we have calculated, our results are not less curious, nor less useful to the advancement of science; and we shall here only observe, that those particular causes may prolong the time of the refrigeration of the globe, and the duration of living nature, beyond the terms we have indicated. But I may be asked is this Theory equally as well founded in every point which serves for its basis; is it certain, according to your experiments, that a globe, as large as the earth, and composed of the same matters, cannot refrigerate from incandescence to actual temperature in less than 74,000 years, and that in order to become heated to the point of incandescence a 15th of this time, that is 5000 years, would be required: and also that it should be surrounded all that time by the most violent fire; if so, there are as you say strong presumptions that this great heat of the earth could not have been communicated to it from a distance, and that consequently the terrestrial matter formerly made a part of the mass of the sun; but it does not appear equally proved that the heat of this body on the earth is at present but 1/50 part of the heat of the globe. The testimony of our senses seems to refute this opinion, which you lay down as a certain truth, for although we cannot doubt that the earth has an innate heat, which is demonstrated by its always equal temperature, in all deep places where the coldness of the air cannot communicate; yet does it result that this heat, which appears of moderate temperature, is greater than that of the sun which seems to burn us? To all these objections I can give full satisfaction, but let us first reflect on the nature of our sensations. A very slight, and often imperceptible, difference in the causes which affect us, produces considerable ones in their effects. Is there any thing which comes nearer to extreme pleasure than grief? and who can assign the distance between the lively irritation by which we are moved with delight, and the friction which gives us pain? between the fire which warms and that which burns? between the light which is agreeable to our sight and that which blinds us? between the savour which pleases our taste and that which is disagreeable? between the smell of which a small quantity will at first be agreeable and yet soon after create nausea? We must therefore cease from being astonished that a small augmentation of heat, such as 1/50 should appear so striking. I do not pretend positively to assert that the innate heat of the earth is really 49 times greater than that which comes to it from the sun: for as the heat of the globe belongs to all terrestrial matter, we have no means of separating it, nor consequently any sensible and real limits to which we might relate it. But even if the solar heat be greater or smaller than we have supposed, relative to the terrestrial heat, our theory would only alter the proportion of the results.