61. Tanks to store water for supplying plumbing fixtures must be set at least 6 feet above the level of the highest point to which water is to be supplied by them.

The choice of material to be used in the construction of a 4-4 tank is governed by the use which is to be made of the water, and also by the size of the tank.

62. For storing drinking water, circular wooden tanks, made of well jointed staves, thoroughly hooped, are used whenever circumstances will permit. Cedar is commonly used in their construction, since it is easily made watertight and is very durable. Wooden tanks are chiefly used for out-of-door purposes, being often placed above the roofs of the buildings they are intended to supply.

Tanks of extra large capacity are frequently made of wrought-iron plate, and are circular in form. They may be rectangular in form if desired, but for equal capacity rectangular tanks are more expensive, because of the elaborate system of bracing which is required to keep them in proper shape. Iron tanks require protection against frost; for, since iron is a good conductor of heat, the heat of the water is rapidly transmitted to the outer air, and the water contained in the tank freezes. This does not occur to such an extent in wooden tanks.

63. Rectangular wooden tanks are very difficult to keep water-tight; consequently, they are usually lined with sheet metal. In a lined tank, the wooden sides and bottom have only to support the sheet-metal lining and resist the hydrostatic pressure. They are not required to be watertight.

The sides and bottom should be 1 1/2 inches thick and upwards, according to the distance between the supports. The supports should be placed so closely together along the sides and ends, and across the bottom, that the planking will not spring between them.

In Fig. 32 is shown two methods of bracing the sides of a rectangular tank. The sides are prevented from bulging outwards by vertical posts A and A1. In the method shown at the left, the post A is secured in position by mortise and tenon joints C to the horizontal timbers D, D, and are wedged tight by wooden wedges E, E. This kind of bracing is not so strong as that which ties the post A1, which is held in position at top and bottom by wrought-iron bolts or rods B, B. The lower rod should be stronger than the top rod. Should the tank be deep, extra rods should be run through the body of the tank to tie in the center of the vertical posts. Iron fish-plates or large washers F must be used to prevent the nuts from cutting into the wood. All the bracing timbers must run crosswise with the lining boards G.

Fig. 32.

The ends of the tank must be supported by posts and tie-rods, as the sides are. The bottom tie-rods will necessarily pass through the sills. The sills must never be notched to pass the rods, but holes for that purpose should be bored along the center line of the timber. It should be remembered that the pressure upon the sides of a tank depends wholly upon the depth of the water, and is unaffected by the width of the tank, while the pressure upon the bottom depends upon its area and the depth of the water. Thus, with an equal depth of water, a tank but 1 inch wide or less will have exactly the same bursting pressure upon its sides as one 10 feet wide or more.

Care should be taken to provide a floor, or other supports, beneath a tank of sufficient strength to support it without sagging.

64. Tanks above roofs are usually fitted up a few feet above the roof in order that connections may be conveniently made underneath, and that the tanks may be protected from frost by suitable casings. Tanks in sheltered positions are usually set on flat floors, etc., with the pipe connections leading from their sides.

A tank which is outside, or on top of a building, should be provided with a wooden housing, to protect it from frost and from the heat of the sun. Care must be taken in locating a tank to avoid the vicinity of soil pipes and vent pipes, and all possible sources of contamination by foul air. All storage tanks, either inside or outside of buildings, should be provided with tight covers to prevent the entrance of dust, etc. The covers should be carefully ventilated, close-meshed brass gauze being used for covering the apertures used for ventilation.

65. The overflow pipe should be made with a wide mouth. The supply pipes to the kitchen boiler and to the cold-water system, are attached to the bottom. The end of the cold-water supply pipe should be extended at least 3 inches higher than the end of the pipe to the boiler. This insures that the cold water will fail at the faucets before it fails at the boiler, and that the boiler shall be supplied as long as the tank contains any water.

If the tank is supplied from the street mains, the supply should be controlled by means of a ball-cock and float; if supplied by a pump, the water should be delivered over the side of the tank.

