CHAMBERS'S INFORMATION FOR THE PEOPLE. 



application in the centre of gravity of the space 

 from which the liquid is displaced. This point 

 may be called the centre of buoyancy. 



We may suppose that the space abd is occupied 

 by the immersed part of a floating body aebd (fig. 

 17). The supporting force, cb, is still the same as 

 in the former case, and acts at c, the centre of 

 gravity of the displaced water ; the weight of the 

 body must also be the same ; but its point of 

 application is now c', the centre of gravity of the 

 whole body. When the body is floating at rest or 

 in a state of equilibrium, this point must evidently 

 be in the same vertical line with c ; for if the two 

 forces were in the position of cs, <fg (fig. 18), they 

 would tend to make the body roll over. The line 

 passing through the centre of gravity of a floating- 

 body and the centre of gravity of the displaced 

 water is called the axis of flotation. 



The equilibrium of a floating body is said to be 

 stable, when, on suffering a slight displacement, it 

 tends to regain its original position. The condi- 

 tions of stability will be understood from the 

 accompanying figures. Fig. 19 represents a body 



floating in equilibrium, G being its centre of 

 gravity, B its centre of buoyancy, and AGB the 

 axis of flotation, which is of course vertical. In 

 fig. 20 the same body is represented as pushed or 

 drawn slightly from the perpendicular. The shape 

 of the immersed portion being now altered, the 

 centre of buoyancy is no longer in the axis of the 

 figure, but to one side, as at B. Now, it is evident, 

 that if the line of direction of the upward pressure 

 that is, a vertical line through B meets the 

 axis above the centre of gravity, as at M, the 

 tendency of the two forces is to bring the axis 

 into its original position, and in that case the 

 equilibrium of the body is stable. But if BM 

 meet the axis below G, the tendency is to bring 

 the axis farther and farther from the vertical, 

 until the body get into some new position of 

 equilibrium. There is still another case ; the line 

 of support or buoyancy may meet the axis in G, 

 and then the two forces counteract one another, 

 and the body remains in any position in which it 

 is put ; this is called indifferent equilibrium. In 

 a floating cylinder of wood, for instance, B is 

 always right under G, in whatever way the cylinder 

 is turned. 



When the angles through which a floating 

 body is made to roll are small, the point M is 

 nearly constant. It is called the metacentre; and 

 its position may be calculated for a body of given 

 weight and dimensions. In the construction and 

 lading of ships, it is an object to have the centre 

 of gravity as low as possible, in order that it may 

 be always below the metacentre. With this view, 

 heavy materials, in the shape of ballast, are placed 



in the bottom, and the heaviest portions of the 

 cargo are stowed low in the hold. 



SPECIFIC GRAVITIES OF BODIES. 



If a lump of sulphur is weighed in air, and 

 found to be two ounces, when weighed in water it 

 will be found to weigh only one ounce. Now, as 

 we know that the weight which a body loses in 

 water is exactly the weight of an equal bulk of 

 water, we infer that a body of water equal in size 

 to the lump of sulphur weighs an ounce. Bulk for 

 bulk, then, sulphur is twice as heavy as water, or 

 their weights are as 2 to I. The specific gravity 

 of a body, then, is its weight compared with that 

 of water. In tables of specific gravities, that of 

 water is generally expressed by r, but sometimes 

 also by 1000. Thus, with water as i, the specific 

 gravity of sulphur is 2 ; of iron, 7 ; with water as 

 icoo, these substances would be 2000 and 7000 

 respectively (see No. 13). The process of deter- 

 mining specific gravity varies with the nature of 

 the substance ; it requires delicate apparatus and 

 delicate handling. 



Instruments for readily indicating the specific 

 gravities of liquids are called hydrometers, from 

 two Greek words signifying water and measure. 

 The name areometer (Gr. measurer of rarity) 

 is also applied. Hydrometers are 

 of various kinds ; but the general 

 principle on which they act will be 

 understood from the accompanying 

 figure, b is a glass ball, with a 

 smaller ball, c, below it, containing 

 shot, in order to make the instru- 

 ment float upright ; the fine stem, 

 ed, being uniform and graduated. 

 Suppose the hydrometer first im- 

 mersed in water, it sinks until the 

 water displaced equals the weight 

 of the whole instrument ; if placed 

 next in a liquid rarer than water, it 

 must sink farther, so as to displace 

 more of the liquid, before it is in 

 equilibrium. By knowing what part 

 of the volume of the whole instru- 

 ment each division of the stem is, we get the com- 

 parative volumes of water and of the liquid that 

 sustain the same weight ; and then the specific 

 gravities are inversely as the volumes. It is 

 evident that the thinner the stem, the more deli- 

 cate will the instrument be. Hydrometers are 

 made to indicate a difference in specific gravity of 

 i part in 40,000. 



The heaviest substance known is platinum, 

 whose specific gravity (when rolled) is 22 ; the 

 lightest is hydrogen gas. The standard of com- 

 parison for gases is atmospheric air, which, at 

 60, is 800 times lighter than water. Its specific 

 gravity is taken at 1000. Hydrogen is fourteen 

 times lighter than air, while carbonic acid and 

 chlorine are more than twice as heavy as air. 

 The vapour of iodine is eight times the weight of 

 atmospheric air. 



Spirits, such as brandy, whisky, gin, &c. consist 

 of alcohol and water in varying proportions ; and 

 as the excise-duty is charged according to the 

 amount of the alcohol, it is important to be able 

 to tell at once what proportion or percentage of 

 alcohol is present in any spirit. Alcohol, or spirit 

 of wine, without any water absolute alcohol, as 



Fig. 21. 



