Section 15. And be it further enacted by the authority aforesaid, That the letter mail shall, during the quarantine season, leave the Lazaretto for the health office in the city of Philadelphia, and return twice in each juridical day, at such hours as the board of health shall determine, and shall be free for the transmission of letters to and from persons who may be within the bounds of the Lazaretto, or on board of vessels which may be detained there; and it shall be the duty of the health office, immediately after the arrival of the Lazaretto mail, to deposit in the post office the letters received thereby, excepting those which shall be addressed to the board of health, and the officers connected therewith.