To give notice when to stop pumping, a small overflow pipe, or telltale, is connected at high-water level, and its outlet is placed at some visible point near the pump. When the tank is properly filled, water will flow from the telltale and thus indicate the fact to the operator at the pump.

66. Tanks receiving their water supply intermittently, as from pumps which have to supply a residence with water and also supply water to a barn, and for sprinkling lawns, etc., are usually provided with two outlet pipes, one of which is connected at a higher level than the other. The object of this arrangement is to cause the supply to the barn, etc. to fail first, as the tank becomes exhausted, and to hold a definite quantity of water in reserve for the use of the dwelling. The stoppage of the water supply to the barn serves as a notice to the attendant to start the pump.

If rain water be led directly from the roof gutters into a tank, the conducting pipes should not only have strainers in the gutters, but should also have a movable basket strainer of fine mesh, hung on the mouth of the pipe, where it can easily be taken off and cleaned. The coarse strainer in the gutter will keep back leaves and twigs from trees, birds, etc., while the small mesh strainer hung on the tank end will catch all the smaller matter, which would otherwise accumulate in the bottom of the tank, decompose and contaminate the water.

67. All tanks should have a washout cock of large caliber attached to the lowest part of the bottom, so that mud and sediment can be scrubbed and conveyed through it to some convenient point of discharge. Or, the overflow pipe may be taken through the bottom of the tank and continued up to 2 or 3 inches below the top, with a hollow brass waste plug and socket connection at the bottom, so that the standing overflow within the tank can be lifted out of the ground socket, and the tank thus emptied. The upper end of the standing overflow should have a funnel mouth.

Large tanks are usually provided with a device to show the level of the water in them. A float commonly made of wood rests on the water, and is connected by a wire or cord and guide pulley, to a small weight or pointer which slides over the graduated scale, which is located at some convenient point of observation.

68. The size of tank required for any building depends chiefly upon the nature of the water supply, the character of the building, and the rapidity with which the water will be used.

If the water supply is from street mains, the tank capacity for residences should equal at least a 2-day supply, estimating at the rate of 25 to 30 gallons of water for each occupant.

If the tank is supplied by a windmill, hot-air engine, or other pump, and if it is located in the country, the tank capacity should be much greater; it should be good for a 5-day supply at least.

69. The amount of water which is actually required per day, has been found by measurement to be about 25 gallons for each person, large or small.

This amount is approximately made up as follows: 1 quart for drinking. 1 quart in food.

1 gallon for washing dishes and cooking utensils.

2 gallons for house cleaning.

3 gallons for washing clothes. 5 gallons for toilet purposes.

The remainder is used for bathing and water closets. A horse will drink about 7 gallons per day and will need 4 gallons for washing.

A carriage will require from 9 to 16 gallons for washing. A cow will drink 5 to 6 gallons per day.

70. If water is to be stored in a tank for drinking and cooking purposes, great care must be taken to make the tank lining of a material that will be insoluble in the class of water contained. It is found that sheet copper tinned on the inside forms an excellent lining, and is often used in wooden tanks.

Tanks made of impervious materials, such as porcelain, glass, slate, stoneware, etc., are also used for storing drinking water. The slabs of which the tank is composed are usually made water-tight at the seams with red or white lead. and are held in position by staybolts, which bind the opposite sides and resist the water pressure.

71. A sheet-metal tray, or safe, is usually placed under water tanks in buildings to prevent damage by leakage or overflow. The safe is usually turned up 3 or 4 inches all around its edges, and is provided with a 1 1/4 or 1 1/2 inch safe waste pipe discharging to the atmosphere at some convenient point, such as over and into a sink.

Sheet lead, zinc, and galvanized iron are unsuitable for tank linings, because they will poison the water. Tanks may be constructed of plain black sheet or tank iron, or of cast iron. These will not injure the water unless it stands a long time, in which case it will be discolored, and will acquire a nauseous taste.

If rain water, or other soft water, is to be stored in a lead-lined tank, the lining should first be thoroughly covered with a coat of lime wash. This will form a coating of carbonate of lead, which will retard the corrosion of the lead and the contamination of the water.