Not long ago, I ran across a meaning of "forestall" I had never heard before. The punishable offence of buying up all of a commodity before it could get to the market, in order to resell it at much higher prices.
Now, Dana uses it in a way not entirely clear to me. He is telling about a Yankee shyster defrauding a formerly wealthy Mexican family:
"I could not but feel a pity for him, especially when I saw him by the side of his fellow-passenger and townsman, a fat, coarse, vulgar, pretending fellow of a Yankee trader, who had made money in San Diego, and was eating out the very vitals of the Bandinis, fattening upon their extravagance, grinding them in their poverty; having mortgages on their lands, forestalling their cattle, and already making an inroad upon their jewels, which were their last hope."
It seems to mean that he somehow persuaded them to sell him the cattle at a low price because they needed money quickly, and then was able to sell them at a higher price.
But the English law prohibiting forestalling seems to have been intended to protect the buyers at the market.
In this case there was never and shortage of cattle, so the buyers were not compelled to buy at the higher price.
Incidentally, Dana was knowledgeable about law, because his father was a lawyer who became Chief Justice of U.S. Supreme Court. Dana himself later had a highly regarded lawyer. I forget his highest position. He did become active in changing the laws to protect the rights of seamen.