Chap. XVII.

THEORY OF THE MOTION OF FLUIDS.

1151

several varieties of Barker’s mill; one consists of a number of tubes arranged within thecircumference of a truncated cone, so that when the water is introduced into the upper endsof these tubes, and allowed to flow out at the lower, a motion is produced around the axis ofthe cone. Another contrivance introduces the water fromthe mill-stream into an annular cavity of a fixed cylindri-cal vessel, in the bottom of which are several inclinedapertures, for the purpose of making the water flow outwith a proper obliquity into an inferior vessel, which ismovable and of the form of an inverted frustum of acone ; this latter moves about an axis which passes throughthe centre of the fixed vessel; it has a number of tubescontrived around its circumference, which do not extendto the top, and are bent at the lower ends into rightangles. When the water is introduced into the uppervessel, and delivered into the tubes of the lower as it issuesfrom their horizontal extremities, motion is conveyed tothe conical drum.

The theory of this engine is not thoroughly under-stood, and Davies Gilbert , Esq., when President of theRoyal Society , decided against any mechanical advantagebeing produced by it. In the Philosophical Transactions for 1827 are several statements and calculations on thesubject.

Hachette has given us in his Cuurs de GeometrieDescriptive the figure of a machine by which the powerof lateral pressure may be measured : as the water fromthe constant reservoir pours into the cup and turnsround the spindle, it is made to raise a weight hungover a pulley, which weight at once expresses its power.

Wind is produced by a fall of water, and Venturi hasshown that the air is dragged down upon the principle ofthe lateral communication of motion in fluids; and he haspointed out the best mode of constructing the machine,so as to produce the greatest quantity of air.

According to the author just quoted, rain-water flowsout of gutters by a continued current; but during its fallit separates into portions in the vertical direction, andstrikes the pavement with distinct blows ; the air whichexists between the vertical and horizontal separations ofthe water which falls is impelled and carried downwards;other air succeeds laterally, and a current is producedround the place struck by the water: blasts, or veutaroli,are also produced by the difference of temperature be-tween the air of a cavern and the external air.

The Tide Wheel is put in motion by the ebbing and flowing of the tide, and in con-sequence of the variable head of water, the sluices are different to those of other wheels.In the tide mill erected at East Greenwich, on the Thames, the water-wheel has itsaxle parallel to the side of the river, or to the sluice-gates, through which it enters:the length of the wheel is 26 feet, its diameter 11 feet, and its number of float-boardsthirty-two; the latter do not each run on one plane from one end of the wheel to the other,but the whole length of the wheel is divided into four equal portions, and the parts ofthe float-boards belonging to each fall gradually, one lower than another, each byone-fourth of the distance from one board to the other, as measured on the circumference :by this means all jerking is prevented, and the action of the water on the wheel equalised.

The wheel with its incumbent apparatus weighs 20 tons, the whole of which is raisedby the impulse of the flowing tide when admitted through the sluice-gates : it is placed inthe middle of the water-way, with a passage on each side 6 feet wide, through which thewater flows into the reservoir.

After the tide is mounted to its extreme height, which is 20 feet above low water-mark,the water runs back from the reservoir into the Thames, when a contrary motion is givento the tide wheel : the most ingenious part of the machinery is that by which the wheelis raised and depressed.

Suppose A B to be the section of the water-wheels, and P the vertical shaft, which car-ries two equal wallower wheels, E, F, so placed that either of them may, as occasion requires,be driven by the first wheel D; and the first wheel acting upon the wallowers F and E atpoints diametrically opposite, will, although its own motion is reversed, communicate therotary motion to the vertical shaft always in the same direction. In the figure, the wheel

Fig. 1744. LATERAL PRESSURE.