ASTRONOMY: C. G. ABBOT 
Proc. N. a. S. 
proached and asked to take over the work. The establishment of such new 
observatories seems not to be urgent for another year, until the results 
obtained in Chile by the Smithsonian Expedition shall have been com- 
pared with the climate of Argentina, as is now being done by Mr. Helm 
Clayton, so as to see if there is a well-justified expectation that solar ob- 
servations of this kind will be useful in meteorology. 
2. Autographic Cloud Observers. — Looking to the possible establishment 
of such solar constant observing stations, the most important preliminary 
investigation relates to the choice of the most favorable situations. Ex- 
perience hitherto has shown that the reported results of observations of 
cloudiness by meteorological services do not give a sound index of the 
availability of situations for this exacting work. In all of our experi- 
ments we have invariably been disappointed in not observing as many 
days as the reports of the cloud observations have led us to expect. Cer- 
tain portions of the earth, it is true, have the reputation of being cloud- 
less, among them the Chilean Desert, the Central and Western Provinces 
of Australia, some parts of South Africa, Upper Egypt, the Desert of 
Sahara, the Desert of Arabia, regions in British India, and possibly others. 
What would facilitate the establishment of solar observing stations more 
than anything else, in case they should seem desirable, would be to have 
a long series of cloud observations obtained by strictly automatic processes, 
independent of the judgment of observers, for a considerable number of the 
most promising regions, such as those I have mentioned. Such observa- 
tions should cover the whole period when the sun is above the horizon. 
Dr. Humphreys has proposed to me an instrument for such a prelim- 
inary investigation of cloudiness. It is based on the principle of photo- 
graphing a reflecting ball from above. The sensitive surface would be that 
of a moving film or plate which would be exposed for an instant once in a 
quarter or half an hour, as would be most convenient, and would, there- 
fore, give a series of photographs of all the clouds which occurred in the 
sky at the instants of observation. If this or some other suitable automatic 
device for photographing the whole sky could be prepared and employed 
by the meteorological services of the world which operate in the most 
cloudless regions, the work of establishing solar constant stations, if 
these should be needed, would be materially advanced. Otherwise, there 
is no method except to go to a region that is recommended and be dis- 
appointed and sorry that the region was chosen but without knowing 
where else to go, just as has occurred before. This is a disheartening, 
costly and time-consuming process, and might, perhaps, be avoided in 
the way above suggested. 
3. Pyrheliometric Observations. — While the measurement of the solar 
radiation by means of spectrobolometric work is the only thorough way 
available in which to determine the variation of the -Sun, situated as the 
observer must necessarily be beneath an atmospheric sea full of haze and 
