By a letter from a Gentleman at Boston, we are informed that an armed sloop from the eastern part of New England, with 40 men on board, had taken the brigantine Loyal Briton, at St. John's river, in the bay of Fundy, laden with oxen, sheep, &c. from Boston. Also that the Americans had burnt his Majesty's fort at the mouth of the river, and carried off three soldiers with their arms and provisions.