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The Cornell Reading-Courses 



Hotbeds of concrete 

 In the case of concrete, a form of plank or of boards is built, having 

 for its inside measurements — which are those of the inside wall of con- 

 crete — the same inside dimensions as for the all-wood bed. The wall 

 of concrete, however, should be at least 3 inches thick at the top and it 

 is generally best to have it 4 or 5 inches thick at the bottom, the inside 

 face being perpendicular and the outside face sloping slightly. The depth 



Fig. 16. — Hotbed. Pit of concrete, top of wood. This will last a lifetime 



and the difference in height of the front and back walls, with the ends to 

 correspond with the slope of the sash, are the same as for the bed pre- 

 viously described. One additional construction is needed — that of 

 several quarter-inch bolts, 7 inches long, set upside down in the top of 

 the concrete. These bolts are for fastening the plank sill to the concrete. 

 After the hole is dug and the form constructed, it is necessary to mix 

 the concrete and fill the form. The following proportions make a good 

 mixture: 100 parts of cement, 200 parts of sand, and 400 parts of fine 

 gravel. The gravel is placed on a board platform and spread out 3 to 4 

 inches deep. Over this the sand is scattered, and the cement is laid over 



