The second cause, which seems not to have been thought of, is the motion of the moon round the earth. This secondary planet performs its evolution round the earth in 27 days and one third, and being 85,325 leagues distance, it goes over a circumference of 536,329 leagues in this space of time, which makes a motion of 817 leagues in an hour, or from 13 to 14 leagues in a minute. Although this rout is, perhaps, the slowest of all the celestial bodies, yet it is rapid enough to produce on the earth, which serves for the axis or pivot to this motion, a considerable heat by the friction which results from the weight and velocity of this planet. But it is not possible to estimate the quantity of heat produced by this exterior cause, because hitherto we have had nothing which might serve us for a term of comparison. But if we ever can discover the number, magnitude, and velocity, of all the planets which circulate round the sun, we shall then be able to judge of the quantity of heat which the moon can give to the earth, by the much greater quantity of fire which all these vast bodies excite in the sun. For my own part I am greatly inclined to think that the heat produced by this cause in the globe of the earth, forms a very considerable part of its own heat: and that, in consequence, we must still extend the limits of time for the duration of nature. But let us return to our principal object. We have observed that the summers are very nearly equal in all climates of the earth, and that this truth is founded on incontestible facts; but it is not the same with respect to winters; they are very unequal, and vary in different climates, as we remove further from that of the equator, where the heat in winter and summer is nearly the same. I think I have already explained in a satisfactory manner the cause of this, viz, the suppression of the terrestrial heat. This suppression is, as I have said, occasioned by the cold winds, which fall from the air, bind the earth, freeze the waters, and shut up the emanations of the terrestrial heat during the time the frosts remain; so that it is not at all surprising that the cold in winter is in fact so much the greater as we advance further towards the climates where the mass of air, receiving the rays of the sun more obliquely is for that reason colder.