Chap. XVII.

THEORY OF THE MOTION OF FLUIDS.

1153

In the tide-wheel contrived by Mr. Drydcn, fig. 1746., the floats are all set atone and thesame angle with the respective radii of the wheel, and have an opening of an inch betweeneach float and the drum board or soling of the wheel : by this arrangement, the wheel is notimpeded by the tail water, and as the bucket rises out of the water, there can be no vacuumformed in it, there being a full supply of air, in consequence of which the wheel is free.Dr. Gregory in his Mechanics has given a clear description of the tide mill, to which wemust refer those readers who may be desirous of further information.

The Breast Wheel, now generally made of iron, differs from the undershot in requiring

Fig. 1747. ELEVATION OF IRON BREAST-WHEEL.

to be closely boarded, or, as it is called, closely so! d round its circumference; the sides arealso close shrouded, or made so as to retain thewater, thus forming in effect a bucket-wheel.

The Pentrough , for conducting the water, hasa rack and pinion, which raises or lowers theshuttle that regulates the supply.

Of Bodies plunged in Water. — M hen a bodyis placed lightly on the surface of still water,one of three things will happen,—

1st, If the specific gravity of the body be lessthan that of water, it will swim, and only dis-place a volume of water equal to its own.

2dly, If the specific gravity be equal to that of water, it will sink totally, and rest between

in. ,i:iiiiunii77m!niiiiini

Fig 1748.

PLAN OF BREAST-WHEEL.

two waters.

3dly, If the specific gravity be greater than that of water, it will descend, and be driventowards the bottom with a force expressed by the excess of its weight over that of thevolume of water the place of which it occupies.

To demonstrate thefirst case, suppose avessel of a prismaticform A B C D, placedon the surface ofwater, and that morebe gently poured intoit to the height E F,which we shall consi-der as an increase ofthe column belowG A DII, which beingheavier than the othersof the same base willdescend and cause

Fig. 1749.

4 E