PROPAGATION BY MEANS OF SEEDS AND SPORES 43 



tion of moisture during the process of steaming. These cover boards 

 are further secured by two boards or planks which are laid over them on 

 the outside, at right angles to them, extending the length of the box, and 

 which are very securely nailed at the ends (Fig. 21). The cover boards 

 are then nailed from the inside to the outside plants, the object being to 

 prevent the swelling and warping of the cover boards. A good view of 

 the inside construction of the box is shown in Fig. 2. 



''Two ring-bolts are set in each side of the frame on the top, one near 

 each corner. Through the rings ax handles or bars are thrust to serve as 

 handles in moving the pan along the bed. 



"A strip of |-inch felt packing, 2 inches wide, is placed along the lower 

 edge of the frame, on the outside, to prevent the escape of the steam. The 

 hoop iron is then laid on the packing and nailed every 4 inches, one-half its 

 width extending beyond the lower edge of the frame, as shown in Figs. 21 

 and 23. When the packing cannot be had, the iron is nailed to the inside 

 of the frame instead of the outside. Instead of the hoop iron, 2-inch 

 angle iron is sometimes used. This is fastened to the lower side of the 

 frame, to form a tight joint when the box is laid on the soil. A piece of 

 thin packing serves to prevent the escape of steam between the iron and 

 the bottom of the wooden frame to which it is nailed. Angle iron, because 

 of its heavier character, will not bend and is better than hoop iron, but it 

 requires careful working to fit it to the frame. 



"In the middle of one end of the frame is set a f-inch pipe-threaded nipple, 

 6 or 7 inches in length, through which the steam is delivered into the pan. 

 The nipple should project 3 inches from the box, to afford a convenient at- 

 tachment for the hose, and it should be held securely in place on the 2 by 4 

 inch end frame by lock nuts, closing on leather gaskets, as shown in Fig. 23. 



"The pan is placed on the bed, open side down, the hoop iron or angle 

 bar cutting into the ground to form a knife joint, and the steam leaving the 

 boiler under pressure enters the pan and quickly penetrates the soil. 



"The boiler is placed close to the bed and where practicable at an equal 

 distance from each end. Steam traction engines and portable boilers have 

 a marked advantage, because they can be easily moved as the work pro- 

 gresses, allowing the use of a short pipe. 



"In sandy soils, after 30 minutes' steaming, the temperatures to be 

 expected in the upper 2 inches of soil directly under the pan are approxi- 

 mately 208 to 212 F., at 3 to 4 inches 170 to 180, and at 6 inches 120. 

 Two hours after the removal of the pan the temperature at 6 inches should 

 be about 160 F. If a thermometer is not available, the efficiency of the 

 steaming operation can be easily determined by burying a potato 4 inches 

 under the surface of the soil. The potato should be well cooked when 



