When we speak of a steamship as being of such a tonnage, we do not always thereby convey a correct idea as to her size, for there is a decided difference between one kind of tonnage and another. When we say a vessel displaces so much water, we know that her weight is exactly that amount of tons; but the tonnages which are given in a vessel’s certificate after being surveyed are of a totally different character. The Board of Trade recognises three measurements of tonnage. First of all, comes the under-deck tonnage. The “tonnage-deck” is the second deck from below when the ship has more decks than one, and the length for the purposes of tonnage-measurement is taken along this deck. This length is divided into a number of equal parts, and the transverse sectional areas are found, deductions being allowed for the thickness of the ceilings. The gross tonnage of a ship consists of the under-deck tonnage plus the tonnage of all the closed-in spaces above the tonnage-deck, excepting the spaces fitted with machinery, wheel-house, shelter for deck passengers, galleys and w.c’s. If the poops, bridges and forecastles are fitted with doors or some other means of closing them permanently, they have to be measured into the gross tonnage; but if they are not of a permanent character, they are exempt. Thus, the gross tonnage of a steamship might include the under-deck tonnage, the space between decks, the poop, the bridge, the forecastle, the captain’s and the officers’ quarters, the chart-room, the light and air space, and so on.