We have observed, that the incubation of yellow fever, is from ten to twelve days. It will be prudent then, in the heated term, not to remain in port more than six days. English vessels have the habit of not remaining more than three days, but they cruise in the vicinity, or go to Jamaica, so that they are always exposed. The best plan would be to leave the waters of the Gulf, which is the true centre of infection. When, despite every precaution, there are sick men on board, and their condition inspires apprehension, they should be put ashore at once, the vessel whitewashed, and sail set for other latitudes to the north. Commercial are not under the same conditions as government vessels. On the former all is sacrificed to speculation. The crew is lodged as closely as possible, and ten or twelve men are often seen in a space, where four men can scarcely lie down. The government should require the lodgment of the crew to be on deck, so as to allow free circulation of air. Their food should be wholesome, and well chosen, and instead of giving the crew fresh meat every day, it is better to continue giving them salt meat which is more wholesome, and more nourishing. Let us add, that the meat of the country, bought by merchantmen, is not of first quality. There are some captains, who, to shun the invasion of the disease, engage blacks to load and unload their vessels, and during this time, leave their crews inactive. This plan is of no avail. The crew is generally composed of strong men, habituated to bodily exercise, which facilitates transpiration. They require, therefore, moderate labor. The water of the country is bad, and it would be good to add to it a little brandy or rum. This drink is better than wine, or beer, which are adulterated and often give colic. Prophylactic remedies have been much recommended. In latter times, chlorine on board ships was extolled, and has fallen into disuse, like others I have tried without satisfaction. I have seen a captain, who, convinced of possessing the universal panacea, neglected his sailors, and they became gravely sick. Prophylactics can not have any action on a disease which is in the air. Hygienic precautions and cleanliness, are worth more than these pretended preventives.