94 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



Figure 1. — March of solar vari- 

 ation, 1920-34. 



It is not sufficient to measure the solar 

 rays as a whole. They must be resolved 

 into their spectrum in order to evaluate 

 the losses from water vapor, ozone, and 

 other atmospheric absorbents. The spec- 

 trum includes not only the seven familiar 

 colors as described by Sir Isaac Newton, 

 but beyond the violet and beyond the red 

 lie long regions, dark to our eyes, but 

 containing a substantial proportion of the 

 energy of the solar beam. To measure all 

 these rays we use the bolometer, invented 

 by Langley. It is an electrical thermom- 

 eter so sensitive that a millionth of a 

 degree of heat is easily measured by it. 



Observations with the bolometer and 

 other instruments occupy the observers 

 for about 3 hours each morning. From 5 

 to 8 hours of computing follow before 

 they are ready to send their telegram to 

 Washington, announcing their result for 

 the intensity of the solar rays on that day. 

 The measures are expressed in heat units 

 called calories. The calorie is the amount 

 of heat required to warm a gram of 

 water 1° C. Those more familiar with 

 English units may recall that there are 

 about 28 grams in 1 ounce, and 1° C.= 

 1°.8 F. The intensity of solar radiation 

 as it is in free space just outside our at- 

 mosphere at mean solar distance is found 

 to fluctuate about a mean value of 1.94 

 calories per square centimeter per minute. 



Having now accumulated for more than 

 15 years measures of the solar emission 

 (which has long been called the " solar 

 constant ", though as we shall see it is 

 variable), we have examined its variations 

 to detect periodicities. This search has 

 now revealed 12 periodicities by the com- 

 bination of which the sun's heat available 

 to warm the earth fluctuates through sev- 

 eral percent. Figure 1 illustrates this 

 fluctuation since 1920, as far as it appears 



