8o8 THE POPULAR SCIENCE MONTHLY. 



cial 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 char- 

 acter 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 in- 

 terior, 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 preces- 

 sion 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 peri- 

 helion, 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 char- 

 acter 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, in- 

 creasing in proportion to the distance of the sun." M. Arago 

 thinks tliat it would take ten thousand years for variation in 

 eccentricity to effect a change of temperature in the earth meas- 

 urable 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, 



