"[C]onsider... the uniform movement of heat in the simplest case, which is... an infinite... solid body formed of some homogeneous substance... enclosed between two parallel and infinite planes; the lower plane A is maintained... at a constant temperature a... the upper plane B is... maintained... at... fixed temperature b, ...less than... a; the problem is to determine... the result... if... continued for an infinite time. ...In the final and fixed state... the permanent temperature... is... the same at all points of the same section parallel to the base... [D]enoting by z the height of an intermediate section... from the plane A... e the whole height or distance AB, and... v the temperature of the section whose height is z, we must have v = a + \frac{b - a}{e} z. ...[I]f the temperatures were at first established in accordance with this law, and... the... surfaces A and B... always kept at... temperatures a and b, no change would happen."