THE origin of the shipping ton is the tun of wine. In European ports the practice grew up of describing a ship’s capacity for cargo in terms of the number of tuns of wine which could be carried. A tun of wine consisted of two butts, and was equivalent to 40·3 cubic feet; adding the size of the casks, and the loss of space due to their irregular shape, the space occupied by a tun comes to about 60 cubic feet.1 Originally the number of tuns was not reached by any process of measurement: the capacity of a ship which had carried wine would be known by experience, and practical men would acquire sufficient knowledge to judge the capacity of other ships by their appearance and build. It is such estimates as these that are available in regard to the period we are considering: they are all given in round numbers, and obviously are not intended to be accurate to a tun; on the average, they are probably fairly close to the truth, though there are doubtless errors in particular observations.
Towards the end of the sixteenth century the number of tuns for particular ships became an important question in England, because it determined the amount of the subsidy paid for ship- building, and estimates such as had hitherto sufficed became subject to bias; a system of measurement was therefore worked out to supersede such estimates. At the first attempt the number of tuns which could be carried in a particular ship was deter- mined by experiment; the length, breadth, and depth of the ship were then measured, and the volume in cubic feet calculated; the rest was a sum in simple proportion. It was found that each actual tun required about 97 cubic feet of volume measured on the lines adopted, and this relation was made the basis of a general rule ; find the volume of a ship by a particular set of measure- ments and divide by 97.
The effect of this rule was to alter the definition, but not at first the size, of the tun. The unit was still the space required by a tun of wine, and this space was found to be 97 cubic feet measured in a particular way. This system has survived up to the present day, but both the method of measurement and the divisor adopted have been varied from time to time. A " registered ton " is now defined as 100 cubic feet of space measured according to the precise specifications made under the Merchant Shipping Acts ; it is a conventional unit, which has lost its original relation to the tun of wine, and its present relation could be determined with precision only by filling a series of modern cargo boats with tuns, a measure which nobody is likely to undertake. For our purposes, the important point is that the changes in the method of measurement have been in the direction of a closer approximation to the actual cargo space. The original method of measuring the length, breadth, and depth of the ship as a whole could give the actual cargo space only if the ship was rectangular and there were no internal structures below the top- most deck ; the fact that 97 cubic feet (measured in this way) were required to accommodate 60 cubic feet (actual) shows that the cargo space was greatly overstated. This overstatement has now disappeared, because each deck is measured separately, and allowance is made for the curve of the sides ; hence substantially less than 100 cubic feet (a measured ton) would now be required to accommodate a tun of wine, and in fact the volume of ordinary goods carried in recently-built vessels is, generally speaking, in excess of the volume given by the measurement. It follows that we shall not understate the cargo-capacity of Indian ships of the sixteenth century if we say that a tun of wine would require from ${\frac{4}{10}}$ to ${\frac{6}{10}}$ of a modern registered ton, instead of requiring a whole measured ton as was the case when measurement was first introduced. At this rate 1000 tuns of wine would require from 400 to 600 modern tons of shipping ; and we should reduce the " tunnage " calculated for Indian trade in the sixteenth century by from ${\frac{2}{5}}$ to ${\frac{3}{5}}$ in order to compare it with the returns of shipping published at the present day. In the text I have arrived at 60,000 tuns as the probable maximum volume of Indian commerce : this would require from 24,000 to 36,000 tons present measurement, and since this figure has to be compared with about $6{\frac{3}{4}}$ million tons (the volume of Indian trade before the War), it really matters very little which fraction we take.
A word must be added regarding the modern distinction between gross and net tons. The shipping statistics which have just been quoted are given in net registered tons, but landsmen’s ideas of ships are usually based on gross tons,1 and the difference between the two is important. The gross tonnage of a ship includes space occupied by machinery, etc., and not available for cargo; the net tonnage excludes the space so occupied. The relation between gross and net tonnage varies greatly among different classes of ships, but for modern cargo boats we shall not be very far from the truth in taking the net tonnage as on the average 60 per cent of the gross. If, then, we conclude that the Indian sea-borne commerce at the end of the sixteenth century could be carried in from 24,000 to 36,000 tons net, we may say that it would require from 40,000 to 60,000 tons gross, or at most one modern cargo-boat of moderate size sailing in each month of the year.