Modern demands on the finished products of steel have necessitated rapid strides in the art of heat treatment of the metal. As the subjects of forging, hardening, tempering and annealing are so closely correlated it has seemed wise to add to Forge Practice a certain amount of material devoted to the other branches of the art. The introduction of heat measuring and hardness testing instruments, together with various other modern appliances, and up to date systems of doing work have made necessary a broader knowledge of heat-treating methods than was formerly the case: for after all the most important factor is the man doing the work.