Fig. 133 represents a small hydraulic engine—The Compton Hydraulic Motor—attached to and operating a gas-compressor. It shows a style of water motor in large use in connection with city water-mains. A pressure of 15 to 20 lbs. per square inch is sufficient to operate it; the motor here illustrated occupies a floor space of 9 x 23 inches; it will supply gas burners to the extent of 6,000 candle-power. [Illustration: FIG. 133.] The valve motion on the motor is unique in this, the outlets and inlets have a positive motion by which they are simultaneously opened and closed by the motion of the piston; this valve motion is designed to overcome the back pressure; it has a governor, incorporated in the valve-motion for the purpose of maintaining uniform pressure on the main pipes. HYDRAULIC PACKINGS. Generally speaking a packing is a contrivance or a material to close a joint. Various greasy materials with gaskets, flax, hemp, etc., are used in joints which are screwed down, also collars of rubber, red lead, luting, graphite, etc. [Illustration: “U” PACKING—FIG. 134.] A most important part in the practical working of nearly all water-pressure machines is the leather collar, the invention of which by Bramah removed the difficulties which had been experienced in making the large ram work water-tight when submitted to great pressure. It consists of a circular piece of stout leather (see cut page 20), in the center of which a circular hole is cut. This piece of leather is thoroughly soaked in water and is pressed into a metallic mould and so that a section of it represents a reversed ^U^, and is fitted into a groove made in the neck of the cylinder. This collar being concave downwards, then in proportion as the pressure increases, the edge nearest the ram being trimmed down, it fits the more tightly against the ram plunger on one side and the neck of the cylinder on the other. It should be saturated with Neatsfoot or Castor oil so as to be impervious to water. [Illustration: CUP PACKING—FIG. 135.] When the least amount of friction possible is desired in the operating of a hydraulic plunger, there is no form of packing which can surpass a properly prepared and applied Leather “^U^” Packing (Fig. 134), and in practice its position is according to conditions, either in a groove near the upper end of the cylinder, or at the lower end of the ram. When for any reason it is not desired to use the outer lip of the packing, the resulting form is known as a Cup Packing, (Fig. 135), and when the inner lip is used then we have the Hat or Flange Packing. Fig. 136. [Illustration: FLANGE PACKING. FIG. 136.] When the water pressure is not over 2,000 lbs. to the square inch, and a greater allowance for friction is not important, a fibrous packing can be used, which is easier of application than these for large sized cylinders.