A tonnage figure in an Age of Sail source usually refers to tons burthen, a formula-based estimate of carrying capacity by volume rather than the vessel's displacement. The period rule shown here multiplies length from stem to sternpost by maximum breadth and half that breadth, then divides the result by 94. This is neither a modern measure of weight nor an especially exact measure of capacity. The distinction matters when comparing ship lists, because similar-looking tonnage figures can answer a different question from modern displacement data.
Notable Point · 8:27-9:28
Why tons burthen is not displacement
The primer separates tons burthen, an estimate of cargo capacity, from a ship's displacement and explains why period measurements need careful reading.