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pie space to allow several cabins to be 

 grouped at the bottom, one beside the other. 



My calculations result in the structure 

 which I will now describe. With strips of 

 boarding a good finger's-breadth thick, which 

 will later reduce evaporation, the carpenter 

 builds me a square, hollow prism, measuring 

 some ^6 inches in height. Three of its 

 sides are permanently fastened with nails; 

 the fourth consists of three shutters of equal 

 size held in place by screws. This arrange- 

 ment will enable me to inspect at will the top, 

 the bottom or the middle part of the appara- 

 tus without shaking the contents. The in- 

 ner side of the prism measures nearly 4 in- 

 ches each way. The lower end is closed; 

 the upper end is free and has a ledge on 

 which rests a wide, projecting tray, repre- 

 senting the surroundings of the natural bur- 

 row. The tray is covered by a wire-gauze 

 dome. The hollow column is filled with 

 moist sandy earth, suitably packed. The 

 tray itself receives a layer of the earth, a 

 finger's-breadth in depth. 



There is one indispensable condition to be 

 observed : the earthy contents of the appara- 

 tus must not get dry. The thickness of the 

 planks prevents this partly; but it is not 

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