PREFACE 



The prosecution of the researches described in the following pages 

 has been rendered possible by several grants from the Hodgkins 

 Fund of the Smithsonian Institution, Washington, for which I here 

 desire to express my deep gratitude. 



I also stand indebted to various gentlemen for friendly help and 

 encouragement. 



In the first place, I wish to express my sincere thanks to my 

 esteemed friend, Dr. C. G. Abbot, Director of the Astrophysical 

 Observatory of the Smithsonian Institution, for the great interest 

 he has shown in my researches. His aid and suggestions have ever 

 been a source of stimulation and encouragement, while his criticisms 

 of my work have never failed to be of the greatest assistance to me. 



Other scholars, to whom it is largely due that the observations 

 upon which this study is based have been so far brought to a success- 

 ful termination that I have been able to draw from them certain con- 

 clusions of a general character, are Dr. E. H. Kennard, of Cornell 

 University ; Professor F. P. Brackett, Professor R. D. Williams, and 

 Mr. W. Brewster, of Pomona College, California. To all these gentle- 

 men I wish to express my sense, of gratitude and my earnest thanks 

 for the valuable assistance they have afforded me in my investiga- 

 tions during the expedition to California. 



Ultimately, the value of the observations of nocturnal radiation 

 here published will be greatly enhanced by the fact that the tempera- 

 ture, pressure, and humidity of the atmosphere, up to great eleva- 

 tions, were obtained experimentally by balloon observations made 

 during the expedition from points at or near my observing stations. 

 These observations, made by the United States Weather Bureau 

 in cooperation with the Smithsonian Institution, are given in 

 Appendix I. 



It is also of advantage that observations of the solar constant of 

 radiation, the atmospheric transparency for solar radiation, and the 

 total quantity of water vapor in the atmosphere (as obtained by 

 Fowle's spectroscopic method) were made at Mount Wilson during 

 the stay of the expedition. A summary of these results forms Ap- 

 pendix II. 



