Page:Popular Science Monthly Volume 3.djvu/499

Rh. De Morgan thus quaintly indicates his interpretation of one particular expression of Sir G. Airy's opinion: "In the report to the Greenwich Board of Visitors, for 1867, the Astronomer Royal, speaking of the increase of meteorological observatories, remarks: 'Whether the effect of this movement will be that millions of useless observations will be added to the millions that already exist, or whether something 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 2¾d. 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 natural 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 surface, 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-changes remains 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 circumstance 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 varying 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 inconsiderable distance, but by more than 3,000,000 miles. Accordingly, on a summer day in our hemisphere, we receive much less heat than is received on a summer day in the Southern Hemisphere. Or, instead of comparing our summer heat with summer heat in the Southern Hemisphere, 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, came upon the sun's face as seen on December 30th at