The water from a reservoir is usually drawn off by means of pipes which are laid inside a masonry culvert built under the dam. The pipes can thus be inspected. The culvert is blocked at its upstream end by a thick masonry wall through which the pipes pass. Accidents which have happened in the past have been due to weakness of the culvert or to water finding its way along the outside of the masonry. The culvert can be made of proper strength, and it should have a thick coating of clay puddle which is worked into the clay-puddle core-wall of the dam. If the core-wall is of masonry or concrete, the masonry of the culvert is properly joined to it. In many cases the culvert and pipes are taken through a cutting or tunnel and not under the dam. At the upstream end of the culvert there is a masonry tower--access to it is obtained from the top of the dam by a foot-bridge,--and from it valves for opening and closing the pipes are worked. If the reservoir is for the water supply of a town, it is arranged, by means of a vertical pipe, that the draw-off can be at various levels so that the surface-water can always be used. In the case of some of the towers at the reservoirs whence Birmingham is now supplied, the vertical pipe consists of a number of steel cylinders with gun-metal faces which are so accurately made that the joint is water-tight when one cylinder merely stands on another. The draw-off is obtained from a given level by lifting a particular number of cylinders. Sometimes the tower is made of reinforced concrete. When it is lofty it should be strong enough to resist a strong wind, blowing when the reservoir is empty. 4. =Masonry Dams.=--For heights much exceeding 110 or 120 feet a masonry dam may be cheaper than an earthen dam; and in case a flood occurs while work is in progress the masonry might suffer little injury, while earthwork might be swept away completely. Masonry dams are usually built of random rubble masonry with faces of dressed stone. Such masonry weighs about 140 lbs. per cubic foot, and is ordinarily quite safe when subjected to pressures of 20 tons per square foot, but in a masonry dam a high factor of safety is necessary, and 15 tons per square foot may be allowed. In a wall of such masonry with both faces vertical, the pressure, owing to the weight of the wall, will reach the above limit when the wall has attained a height of about 220 feet. In a masonry dam, although the masonry is always of the best quality, it is a rule to calculate the dimensions so as to give no tension on any part of the masonry. Any crack or opening of a joint, occurring perhaps before the masonry had hardened, would let in water, and its pressure would tend to gradually extend the crack and eventually to overturn the portion above the crack.