ACTINOMETRIC MEASUREMENTS ys) 
relative to the sun. Most important in the use of the 
instrument is a careful examination of its dependence 
on temperature. Under all circumstances, rather fre- 
quent comparisons with actinometers for solar radia- 
tion measurements must be made. It is difficult to 
avoid errors of less than about +10 per cent for in- 
dividual days and less than +5 per cent in monthly 
means. 
4. Measurements of the outgoing “effective” radia- 
tion. i 
Instruments: Angstrém pyrgeometer with electrical 
compensation. Some of the errors inherent in this in- 
strument are the same as for the compensation pyr- 
heliometer. A variable edge effect introduces errors up 
to about +3 per cent in single measurements. No 
recording device of suitable design can yet be recom- 
mended for general use. Some theoretical investigations 
by Prohaska and Wierzejewski [9] on the Bellani instru- 
ment for measuring incoming radiation seem to support 
the view that the Angstrém “tulipan” instrument 
founded on a similar principle, that is, overdistillation 
of ether under the influence of outgoing heat radiation, 
may adequately serve its purpose of performing time 
integration of “effective” radiation. The necessary pre- 
cautions seem, however, to be too numerous to allow a 
more general use. 
5. Records of hours of sunshine. It is recommended 
that the duration of sunshine be recorded at all stations 
where the total radiation from sun and sky is recorded. 
Special studies of the relationship between the duration 
of sunshine and the incoming radiation at various 
stations will then provide a possibility of computing 
the incoming radiation from the hours of sunshine 
Q, according to some formula, for instance, Q, = 
Qe + (1 — a)S], where the constants Qo) and a must 
be determined. Such studies may give us a means of 
interpolating radiation values for localities between ac- 
tinometric stations from the number of hours of sun- 
shine recorded at a much larger number of places. 
Instruments: Most widely used are the sunshine 
recorders of Campbell-Stokes and the recording black- 
bulb thermometer used by the United States Weather 
Bureau. It should be emphasized that these instruments 
can under no circumstances be considered as instru- 
ments of precision. It is more important to correlate 
the data from the various types of sunshine recorders 
with the radiation balance than to attempt to stand- 
ardize or correct the instruments so that a precise 
measurement of the loosely defined ‘‘hours of sunshine”’ 
may be obtained. With this guiding principle in mind, 
one may allow rather wide variations in the type of 
instrument. 
Third Order Actinometric Stations. These stations 
should be equipped with sunshine recorders as well as 
with simple recording instruments such as the Moll- 
Gorezynski, Kimball-Eppley, or the Robitzsch type. 
The pyranometers should, however, be checked at regu- 
lar intervals by comparison with secondary standard 
actinometers, either brought to the stations during 
inspection or kept at the station for use on special 
occasions. The frequency of these check measurements 
must, to some extent, be chosen on the basis of ex- 
perience concerning the constancy of the recording 
device, which probably depends on the climate. 
Regular Meteorological Stations Equipped with Sun- 
shine Recorders. In general, it seems advisable that all 
actinometric stations of the second and third orders 
should also be meteorological stations of at least the 
second order. This will facilitate studies of the relation 
between radiation and other meteorological elements. 
A rather wide network of stations recording simply 
sunshine duration is desirable so that they may act as 
interpolation stations with respect to the radiation 
balance. However, it is also recommended that other 
meteorological observations be made at such stations, 
at least observations of cloudiness, temperature, and 
precipitation. 
CONCLUSIONS 
Critique of Routine Actinometric Measurements as 
Hitherto Organized. It is generally realized that actino- 
metric measurements are still in a state in which an 
organization according to systematic and generally ac- 
cepted rules is lacking. The observations are, as a rule, 
limited to certain single observatories, and there is, in 
general, no possibility for a synoptic treatment of the 
results. Only very seldom are the observations made in 
a manner which permits separation of scattering and 
absorption in the atmosphere. 
The recording devices, such as the Moll-Gorezynski 
instrument or the Robitzsch instrument, when used 
at field stations, are in many cases too infrequently 
checked, a fact which especially for the latter instru- 
ment is rather fatal, since its indications are highly 
dependent on the manner of exposure, and its constant 
is highly variable with temperature. Many obser- 
vational data of no value have been collected in this 
way. 
Measurements of the effective radiation, important 
as they may seem, are almost totally lacking, with the 
exception of the results from only a few observatories. 
Regular checks of the instruments in use are seldom 
made. 
It is my opinion that our knowledge and experience 
of the instruments available are such, after the work 
on the subject particularly by the Smithsonian Institu- 
tion and the International Radiation Commission in 
close collaboration with the Observatory of Davos, 
that the time is ripe for a systematic organization of an 
international actinometric station network within the 
meteorological organization. The technical and instru- 
mental problems which still need to be solved and the 
improvements to be expected need not delay an organi- 
zation which now seems highly desirable. 
Special Problems. We have already briefly indicated 
the problems which such an actinometric network would 
primarily help to solve. They include the important 
problem concerning the energy exchange within the 
earth’s atmosphere and the factors influencing it. If we 
consider a given vertical column extending from the 
