592 WIIEEL gravelled roads beaten hard, ^5 and on the best paved or hard earth roads, about -fa. Of the most important of Morin's results the fol- lowing is a brief summary: 1, the resistance to wagons on solid metalled roads or pave- ments, taken with reference to the axle, and in a direction parallel to the ground, is sensibly proportional to the pressure or total weight of vehicle and load, and inversely proportional to the diameter of the wheels ; 2, on such roads the resistance is very nearly independent of the width of the tires; 3, upon compressible bottoms, such as earths, sands, gravel, &c., the resistance decreases with increased width of tire ; upon soft earths, such as loam or sand, the resistance is. independent of the velocity; 5, upon metalled roads and upon pavements the resistance increases with the velocity, but the resistance is less as the wagon is better hung (with good springs) and the road more smooth ; 6, the inclination of the line of draught (the direction in which the pull of the team takes effect) should approach the horizontal for all roads, and for common wagons so to as the construction will admit. These results or laws are to be regarded as approximations, and as, practically, varied with the conditions. It is usual not to bring the line of draught near- er than by about 15 to the horizontal. (See the chapter on "Draught of Vehicles" in Mo- rin's "Fundamental Ideas of Mechanics, and Experimental Data," New York, I860.) Car wheels were at first made like ordinary spoke wheels, and wore guided by flanges on the rails, as on the Sheffield colliery railroad in 1767, where the rails were of cast-iron. In 1789 car wheels were made with flanges to run on an edge rail, that is, a rail which rest- ed upon its edge instead of lying flat. These rails were first made of cast iron and used at Loughborough, England. Car wheels with cast-iron hubs and rims and wrought-iron spokes were patented by Stevenson and Losh in 1816. A wrought-iron tire was shrunk on to the rim, and secured in its seat by a dove- tailed depression. At the present time there are in use in the United States the following kinds of car wheels, which may be mentioned as examples of different modes of construc- tion. The Washburno wheel, patented by Sax and Kear, is composed of a cast-iron centre surrounded by a steel tire. It is made by first raising the tire to a white heat and almost to the fusing point, and casting the centre into it, which causes the union of the two parts. These wheels are heavy and durable, and ca- pable of running 200,000 miles. As is the case with all car wheels, the centre may be com- posed of a hub and spokes, or of a hub and disk. In Moore's patent there is a packing of wood for diminishing the jar between the cast-iron centre and the steel rim, the latter being put on with hydraulic pressure. There are many wheels now made with a cast-iron centre and a steel tire shrunk on. Atwood's patent wheel, recently introduced, consists of a cast-iron centre and steel tire, between which there is a highly compressed oakum packing for the purpose of diminishing the jar. The kind in most general use is the chilled iron wheel, made of different patterns by different car wheel companies, but in all cases cast into a cold cast-iron mould, which chills the sur- face of the rim. This wheel has no separate tire, and cannot in consequence be "turned up " in a lathe when the rim becomes worn, as all steel-tired wheels can. The usual mileage of chilled iron wheels is 60,000 miles. The Hamilton steel wheel company are introducing a wheel made of a mixture of cast steel and cast iron, melted and cast together, whereby the strength is supposed to be increased. It is intended as a substitute for the chilled iron wheel. Car wheels are also made entirely of cast steel, which can, like steel-tired wheels, bo " turned up " when worn. The driving wheels of locomotives are made with cast-iron cen- tres and steel tires shrunk on. Formerly car wheels were keyed on their axles, but now the hole in the hub is turned slightly smaller than the perfectly cylindrical axle, which is then thrust in with hydraulic pressure. Wheels so treated have the advantage of never getting loose, while the keyed wheel always will in time with constant use. The bearings of loco- motive wheels, both small and drive wheels, are on the inside, but truck wheels have the bearings on the outside. The bearing con- sists of a brass or other composition box in which the wrought-iron axle turns. Water wheels are intended to impart to connected machinery the moving force due to the weight or momentum of water, or to both these com- bined. They are divided into two general sorts, according as they have horizontal or vertical axes. The latter, most of which are also reaction wheels, are considered under TUR- BINE. The former class, or those with hori- zontal axes, include the earliest known forms of water wheel; and they are generally the simpler in construction. If the natural current of a stream be employed, it readily appears that, supposing the conditions of depth and friction along the bed to correspond, the mov- ing force in any case will be as the product of the volume of water that would in a given time strike and act on the float boards of a wheel into the amount of fall, or the descent of the stream within a given distance. But owing to the irregularities in the volume and velocity of streams at different seasons, and the loss of momentum by friction against their beds, it becomes desirable to accumulate and retain a certain supply or head of water, and to develop the impelling force of this, duo to volume and gravity, at a fixed position. These results are secured by constructing a weir or dam across the stream, and allowing the water collecting in a pond above this to fall upon the wheel at one of three points, which points of applica- tion of the force give to the wheel the name of overshot, undershot, or breast wheel. The
