=181. Joints.=—Pipes may be laid with open joints, mortar joints, cement joints, or poured joints. Open joints are used for storm sewers in dry ground close to the surface. Mortar and cement joints are commonly used on all sewers except in special cases. Cement joints are more carefully made than mortar joints and result in a greater percentage of water-tight joints. Poured joints are used in wet trenches where it is necessary to exclude ground water from the sewer. A specification used in some cities for open joints is: Pipes laid with open joints are to be laid with their inverts in the same straight line and shall be firmly bedded throughout their length on the bottom of the trench. No cement or mortar is to be used in the joints. Not more than ⅛ inch shall be left between the spigot end of the pipe and the shoulder of the hub of the pipe into which it fits. The joints shall be surrounded with cheese cloth, burlap, broken pipe, gravel or broken stone. The purpose of the cheese cloth, etc., is to prevent fine earth from sifting into the pipe until the cheese cloth or other material has rotted away, by which time the earth has become arched over the opening. Mortar joints are specified by Metcalf and Eddy as follows: Before a pipe is laid the lower part of the bell of the preceding pipe shall be plastered on the inside with stiff mortar of equal parts of Portland cement and sand, of sufficient thickness to bring the inner bottoms of the abutting pipe flush and even. After the pipe is laid the remainder of the bell shall be thoroughly filled with similar mortar and the joint wiped inside and finished to a smooth bevel outside. In some work a wood block or a stone is embedded in the mortar at the bottom of the joint to bring the spigot in place concentric with the next pipe. Cement joints are specified in the Baltimore specifications as follows: