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Essentially it says that, "They shot through one side [either the left, or right, side] of the main mast [of the ship http://tinyurl.com/py7956s ] with a cannonball made of cast iron, and that all the cannonballs they fired were made of cast iron."
The reason he is stressing the fact that they were made of cast iron is because, at the start of the 1600s that was the best and most accurate cannonball. Before that time various materials were used for cannonballs, mainly stone, I think.
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