CON STRUCTION TANKS AND POOLS 221 



these points. The corners are always the weakest sections. As the 

 concrete is poured, lay in extra right-angle pieces of 12 inches total 

 length, that is, 6-inch elbows. 



The pouring of the concrete makes a great strain on the forms. 

 These need to be thoroughly braced in the beginning, for it is impos- 

 sible to improvise means of looking after these points once the pour- 

 ing has started and the walls have bulged or the corners sprung. 

 The outside bottom stakes are merely driven into the ground and not 

 otherwise fastened. Outside corners, in addition to being nailed, are 

 secured by pieces of tin, nailed through. Inside corners are held by 

 iron elbows (to be had at hardware stores). They are secured by 

 screws. The nails fastening side-cleats should be driven through and 

 turned over. Concrete walls had best be thicker at the bottom, put- 

 ting all the slope on the inside. A good inside depth for ordinary 

 purposes is 15 inches, filling to 13^ inches with water. If an over- 

 flow is to be made through side, make a removable core of soft wood 

 and saturate it with water so that it will contract upon drying. For 

 outdoor pools an inside slope of 2 inches is about right. This would 

 be, for a 7 X 9-foot pool, a top thickness of 4 inches, with 6 inches at 

 the bottom. Base, 6 inches thick. For larger sizes add 1 inch of 

 thickness to walls and base. Indoor pools require no slope. In mak- 

 ing all calculations, allow ^ inch for thickness of finishing coat. 



The forms should be leveled up and made absolutely true in the 

 beginning. It is a bad plan to depend upon the finishing coat to 

 correct inaccuracies. It seldom does it. Some considerable care is 

 necessary to establish the four corners at perfect level. Select one 

 corner and measure each of the other three from that. If the spirit- 

 level is attached to a board, its trueness should be tested by reversing 

 the ends. In any case, the level should be reversed on each test. 



The concrete is mixed 1 part cement, 2 parts sand, 4 parts Y^-'mch. 

 crushed stone or round stone. This is poured to within >< inch of 

 top of form, beginning with the base, and must be well tamped to 

 avoid bubbles and open spots. Those wishing to secure the best pos- 

 sible results first place a thin skin of cement over the cinders, so the 

 water in concrete will not seep into the cinder base, it being desir- 

 able that all concrete work dry slowly in order to crystallize perfectly.- 

 This preliminary skin is as thin as possible and is applied the day 

 before. It is composed half each of sand and cement. If this is used, 

 the lower layer of reinforcement may be laid directly upon it. 



The finishing coat should be applied within twenty-four hours 

 after concrete work is done, and is made 1 part cement, 2 parts sand. 

 Finish one side at a time, top edge first, inside surface next, then out- 



