570 COTTAGE, FARM, AND VILLA ARCHITECTURE. channel or wheel-course through an aperture level with the bottom of the wheel course. In this way the water is lu-ged by the pressure of the water in the dam, and rushes out from the aperture in a stream or spout, with a velocity proportionate to the perpendicular pressure, striking the float-boards of the wheel, so as to urge them forward. Such is the form of the undershot wlieels still generally employed in France and other parts of the Continent ; but in England they have long been superseded l>y more effectual applications of the water, and are very rarely met witlu Undershot wheels of this description are generally called groundshot wheels, because the water shoots along th" ground or floor of the channel in which the wheel works. Fig. 1101 is the vertical section of an undershot wheel, as now in use ; in which a is the axis, which, if not of metal, should be made of hard and durable wood, of a length and size proportioned to the size and weight of the wheel. Into each end of this axis a gudgeon or centre-pin should be fixed, for the wheel to turn upon. There are two methods of fixing the gudgeon into a wooden axis : one is by forming the gudgeon with a cross, which is let into the end of the tree, and fastened by screws, the wood being compressed round the cross by two or three iron hoops, fitted on the end of the tree, and wedged. The other method is, to make a strong iron box in a piece with the gudgeon, into which box the end of the tree is received and secured with wedges : in the latter case, the box being of an octagonal shape, and the wood being cut to the same form, it cannot slip round with the box. Of late years it has been usual to make the axis of water-wheels of cast-iron tubes, which is a very good plan if they are of sufficient dimensions. In an iron axis, it is advisable to make the bearings of the axis close to the side of the water-wheel ; and to leave the ends of the axis projecting beyond the bearing, in order to attach the cog- wheel. This diminishes the length of the axis between the bearings, and renders it much stronger. The arms supporting the circular rim of the ^-heel h b, are usually eight in number, framed together so as to intersect each other at right angles, and leaving a square oiiening in the centre for the recejition of the axis ; the corners roimd which being filled up by adding pieces of wood to it, and the wheel being fastened on by wedges. The only objection to this mode of framing is, that the arms are weakened by intersecting each other; and support the circular rim of the wheel in imecjual segments: but on the whole it is considered superior to the metliod of mortising diverging arms into the axis, because by so doing the axis is much weakened, and the water, being admitted into the centre of the tree, soon causes it to decay. Another objection is, that an arm cannot be easily replaced without taking all the wheel to pieces. The very best method of uniting the arms to the axis is to have a cast-iron centre-piece, or strong hoo]), to fit on the axis, with a broad projecting flanch round it, against tlic flat surface of which the arms of the wheel are applied, and the intervals between them are filled up by wooden blocks or wedges : the arms and blocks are firmly bound to the iron flancii by iron rings a])plied to the arms on the opjiosite side to the flanch with screw bolts to go through the whole. The circular rim, c, is made of wood put together in two or three thicknesses, the joinings of one ring not coinciding with those of the other ; and there being eight or ten segments in each thickness, according to the size of the wheel. The thicknesses are united by rivets. The arms are attached to the ring by notching them into it, and securing them l)y bolts. Cast-iron rings are now generally used, and with gieat advantage ; because the necessary mortises can be made in the iron without weakening the ring, but the strength of a wooden ring is greatly impaired by the mortises througii it. The number of rings in a wheel must depend on its breadth ; when the %vheel is 4
