Page:Popular Science Monthly Volume 40.djvu/830

808 records of the times of the opening of the vintages as far back as to the fourteenth century, that there has been no real change. The times have varied in the same places, in different years or series of years, during all this period, as much as two months, but there has been no regular variation, or any of a character to support the hypothesis of a constant, secular movement.

M. Arago undertook, about fifty years ago, to measure the value of these cosmical influences on climate, and declared that they were not competent to produce an effect within the period of historical time worthy to be regarded. He found that the present effect upon the surface of the cooling of the earth's interior, which some were disposed to regard, could be comprised within a thirtieth of a degree. Sir William Thomson makes it still less, and limits it to one seventy-fifth of a degree. M. Arago saw no reason for supposing there were differences of temperature in the parts of space, while, if there were, they would affect all the earth alike and not one hemisphere more than another. The variation in the obliquity of the ecliptic, small in its total at the most, could not cause a change of more than a quarter of a degree in two thousand years.

M. Arago likewise depreciated the importance of the precession of the equinoxes and the variation in the eccentricity of the earth's orbit as climatological factors, because, as he showed, during a period of long eccentricity with summer at the perihelion, while the hemisphere may receive a more intense heat during the summer part of the revolution the excess is balanced by the season's being shorter; but the winter will under those conditions be both colder and longer. Sir John Herschel and M. Reynaud have answered him as to this point by saying that character IS given to the season, not by the absolute quantity of heat received, but by its distribution; not its mean temperature but its maxima and mimima of temperature are to be considered, and the greater or less rapidity of the ascent and descent of thermic movements. A difference of four and a half times in eccentricity, such as is possible, might work great changes in these properties; so that in the case considered by M. Arago "half the annual heat would be concentrated into a summer of very short duration, while the other half would be distributed through a long and gloomy winter, made intolerable by the intensity of the cold, increasing in proportion to the distance of the sun." M. Arago thinks that it would take ten thousand years for variation in eccentricity to effect a change of temperature in the earth measurable by the thermometer. No evidence is produced that it has had any effect within the historical period.

Thus, whatever may be the importance of these astronomical causes in determining the climatic features of geological periods,