NO. 17 SMITHSONIAN KXPLORATIONS, I916 I33 



As the sun is the ruler of the earth's temperature, and his rays 

 the dependence of all vei^etation, solar fluctuations of five, or even 

 sometimes ten per cent, such as have been discovered in these studies 

 must be important, (ireat need is apparent of checkini^' and com- 

 pleting" the Mount Wilson work at other favorable stations. In 

 1914 Mr. Abbot went to Australia and urged the erection of an 

 observatory for the purpose there, but owing- to the outbreak of the 

 war, the (Government, though favorably inclined, was unable to take 

 the matter up. Importunately it has recently become possible for the 

 Smithsonian Institution itself to undertake the support of a station 

 in South America for observing solar radiation, and this is expected 

 to be installed in July, 1917 by Mr. Abbot. It is hoped to make the 

 solar radiation observations every day in the vear hereafter either 

 at Mount \Mlson or in South America or at both stations. 



I'urther work was done with a solar cooking appliance at Mount 

 \\'ilson in 1916, but owing to the dehi}- until Sej^tember of materials 

 ordered for it and expected in April, no satisfactory tests have yet 

 been made. Food was cooked in 191 5, including meat, potatoes and 

 other vegetables, and cereals. It is confidently expected to bake bread 

 also when the apparatus is done. 



A great drawback to the solar work done hitherto has lain in the 

 tarnishing of the silvered mirror surfaces used to reflect the sun ravs 

 onto the bolometer. This is the more serious because it affects rays 

 of different colors differently. N'iolet and vdtra-violet ravs are most 

 weakened by the tarnishing of silver. At last a new alloy " stellite " 

 has appeared which does not tarnish, even if exposed for months to 

 sun, rain, snow, and smoke. Two stellite mirrors for the spectro- 

 bolometer were introduced on Mount Wilson in 1916, along with a 

 vacuum bolometer of greatly increased sensitiveness. It is now 

 hoped to determine definitely whether all rays of the sun wax and 

 wane in their intensity ]:)roportionally, as the sun varies, or increased 

 solar radiation is preponderatingly associated with special regions of 

 the spectrum. 



Comparisons were made in 1916, of the pyrheliometers used daily 

 in Mount Wilson work with the standard waterflow jwrheliometer. 

 The results showed that no detectable change of the sensitiveness 

 of the secondary pyrheliometers has occurred. \\'e may be confi- 

 dent that the entire series of observations at Mount Wilson, from 

 1905 to 1 91 6, is expressed on a constant scale of radiation to within 

 one per cent. Numerous and varied measurements show also that 

 this constant scale is the true standard scale of measurement whose 



