semicircle, and let the circumference between the point where the last line cuts it and the point o be divided into ninety equal parts, and let these parts be numbered, and straight lines drawn from the centre to each point of division. In the figure the lines are drawn at every fifth division, and the centre line of the ship coincides with that drawn through the forty-fifth division. In this case the vessel is said to be inclined, or heeled, at an angle of forty-five degrees, which is usually written 45°. In a position half-way between this and the upright the angle of heel would be 22½°, and so on. The reader no doubt perceives that a ship, like any other body, must be supported, and he is probably aware that the support is afforded by the upward pressure of the water. He may also be familiar with the fact that the weight of every body acts upon it as if the whole weight were concentrated at one certain point, and that this point is called the centre of gravity of the body. Whatever may be the position of the body itself, its centre of gravity remains always at the same point with reference to the body. When the centre of gravity happens to be within the solid substance of a body, there is no difficulty in thinking of the force of gravitation acting as a downward pull applied at the centre of gravity. But this point is by no means always within the substance of bodies: as often as not it is in the air outside of the body. Thus the centre of gravity of a uniform ring or hoop is in the centre, where, of course, it has no material connection with the hoop; but in whatever position the hoop may be placed, the earth’s attraction pulls it _as if_ this central point were rigidly connected with the hoop, and a string were attached to the point and constantly pulled downwards. This explanation of the meaning of centre of gravity may not be altogether superfluous, for, when the causes of the loss of the _Captain_ were discussed in the newspapers, it became evident that such terms as “centre of gravity” convey to the minds of many but very vague notions. One writer in a newspaper enjoying a large circulation seriously attributed the disaster to the circumstance of the ship having lost her _centre of gravity_! The upward pressure of water which supports a ship is the same upward pressure which supported the water before the ship was there—that is, supported the mass of water which the ship displaces, and which was in size and shape the exact counterpart of the immersed part of the ship. Now, this mass of water, considered as a whole, had itself a centre of gravity through which its weight acted downwards, and through which it is obvious that an equal upward pressure also acted. This centre of gravity of the displaced water is usually termed the “centre of buoyancy,” and, unlike the centre of gravity, it changes its position with regard to the ship when the latter is inclined, because then the immersed part becomes