LAYING OUT OCTAGONAL WALL. 107 



and a great pressure, although it is only 15 inches thick at 

 bottom and 12 inches at top. This is heavy enough for 

 any-sized octagon, because in this form one side braces 

 against every other side. In a concrete wall under a very 

 long barn it would be proper to have a short pier built 

 against the inside every 50 feet to prevent a side swaying 

 in a strong wind. 



In building the concrete wall the service of a mason is 

 quite unnecessary. You need only good, common laborers, 

 one of whom is learned in mixing the materials in proper 

 proportions. Anyone who is capable of tending a mason 

 can mix the materials and superintend placing them in the 

 boxes. 



PBEPARATIONS FOB LAYING OUT THE WALL. 



If there is moisture to come to the wall, water-lime must 

 be used, and it is well to carry two or three feet above the 

 ground with concrete. The place should also be excavated 

 one or two feet beyond the proposed wall, so as to leave an 

 air-space on the outside, giving the wall a chance to dry 

 and become hard. If, in any case, you go into the slate 

 rock, which is always full of seams charged with moisture, 

 you must not allow the concrete to be built against this 

 rock, for the moisture in the rock coming into the thin 

 mortar will cause the milk of lime to run out and leave an 

 infinite number of fine pores through which water will run ; 

 but if no water is allowed to come to it while drying, it 

 will be water and air-tight. It is also well to have a drain 

 cut lower than the bottom of the wall, on the outside, to 

 carry off any water that might otherwise come against it, 

 which will render the basement dry. 



How TO LAY OUT AN OCTAGONAL WALL. 



The shape of this wall may give some trouble to get it 

 so exact as to receive the lower rim of timber or sills. It 



