Empire of the Seas..a query

[Appleyard]

Watched this tonight and the bit where Dan was in the archives looking through the old lists got me thinking...
All the ships listed had their tonnage included in the description. How was this measured?
In fact how does the gross weight of any ship get measured,obviously you can't stick it on a weighing scale,so does the weight of each individual component get recorded as the vessel is built?
I know that the displacement is equal to the weight of the vessel,but how is it calculated?

 

[sailorman]

volume of water displaced (i think) x weight of water displaced = dead weight of ship

 

[TamarMike]

Various historical tonnages were used based on formulae using overall dimensions but bearing little relation to the displacement in tonnes. (A merchant vessel's register ton was supposedly 100 cubic feet in the hold)

 

[mcframe]

http://en.wikipedia.org/wiki/Tonnage

Not to be confused with the modern-day measurement of "Number of cases of wine from Gosselin sufficient to completely cover the cabin-sole of a cruising yacht"

(I think we got about 14 on a 45' MAB once, but the magnums were stacked in the heads