A letter in the New York Times irom Fort Pickens naya a Br tish Irig;ate belonging to the squadron of observation had arrived otf the fort, the comuiander oí wbich exhibitnd the instrucf.ions rcoeived from bis flag ofBcer they werc as follnws : "1. No port is blockadeed efficiently if any vesse! can unter or depart from itunknown to or in spite cf the guardián men ot-war. ui. An efficiënt blockade neoessitates the complete cuttins; off all ingress and egress, except in regard to harbor islands bttviog no outlet to the sea, eave under tho guns of tho flest. "3. The escape of the third vessel from the blockading squadron significa the invalidity of the blockade," If this implips that the escape of the third vessel is to be followed by a British proclamation directing British ehips to disregnrd the blockade, we shall inevitably have trouble with England, because at least three vessels have ilready sliptlirough the squadron at the mouth of the Mismsnppi. Since when has the number three been fixed by maritime law as the limit of a tolerable blockade !