THE WEATHER AND THE SUN 48 ? 



ion. De Morgan thus quaintly indicates his interpretation of one par- 

 ticular expression of Sir G. Airy's opinion : " In tne report to the 

 Greenwich Board of Visitors, for 1867, the Astronomer Royal, speak- 

 ing of the increase of meteorological observatories, remarks : ' Whether 

 the effect of this movement will be that millions of useless observa- 

 tions will be added to the millions that already exist, or whether some- 

 thing may be expected to result which will lead to a meteorological 

 theory, I cannot hazard a conjecture.' This is a conjecture, and a 

 very obvious one ; if Mr. Airy would have given 2f d. for the chance 

 of a meteorological theory formed by masses of observations, he would 

 never have said what I have quoted." 



The simple combination of terrestrial considerations with the effects 

 due to the sun's varying daily path having thus far failed to afford 

 any interpretation of the varying weather from year to year, it is nat- 

 ural to inquire whether the variations in the sun's condition from year 

 to year may not supply the required means of interpreting, and hence 

 of predicting, weather-changes. We know that the sun's condition 

 does vary, because we sometimes see many large spots upon his sur- 

 face, whereas at others he has no spots, or few and small ones. We can 

 scarcely doubt that these variations affect the supply of heat and 

 light, as well as of chemical action and possibly of other forms of 

 force ; and hence we are certainly dealing with a vera causa, though 

 whether this real cause be an efficient cause of weather-chano-es re- 

 mains yet to be determined. 



It may perhaps be as well to inquire, however, in the first place, 

 whether any peculiarities of weather can be traced to another circum- 

 stance which ought to be at least as efficient, one would suppose, as 

 any changes in the sun's action due to the spots. I refer to his vary- 

 ing distance from the earth. It is known doubtless to all my readers 

 that, in June and July, although these are our summer months, the 

 sun is farther away than in December; and this, not by an inconsider- 

 able distance, but by more than 3,000,000 miles. Accordingly, on a 

 summer day in our hemisphere, we receive much less heat than is re- 

 ceived on a summer day in the Southern Hemisphere. Or, instead of 

 comparing our summer heat with summer heat in the Southern Hemi- 

 sphere, we may make comparison between the quantity of heat received 

 by the whole earth on a day in June and on a day in December. 

 Either way of viewing the matter is instructive ; and I believe many 

 of my readers will be surprised when they hear what is the actual 

 amount of difference. 



We receive in fact, on June 30th, less heat and light than dwellers 

 at our antipodes receive on December 30th, by the amount which 

 would be lost if an opaque disk, having a diameter equal to one-fourth 

 of the sun's, 1 came upon the sun's face a? seen on December 30th at 



1 It is easily shown that such would be the size of the imagined black disk. For the 

 sun's distance varies from about 93,000,000 miles to about 90,000,000, or in the propor- 



