(140.) To this we may add another, which has likewise the essential characters of a _vera causa_, in the astronomical _fact_ of the actual slow diminution of the eccentricity of the earth’s orbit round the sun; and which, as a general one, affecting the _mean temperature of the whole globe_, and as one of which the effect is both inevitable, and susceptible, to a certain degree, of exact estimation, deserves consideration. It is evident that the _mean_ temperature of the whole surface of the globe, in so far as it is maintained by the action of the sun at a higher degree than it would have were the sun extinguished, must depend on the mean quantity of the sun’s rays which it receives, or, which comes to the same thing, on the _total_ quantity received in a given invariable time: and the length of the year being unchangeable in all the fluctuations of the planetary system, it follows, that the total _annual_ amount of solar radiation will determine, _cæteris paribus_, the general climate of the earth. Now, it is not difficult to show that this amount is inversely proportional to the minor axis of the ellipse described by the earth about the sun, regarded as slowly variable; and that, therefore, the major axis remaining, as we know it to be, constant, and the orbit being actually in a state of approach to a circle, and, consequently, the minor axis being on the _increase_, the mean annual amount of solar radiation received by the whole earth must be actually on the _decrease_. We have here, therefore, an evident real cause, of sufficient universality, and acting _in the right direction_, to account for the phenomenon. Its adequacy is another consideration.[41] (141.) Whenever, therefore, any phenomenon presents itself for explanation, we naturally seek, in the first instance, to refer it to some one or other of those real causes which experience has shown to exist, and to be efficacious in producing similar phenomena. In this attempt our probability of success will, of course, mainly depend, 1st, On the number and variety of causes experience has placed at our disposal; 2dly, On our habit of applying them to the explanation of natural phenomena; and, 3dly, On the number of analogous phenomena we can collect, which have either been explained, or which admit of explanation by some one or other of those causes, and the closeness of their analogy with that in question.