THE SLAVE TRADE AT THE NORTH. – A letter to the New York Times says that there are at this time two vessels fitting out at Salem, Massachusetts, for the slave trade on the coast of Africa. The principals in this affair are New York Merchants, who have forwarded to Salem $20,000 in specie as the pecuniary equipment of the vessels. This is not the first instance in the last three months, it is stated, of such an enterprise at Salem. The writer very correctly remarks that if the Government really wishes to stop this infamous trade it must look North as well as South, and to these small New England ports as well as to New York and New Orleans. There will be no difficulty, he says, in identifying the craft at Salem.