CENTRE OF GRAVITY, IN REFERENCE TO SEA-SICKNESS.
Man requiring so strictly to maintain his perpendicularity, that is, to keep the centre of gravity always over the support of his body, ascertains the required position in various ways, but chiefly by the perpendicularity or known position of things about him. Vertigo, and sickness commonly called sea-sickness, because it most frequently occurs at sea, are the consequences of depriving him of his standards of comparison, or of disturbing them.
Hence on shipboard, where the lines of the masts, windows, furniture, &c. are constantly changing, sickness, vertigo, and other affections of the same class are common to persons unaccustomed to ships. Many experience similar effects in carriages, and in swings, or on looking from a lofty precipice, where known objects being distant, and viewed under a new aspect, are not so readily recognised: also in walking on a wall or roof, in looking directly up to a roof, or to the stars in the zenith, because, then, all standards disappear: on walking into a round room, where there are no perpendicular lines of light and shade, as when the walls and roof are covered with a spotted paper without regular arrangement of spot:—on turning round, as in waltzing, or on a wheel; because the eye is not then allowed to rest on the standards, &c.
At night, or by blind people, standards belonging to the sense of touch are used; and it is because on board ship, the standards both of sight and of touch are lost, that the effect is so very remarkable.
But sea-sickness also partly depends on the irregular pressure of the bowels against the diaphragm, as their inertia or weight varies with the rising and falling of the ship.
From the nature of sea-sickness, as discovered in all these facts, it is seen why persons unaccustomed to the motion of a ship, often find relief in keeping their eyes directed to the fixed shore, where it is visible; or in lying down on their backs and shutting their eyes; or in taking such a dose of exhilarating drink as shall diminish their sensibility to all objects of external sense.
Arnott's Elements of Physics.