"If the mass is spherical, and we denote by x the distance... from the centre... t the time... cooling, and by v the variable temperature of the point m... all points... at the same distance x... have the same temperature v. This quantity v is a certain function F(x, t) of the radius x and... time t... such that it becomes constant whatever... value of x, when... [t=0]; for... the temperature at all points is the same at... emersion. The problem consists in determining... [F(x, t)]."