“The changes of the form, and of the position, of the terrestrial orbit are mathematically inoperative, or, at most, their influence is so minute that it is not indicated by the most delicate instruments. For the explanation of the changes of climates, then, there only remains to us either the local circumstances, or some alteration in the heating or illuminating power of the sun. But of these two causes, we may continue to reject the last. And thus, in fact, all the changes would come to be attributed to agricultural operations, to the clearing of plains and mountains from wood, the draining of morasses, &c.
“Thus, at one swoop, to confine, the whole earth, the variations of climates, past and future, within the limits of the naturally very narrow influence which the labour of man can effect, would be a meteorological result of the very last importance.”—pp. 221−224, Memoir on the “Thermometrical State of the Terrestrial Globe,” in the Edinburgh New Philosophical Journal, vol. xvi., 1834.
BARON HUMBOLDT.
“The question,” he says, “has been raised as to whether the increasing value of this ellipticity is capable during thousands of years of modifying to any considerable extent the temperature of the earth, in reference to the daily and annual quantity and distribution of heat? Whether a partial solution of the great geological problem of the imbedding of tropical vegetable and animal remains in the now cold zones may not be found in these astronomical causes proceeding regularly in accordance with eternal laws?... It might at the first glance be supposed that the occurrence of the perihelion at an opposite time of the year (instead of the winter, as, is now the case, in summer) must necessarily produce great climatic variations; but, on the above supposition, the sun will no longer remain seven days longer in the northern hemisphere; no longer, as is now the case, traverse that part of the ecliptic from the autumnal equinox to the vernal equinox, in a space of time which is one week shorter than that in which it traverses the other half of its orbit from the vernal to the autumnal equinox.
“The difference of temperature which is considered as the consequence to be apprehended from the turning of the major axis, will on the whole disappear, principally from the circumstance that the point of our planet’s orbit in which it is nearest to the sun is at the same time always that over which it passes with the greatest velocity....
“As the altered position of the major axis is capable of exerting only a very slight influence upon the temperature of the earth; so likewise the limit of the probable changes in the elliptical form of the earth’s orbit are, according to Arago and Poisson, so narrow that these changes could only very slightly modify the climates of the individual zones, and that in very long periods.”[321]—Cosmos, vol. iv., pp. 458, 459. Bohn’s Edition. 1852.
SIR HENRY T. DE LA BECHE.
“Mr. Herschel, viewing this subject with the eye of an astronomer, considers that a diminution of the surface-temperature might arise from a change in ellipticity of the earth’s orbit, which, though slowly, gradually becomes more circular. No calculations having yet been made as to the probable amount of decreased temperature from this cause, it can at present be only considered as a possible explanation of those geological phenomena which point to considerable alterations in climates.”—Geological Manual. Third Edition. 1833. p. 8.
PROFESSOR PHILLIPS.
“Temperature of the Globe.—Influence of the Sun.—No proposition is more certain than the fundamental dependence of the temperature of the surface of the globe on the solar influence.