Ferrier's tool is a froe, which is a type of woodworking tool used to split wood along the grain. It consists of a metal blade with a handle attached at a right angle. The froe is driven into the end grain of a piece of wood and then twisted or rocked back and forth to create a split. It is an essential tool for any woodworker, as it can be used to make shingles, shakes, and other wooden items.