"The quantity of heat which bodies lose through their surface is subject to the same principle. If we denote by \sigma the area, finite or infinitely small, of the surface, all of whose points have the temperature v, and if a represents the temperature of the... air, the coefficient h being the... external conducibility, we shall have \sigma h (v - a) dt as the expression for the quantity of heat which this surface \sigma transmits to the air during... instant dt. ...h may... be considered as having a constant value, proper to each state of the surface, but independent of the temperature."