398 ENGINEERING ON THE FARM 



A mixture of i : 2 : 4 should be used if it is select aggregate, 

 or 1 : 4 if bank run. The concrete should be mixed fairly- 

 wet and tamped as it is put in place. The thickness 

 of the walls shown in the figure is the minimum. If the 

 location is in clay and a little care is exercised in the exca- 

 vation, only an inside form is necessary. Where the soil 

 is sandy and likely to cave or slip, it is essential that 

 both an outside and an inside form be used. As the tank 

 is out of sight, any rough lumber may be used for the form. 

 The top can be made of 2 -inch pine planks, laid across. 

 Such a top will probably need to be replaced in seven years. 

 The only advantages it has are the cheapness of first cost 

 and the ease with which it is put in place. The best form 

 of top is one made of reinforced concrete slabs, 4 inches 

 thick, 18 inches wide, and with a length equal to the width 

 of the tank plus the thickness of the two walls. These slabs 

 should be reinforced with ^-inch rods placed 4 inches apart 

 and about one-fourth the depth from the bottom. Old 

 horseshoes or bent iron rods may be placed in each of the 

 slabs to serve as handles for lifting, and small pipe, old iron 

 rods, or hayrake teeth may be used for reinforcement. 

 These slabs can be molded at some convenient place, and 

 when three weeks old placed on top of the tank. Their use 

 makes it possible to do all the concrete work at one time 

 and to remove the entire covering from the tank if desired. 

 It avoids the need of placing forms inside the tank for 

 overhead covering. For quantities of material, methods of 

 mixing concrete, etc., see chapter on concrete. 



A wide variation may be made in any of these tanks for 

 the purpose of using materials that are already on hand, or 

 to take the place of materials that cannot readily be pur- 

 chased in the local market. 



There is no great economy in constructing too small a 

 tank. A tank 6 feet long, 3 feet wide, and 3 feet deep is as 

 small as should be used for the smallest family; and the size 

 should be increased whenever it is found too small to hold 



