USEFUL AETS. 



CHAPTER XVI. 



TILLAGE. DRAINAGE. SPIRAL PRINCIPLE. CENTRIFUGAL 



FORCE. 



Systems of cultivating Ground. The Fallow System. Manuring the Ground. 

 Custom of China. Nature's Abhorrence of Waste. What becomes of Dead 

 Animals. Burying-beetles. The Scarabaeus-beetles and their Work. 

 Drainage versus Sewage. Clay Soils and Drains. The Mole, the Earth- 

 worm, Rats, Mice, and Rabbits. The Flexible Drain and the Lobster's 

 Tail. The Turbine Pump and the Ascidian. The Spiral Principle. The 

 Smoke-jack, Kite, and Wings of Birds. Centrifugal Force. Revolution of 

 Planets. The " Governor " of the Steam-engine. The Sling, Amentum, and 

 Mop. The Gyroscope, the Bicycle, and the Hoop. 



SEVERAL times, in the course of this work, we llave touched 

 upon man's dealings with the earth, such as mining and 

 tunnelling. We will now take another side of the same 

 question, and, in connection with Tillage, consider Drainage, 

 whereby superabundant moisture is removed from the earth, 

 and Manuring, whereby the exhausted soil is renovated. 



We will take this subject first. 



It has long been known that it is impossible to get more out 

 of the ground than exists in it, and that when the soil has 

 been so worked as to become unproductive, there are only two 

 remedies. The one is to allow the ground to remain unculti- 

 vated for a time. It must be ploughed in deeply, as if it were 

 to be sown with a crop, and must be left to recruit itself 

 from the air. This is the now abandoned " fallow " system, 

 which used to be in full operation when I was a child. 



As, however, population increased, and with it the per- 

 petually increasing demand for food, land was found to be too 

 precious to be allowed to lie fallow and idle. Then came the 

 system of rotation of crops, potato following wheat, clover 



