It is worthy of notice that although the magnetic needle of the compass turns to the north, it does not do so because it is attracted by the north pole, but because it is repelled from the east and west.
We have long known that if a current of electricity be sent round a magnetic needle, the latter at once turns at right angles to it. On this principle depends the Electric Telegraph. When communication is made by using the handles, a current of electricity is sent round the needles, and causes them to turn at right angles until stopped by a little ivory pin, which prevents them from overshooting themselves.
There is a perpetual stream of electricity passing over the earth from east to west, and in consequence all magnetic bodies are forced to turn at right angles, just as is the case with the magnetic needle.
USEFUL ARTS.
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 Scarabæus-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 have 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 uncultivated 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 perpetually 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 following potato, &c. But, above all, agriculturists learned that in the long-run there is nothing so cheap as manure, i.e. the return to the soil by animals of the elements which these animals took out of it.