The prevention of diseases is an object as much deserving our attention as their cure; for the art of physic is at best but fallible, and sickness, under the best medical management, is productive of great inconvenience, and is attended with more or less mortality. The means of prevention are also more within our power than those of cure; for it is more in human art to remove contagion, to alter a man’s food and cloathing, to command what exercise he is to use and what air he is to breathe, than it is to produce any given change in the internal operations of the body. What we know concerning prevention is also more certain and satisfactory, in as much as it is easier to investigate the external causes that affect health than to develope the secret springs of the animal œconomy. This part of the work, therefore, is chiefly addressed to those who direct the navy either in a civil or military capacity; for the general health of ships depends so much upon the victualling and manning in the first instance, and, afterwards, on the degree of discipline and order which are kept up, that I am persuaded that a certain degree of attention on their part would almost entirely eradicate disease from our fleets. Several remarks in this part of the work will be found so obvious, that it might seem superfluous to mention them. But it has been my intention to omit nothing that I have heard of or observed as a matter of ascertained utility, and, I believe, the most experienced will find either something new, or what they had not before sufficiently attended to. Though the design of it is that of being extensively useful, yet my trouble would be compensated, should it prove the means of health and comfort to a single ship’s company; nay, I should not repent my labour, could I enjoy the conscious certainty of its being the means of saving the life of one brave and good man. The prevention of disease has relation only to the external causes that affect health, and I shall consider these under the four heads of I. AIR, II. ALIMENT, III. EXERCISE, IV. CLOATHING. CHAP. I. AIR. Under this head I shall not only consider the natural state of the air of the atmosphere in point of heat and cold, moisture and dryness, purity and corruption, but also the different artificial impregnations of it from the holds or other parts of a ship, or from the persons of men who have been neglected in point of cleanliness.