90 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



logarithms of intensities as they would be observed outside our at- 

 mosphere. The inclinations of the representative straight lines give 

 the logarithms of the atmospheric transmission coefficients. What 

 I desire to point out is that the process requires taking out about' 

 300 logarithms, besides plotting and extrapolating. 



In the new instrument as shown in the illustration six 16-inch 

 slide rules are arranged to be set at chosen places and at' right angles 

 to a horizontal linear scale of air masses. The observations are set 

 up by reading the crossline of the sliders against the central movable 

 slide-rule scales, these latter being set with respect to the fixed scales 

 on the sides so as to apply a small correction for sensitiveness of the 

 bolometric apparatus. A stretched Avire is then adjusted to fit the 

 six points as thus plotted. On another slide rule fixed at zero air 

 mass one reading of the crossing point of the wire over the fixed scale 

 gives the intensity as it would be outside the atmosphere, and a 

 second reading on the movable scale gives the atmospheric transmis- 

 sion coefficient. No logarithms or computing are required. 



The equipment of the expedition was all boxed ready for ship- 

 ment to South America when circumstances connected with the war 

 with Germany led to a postponement. Under these circumstances it 

 was deemed best to send the expedition to Hump Mountain in North 

 Carolina, a station at 4,800 feet elevation, where it is now located. 

 This location was chosen with a view to its being at' a great distance 

 from Mount Wilson, in a region where Weather Bureau observers 

 reported uncommonly little cloudiness, and easily accessible from the 

 railroad and from Washington. 



The expedition with over 3 tons of equipment went forward in 

 May, 1917. It is in charge of Mr. A. F. Moore, who is assisted by 

 Mr. L. H. Abbot. Two small frame buildings were erected for the 

 observing and living quarters. The apparatus was set up and ad- 

 justed by Messrs. C. G. Abbot, L. B. Aldrich, and A. F. Moore, and 

 gotten ready for observing about June 15. Unfortunately the most 

 cloudy and rainy summer in the recollection of old residents had 

 been experienced up to August 1. Otherwise, everything is highly 

 favorable to excellent solar- constant work. If war conditions war- 

 rant, the Institution still hopes to send the expedition to South 

 America later, where a station is selected at which 300 cloudless fore- 

 noons for observing per year are to be expected. 



Before leaving this subject I desire to call attention to the remark- 

 able paper by Dr. H. Helm Clayton (Smith. Misc. Collections, vol. 

 68, No. 3) on the " Effect of Short Period Variations of Solar Radia- 

 tion on the Earth's Atmosphere." Dr. Clayton shows by the methe- 

 matical method of correlations, free from all influence of personal 

 judgment, that variations of solar radiation observed by us at Mount 

 Wilson in 1913 and 1914 were reflected in variations of terrestrial 



