360 



VARIABLENESS OP SOLAR RADIATION* 



preclude the admission of this hypothesis ; nor does it derive any 

 support from astronomical abservations continued for 2000 years. 



Even an increase of the obliquity of the earth's axis to the ecliptic, 

 without any other change, would produce a great effect in the climate 

 of northern latitudes, by increasing the summer heat ; but the win- 

 ters would be colder than at present. There is, indeed, an annual 

 change in the obliquity of the ecliptic, but it appears to be confined 

 within limits too small to produce a sensible effect on the tempera- 

 ture of any part of the globe. The effects that might be produced 

 by a change of the earth's orbit remain to be noticed. 



A change in the form of the earth's orbit, if considerable, might 

 change the temperature of the earth, by bringing it nearer to the sun 

 in one part of its course. The orbit of the earth is an ellipsis, ap- 

 proaching nearly to a circle : the distance from the centre of the or- 

 bit, to either focus of the ellipsis, is called by astronomers the " ec- 

 centricity of the orhit.^^ This eccentricity has been for ages slowly 

 decreasing, or in other words, the orbit of the earth has been ap- 

 proaching nearer to the form of a perfect circle ; after a long period 

 it will again increase, and the possible extent of the variation has not 

 been yet ascertained."^ From what is known respecting the orbits of 

 Jupiter and Saturn, it appears highly probable, that the eccentricity 

 of the earth's orbit, is confined within limits, that preclude the belief 

 of any great change in the mean annual temperature of the globe 

 ever having been occasioned by this cause. f 



The heat from solar radiation, may possibly have been greater in 

 remote ages than at present. Sir Wm. Herschel inferred, from the 

 variable spots on the sun, that the mean temperature of the earth was 

 increased or decreased in certain years ; or, in other words, that the 

 earth received an unequal annual supply of heat from the sun. We 

 have, however, no data from which to ascertain that there has ever 

 been any considerable change of temperature effected by this cause ; 

 to appeal to the high former temperature of the globe in proof of it, 

 would be to substitute vague hypothesis in the place of facts. 



Beside solar radiation, it is believed by many philosophers, that 

 there is a source of subterranean heat within the earth itself ; this 

 opinion is by no means new, but it appears to have received support 

 from numerous observations and experiments made in a compara- 

 tively recent period. The evidence by which the theory of central 



* Sir J. W. Herschel, in a paper on the subject read to the Geological Society, 

 stales that a variation in the eccentricity of the earth's orbit, from the circular 

 form to that of an ellipse, having an eccentricity of one fourth of the major axis, 

 would produce only an increase of 3 per cent, in the mean annual amount of solar 

 radiation. 



t Un autre phenomene egalement remarquable du systeme solaire, est le peu 

 d'eccentricite des orbes des planetes et des satellites, tandis que ceux des eometes 

 sont tres-allonges. Nous sommes encore forces de reconnaitre ici I'effet d'une 

 cause reguliere, le hazard n'eut point donne une forme presque circulaire aux 

 crbes de toutes les planetes et de leurs satellites."— La Place, sur les ProbabiHt6s. 



