HYDRODYNAMICS. 



liquid, if it be of the tame weight as the liquid, it 

 will muain in whatever part uf tin- fluid it is placed ; 

 If it be heavier, it will sink to the bottom ; if lighter, 

 it will rise to the top. If any body, therefore, be 

 weighed in the air, and then weighed in a liquid, it 

 will low as much in weight as an equal bulk of the 

 liquid weighs. In this manner we determine the 

 relative weights of all bodies, or the proportion which 

 they bear to each oilier in weight, which is called 

 their tpteijSe gravity, (q. v.) Suppose a mass of 

 gold, for instance, to have a certain weight in the air; 

 it would lose, on being weighed in water, about a 

 nineteenth of its weight ; that is, the gold would be 

 nineteen times heavier than water. The instrument 

 and for this purpose is called the hydrostatic balance, 

 (See Balance) .and adonis the easiest and most accurate 

 nethod of comparing all substances, whether solid or 

 fluid. This operation may be performed with sub- 

 nances lighter than water, by attaching them to a stiff 

 pin, fastened to the bottom of the scale, or by suspend- 

 ing some heavy substance of a known weight. The 

 tame principle also enables us to ascertain the specific 

 gravities of different fluids; for, if the same substance 

 be weighed in two fluids, the weight which it loses 

 in each is as the specific gravity of that fluid. (See 

 Hydrometer.) 



Mr Thorn of Rothsay has employed the principle 

 of floating bodies in the regulation of the height of 

 water in mill dams. The accompanying wood cut 

 shows a section of one of his contrivances for this 

 purpose, called a self-regulating sluice. 



The watter sluice. This sluice, when placed 

 upon any river, canal, reservoir, or collection of 

 water, prevents the water within the embankment 

 from rising above the height we choose to assign to 

 it; for whenever it rises to that height, the sluice 

 opens and passes the extra water; and whenever that 

 extra water is passed, it shuts again; so that whilst it 

 saves the banks at all times from damage by over- 

 flow, it never wastes any water we wish to retain. 

 A C B L, part of a canal, river, stream, or collection 

 of water. B C, high water mark, or the greatest 

 height to which the water is to be allowed to rise. 

 B D, a sluice, or folding dam, which turns on pivots 

 at D. E F, a hollow cylinder, having a small aper- 

 ture in its bottom, to which is joined F L, a small 

 pipe always open. I II I, small holes in cylinder 

 E F, on the line of high water mark. G H, another 

 cylinder, waterproof, that moves up and down freely 

 within cylinder E F; and the weight of which keeps 

 the sluice B D shut by its connexion with B K H, a 

 chain fixed to cylinder G H at H, thence passing 

 over pulley K, and having its other end fixed to 

 sluice B D at B. When the water in the canal, 

 river, or pond, rises to the line B C, it passes into 

 cylinder E F, at the small holes I I I I ; and this 

 lessens the weight of cylinder G PI so much that the 

 pressure of the water in front of sluice B D throws it 

 open. When the water subsides, so as not to enter 

 these holes, Uie cylinder is emptied by the tube 

 L; and then the weight of cylinder G H shuts the 

 *as before. The dimensions and weight of this 

 under must of course correspond with the weight 

 ^the column of water pressing upon sluice B D. 

 tin sluice is here represented with the pivots on 

 which it turns at its under edge, but they may be 



I either at the upper or under edge as circum- 

 stances render advisable. The upper edge is ;i. <> 

 here represented on a level with high water mark, 

 but if necessary, it may be placed anywhere between 

 that and the bottom of the pond, or aqueduct, or 

 right below, as on an aqueduct bridge, or similar 

 situation. The cylinders may also be placed on the 

 outside of the dam or embankment, by having a pipe 

 to communicate between them and the water within ; 

 but in whatever situation the sluice or cylinders may 

 be placed, the pipe that communicates between the 

 cylinders and the water within the embankment must 

 always have its opening there exactly at the level of 

 high water mark, or at the greatest height to which 

 the water therein is to be permitted to rise. On this 

 principle a self-acting dam may be raised in any river 

 or stream, up to high water mark, by which means a 

 considerable reservoir will be obtained, whilst during 

 floods the dam will fold down, and no new ground be 

 overflowed. In lawns, or pleasure grounds, through 

 which streams or rivulets flow, these sluices might oe 

 applied to advantage; for by placing one on the bank 

 of each pond, the water within would always be kept 

 at the same height, whether the weather were wet 

 or dry; and hence flowers or shrubs might be planted 

 close to the water's edge, or in it, (as best suits their 

 respective habits,) and their position with regard to 

 water, would always be the same. 



The principles of buoyancy are very ingeniously 

 applied in Mr Farcy's self-acting flood-gate. In the 

 case of common sluices to a mill-dam, when a sudden 

 flood occurs, unless the miller gets up in the night to 

 open the gate or gates, the neighbouring lands may 

 become inundated; and, on the contrary, unless he 

 be present to shut up when the flood subsides, the 

 mill-dam may be emptied and the water lost which 

 he would need the next day. To prevent either of 

 these occurrences, Mr JOHN FAREY, whose talent 

 and ingenuity are well known, has proposed a self- 

 acting flood gate, the following description of which 

 has been given in the Mechanics 1 Weekly Journal. 



A A represents a vertical section of a gate poised 

 upon a horizontal axis passing rather above the centre 

 of pressure of the gate, so as to give it a tendency to 

 shut close, a a is a lever, fixed perpendicular to 

 the gate, and connected by an iron rod with a cask, 

 b, floating upon the surface of the water, when it rises 

 to the line, B,D, which is assumed as a level of the 

 wear or mill-dam, B, C,E, F, in which the flood-gate 

 is placed: by this arrangement, it will be seen that 

 when the water rises above the dam, it floats the 

 cask, opens the gate, and allows the water to escape 

 until its surface subsides to the proper level at B,I>; 

 the cask now acts by its weight, when unsupported 

 by the water, to close the gate and prevent leakage. 

 The gate should be fitted into a frame of timber, H, K, 

 which is set in the masonry of the dam. The upper 

 beam,H, of the frame being just level with the crown 

 of the dam, so that the water runs over the top of 

 the gate at the same time that it passes through it. : 

 to prevent the current disturbing the cask, it is con 



