NO. \y SMITIISONIAX KXPLOKATIONS, I916 I33 



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

 the dependenee of all ve,<^etation. solar fluctuations of five, or even 

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

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

 pleting the Mount \\'ilson 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. Fortunately 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 year hereafter either 

 at Mount Wilson or in South America or at both stations. 



Further work was done with a solar cooking appliance at Mount 

 Wilson in 1916, but owing to the delay until September of materials 

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

 been made. Food was cooked in 191 5, including meat, i)otatoes 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 rays 

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

 of different colors differently. Violet and ultra-violet rays 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 w'ere 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 proportionally, 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 \Mlson work with the standard waterflow pyrheliometer. 

 The results showed that no detectable change of the sensitiveness 

 of the secondary pyrheliometers has occurred. We may be confi- 

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

 1905 to 1916. 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 



