We have no space to indicate, as it deserves, the comparative merits and demerits of this ingenious theory. It would, however, be a great calamity to have geologists accept it without scrutiny. It is, indeed, a part of the business of geologists to doubt such theories until they are verified by a thorough examination of all accessible _terrestrial_ evidence bearing upon the subject. There is no reason to question the reality of the variations in the relative positions of the earth and the sun assumed by Mr. Croll; though there may be serious doubt whether the effects of those changes upon climate would be all that is surmised, since equal amounts of heat would fall upon the earth during summer, whether made longer or shorter by the cause referred to. During the short summers the earth is so much nearer the sun that it receives each season absolutely as much heat as it does during the longer summers, when it is so much farther away from the sun. Thus the theory rests at last upon the question what would become of the heat reaching the earth in these differing conditions. It is plausibly urged by Mr. Croll that when a hemisphere of the earth is passing through a period of long winters the radiation of heat will be so excessive that the temperature would fall much below what it would during the shorter winters; and so ice and snow would accumulate far beyond the usual amount. It is also supposed that the effect of the summer's sun in melting the ice during the short summer would be diminished through natural increase of the amount of foggy and cloudy weather. Adhémar's theory is supposed by Sir Robert Ball, Royal Astronomer of Ireland, to be considerably re-enforced by a discovery which he has made concerning the distribution of heat upon the earth during the seasons culminating in the summer and winter solstices. Croll had assumed, on the authority of Herschel, that a hemisphere of the earth during the longer winter in aphelion would receive the same actual amount of heat which would fall upon it during the shorter summer in perihelion; whereas, according to Dr. Ball's discovery, "of the total amount of heat received from the sun on a hemisphere of the earth in the course of a year, sixty-three per cent is received during the summer and thirty-seven per cent during the winter."[DU] When, therefore, the summers occur in perihelion the heat is more intense than Croll had supposed, and, at the same time, the winters occurring in aphelion are more deficient in heat than he had assumed. This discovery of Dr. Ball will not, however, materially affect the discussion of Croll's theory upon its inherent merits, since it is simply an intensification of the causes invoked by him. We will therefore let it stand or fall in the light of the general considerations hereafter to be adduced. [Footnote DU: Cause of an Ice Age, p. 90.]