108 abbot: solar constant of radiation 



is brought out by taking the mean monthly values of the solar 

 constant measurements at Mount Wilson from the year 1906 

 on, and comparing them with the mean monthly sun-spot num- 

 bers of Wolfer for the same period. From such a comparison 

 it appears that the greater the number of sun-spots the higher 

 will be the solar constant of radiation, and that an increase of a 

 hundred sun-spot numbers corresponds to an increase of about 

 0.07 calories per square centimeter per minute in the solar radi- 

 ation outside the earth's atmosphere. This is a very curious 

 circumstance, when it is recalled that the temperature of the 

 earth is generally lower at sun-spot maximum than at sun-spot 

 minimum, notwithstanding that, if the above result be true, the 

 solar radiation is more intense at sun-spot maximum than at 

 sun-spot minimum. On the other hand, the result is in line with 

 the irregular variability of the Myra type of variable stars. 



Atmospheric transjiiission. In connection with the measure- 

 ments which have been made of the solar constant of radiation, 

 there have been some interesting by-products. Among these we 

 may mention first the determination of the transmission coeffi- 

 cients of the earth's atmosphere for light of all wave-lengths, in- 

 cluding the ultra-violet and the infra-red spectrum, and ranging 

 from wave-length 0.3m in the ultra-violet to wave length 2.5m in 

 the infra-red. These transmission coefficients have been ob- 

 tained by the Smithsonian observers at Washington, Mount Wil- 

 son, Mount Whitney, and Bassour. It is very interesting to com- 

 pare them with the transmission of the atmosphere as computed 

 according to the theoretical considerations of Rayleigh on the 

 cause of the hght of the sky. It is found that by means of these 

 transmission coefficients the value of the number of molecules in 

 the atmosphere may be obtained almost as accurately as by the 

 use of the more common laboratory methods for determining 

 the number of molecules per cubic centimeter of a gas of known 

 density. It is found that the theory of Rayleigh connecting the 

 change of transmission with the wave-length is closely confirmed 

 by the observations at Bassour, Mount Wilson, and Mount Whit- 

 ney. Similar measurements of atmospheric transmission for more 

 limited regions of the spectrum have been made by other ob- 



