Students of early modern maritime history are well aware of the perplexing variety of national units and rules for ship measurement. The adoption of the British 'Moorsom' system by most maritime nations between 1864 and 1885 was therefore a major advance, not least for the cross-national comparability of shipping statistics. [...] main problems with the Moorsom system after 1867, when the modern principle of net tonnage was introduced. The most important questions are whether any substancial discrepancies develeoped between measured tonnage and actual carrying capacity and, if so, whether this was so common that the validity of shipping statistics - which were usually based on register tonnages (κόρος=μονάδα όγκου ίση με 100 κυβικά πόδια) - were endangered. These practical problems affected anly steamers and other machine-propelled ships.