction is rather delicate, that reliance 
on a single calibration involves the serious risks of 
collecting observational data of greatly reduced value. 
For this reason, a standardization, either through com- 
parison with a central standard or through a physical 
determination of the constants of the compensation 
pyzheliometer, should be carried out at regular inter- 
vals. A central actinometric observatory would be the 
proper place for that purpose. 
Second Order Actinometric Stations. Provided that 
absolute standardizations are carried out at a central 
observatory, second order stations do not need to be 
provided with absolute instruments. On the other hand, 
it seems advisable that they be furnished with double 
sets of actmometers for measuring direct solar radia- 
tion, in order that one instrument can, to some extent, 
serve as a check on the other. The following measure- 
ments are proposed. 
1. Measurements of the direct total solar radiation 
at least three times a day when the sky is clear. 
, Instruments: Smithsonian silver-disk pyrheliometer, 
Angstrém compensation pyrheliometer, or an instru- 
ment of the Michelson type. These are the instruments 
which have been most carefully examined with respect. 
to various errors. However, other instruments may also 
be used, provided they are carefully standardized. 
2. Measurements of direct solar radiation within 
special regions of the spectrum with the aid of colored 
glass filters of the type RG 2 or OG 1, through which 
the infrared radiation may be separated from the visible 
and ultraviolet (at least three times a day with a clear 
sky). 
Instruments: Same as under 1. 
3. Continuous measurements of the total mcoming 
radiation from the sun and sky. 
Instruments: Moll-Gorczynski actinograph with re- 
cording galvanometer, Kimball-Eppley actinograph, 
Angstrém pyranometer with photographic recorder, Ro- 
bitzsch actimograph, or instruments of similar construc- 
tion. 
Screening these instruments from direct solar radia- 
tion and comparing the reduction of their reading with 
the direct solar radiation, measured simultaneously, 
provides a check on these recording mstruments. With 
this method, the first three instruments mentioned 
above will easily yield results accurate to about +5 
per cent for individual days and +3 per cent for longer 
periods. 
With regard to the Robitzsch actinograph, the con- 
struction of which is similar to a common thermograph 
with bimetallic elements, the following remarks may 
be made. The instrumental “constant” (the factor by 
which the deflection on the recording drum must be 
multiplied in order to give radiation values) is highly 
dependent on temperature. A temperature rise of one 
centigrade degree generally corresponds to an imcrease 
in the constant of about 1 per cent. Furthermore, the 
sensitivity is to some extent dependent on the elevation” 
of the sun as well as on the orientation of the mstrument 
