Particular attention to ventilation is necessary in frigates, for almost all that part in which the men sleep is excluded from the air, and they are therefore very uncomfortable in the West Indies unless small scuttles are cut in the sides. But if this should be objected to as weakening or endangering the ship, there is a good contrivance for the same purpose, which I met with on board of the Nymphe frigate. It consists of a square wooden pipe, of about nine inches in the side coming from between decks, running along the side of the ship, and opening over the gunwale of the forecastle. There was one on each side. SECT. IV. Means of guarding against INFECTION and BAD AIR. Infection never prevails to such a degree, as to affect every person indiscriminately who is exposed to it. Even where the plague and small-pox prevail to the greatest degree, there are some persons who, though susceptible of these diseases, yet escape them. There are certain other infections of a weaker nature, as was before observed, and these will remain entirely inactive, till they find constitutions so disposed as to be fit subjects of their action. The seeds of disease may be compared to those of vegetables, which lye dormant, unless they happen to fall into a situation peculiarly adapted for exciting their activity. It is very difficult to account for this uncertainty in the operation of infection, but it is extremely providential, that under the most calamitous state of sickness, there are always some who are in health and who survive, for the necessary purposes of life. If this were not the case, it might happen that every person on board of a ship might perish from sickness in the course of a voyage, a circumstance which I believe has never been known to happen. There is an endless variety in the constitution of the human frame, both in mind and body, as well as in the features of the face. There are, perhaps, no two individuals in the world in whom the same effect precisely is produced by the same food, air, medicine, poison, or passions of the mind. The different effects of infection, therefore, upon different people, seem to depend, in many cases, on peculiarities of constitution too obscure to be explained; but there are also known circumstances which resist or encourage its effects. The great power of habit[61] in taking off the effect of infection, has already been mentioned, and it would appear that novelty gives an increased energy and activity to all impressions, as well as those on the senses. If a person, therefore, escapes the first attack of infection, he will be more likely to continue exposed to it with safety in future.