CHAPTER 10
WATER SCULPTURE
149.Rainfall. -The water of the earth"s surface is constantly evaporating, rising into the air and being distributed by the winds. Much of this water is blown over the land, where it is condensed and falls as rain. Some portions of the land receive much and some little of this a?rial water circulation. When winds from warm seas, where the evaporation is great, strike lofty mountain ranges, the land upon theA HOT SPRING IN THE YELLOWSTONE.
windward side has a large rainfall, but that upon the lee side compara- tively little. This is particularly well shown in the northwestern part of the United States. Since continents, as a rule, have their mountains near their borders it happens that most continental interiors are com- paratively dry.
Regions over which the prevailing winds blow from a colder to a warmer latitude have little rainfall, as the air is continually having its capacity to hold moisture increased by its rise in temperature. This is well illustrated in the Sahara region. Accordingly, the amount of rainfall in different parts of the earth varies greatly. In eastern India south of the Khasi Hills a "record" fall of over 50 feet was recorded in one year, while in desert regions a year may go by without any fall of rain.
Fig. 111.
In the United States the greatest rainfall, over 80 inches, is found along the northwest coast and the least in the Basin Region of Utah, Arizona and Nevada. Whether rain falls in large or small quantities, its effect is always marked. Without it the surface of the ground is a parched and barren waste of dust and rock, with it, a green and varied expanse of never failing beauty.
150.The Sphere of Activity of Rain. -When rain falls upon theground, it may do one of three things. It may evaporate immediately from the surface and return to the air; or it may run rapidly off the surface and quickly join the streams and rivers which bear it to its final goal, the sea; or it may sink into the ground. In this last case part of it returns gradually through capillary action to the surface where it is again evaporated; part finds its way into springs; and part sinks deep into the soil and rock.
Which of these courses the greatest part of the rainfall will take depends entirely upon the condition of its fall and the kind of surface upon which it falls. If the rainfall comes down rapidly, the larger part of it will immediately run off; if it comes down gently, much of it willFLOWING ARTESIAN WELL.
sink into the ground. If it falls in forest regions or where there is much verdure, its flow will be impeded by the plants and roots. If the surface upon which it falls is hard-packed and impervious, most of it will run off, but if it is loose and easily penetrated, much of it will sink into the soil. Even in the dry parched sands of the desert, however, the rain falls sometimes in such cloud- burst torrents that it runs off in rushing streams.
151.Sub-Surface Water or Ground Water. -The rain that sinksinto the ground descends slowly along the little cracks or between the particles of soil until it reaches a point where it can sink no further, or until it finds an opening through which it can flow out to the surface at a point lower than where it entered. Here it may ooze slowly out, or it may be concentrated in a spring.
If the water which comes to the spring has penetrated below the surface far enough to get away from the heating effect of the sun, it will be comparatively cool when it again emerges, and it will form a cold spring. If, however, in the region where the spring occurs the rocks are hot at the depth to which the water penetrated before it found a crack through which it could cometo the surface of the land, then it
will become heated and will form a hot spring.
A LIMESTONE CAVE.
As the crust of the earth is in many places composed of rocks in layers, the rain often falls upon the top of a folded porous rock layer,MONTEZUMA"S WELL.
This famous water hole is due to the dissolving of the underlying rock layer.
below which is a rock through which it cannot penetrate. The water will then accumulate throughout the porous rock. If this rock layer in another part of its extent is overlaid by an impermeable layer, its water is held in by the impermeable rocks above and below, and so is under hydraulic pressure. When a hole is made in the upper rock layer (Fig. 111), the waterSINK-HOLE IN TENNESSEE LIMESTONE.
GREAT NATURAL BRIDGE, UTAH.
will flow to the surface and if the pressure is sufficient, it may gush outof the hole.
Borings of this kind form what are called artesian wells. These are of great importance in many regions where it is difficult to obtain sufficient surface water. In someof our western states the water from artesian wells has been obtained in sufficient quantity for extensive irrigation. Although this water often contains minerals in solution, it is free from surface contamination and is thereforeusually healthful for drinking.
In some places the surface
NATURAL BRIDGE, SAXONY.
water penetrates into layers of rock which it can dissolve, such as salt or limestone. Here it forms caves and caverns, the solid material which occupied the place of the cave having been carried away in solution by the water. There are thousands of caves of this kind but perhaps the most noted in this country are Mammoth Cave with its nearly 200miles of underground avenues and grotesquely sculptured halls, and Luray Cave with its wonderful stalactite and stalagmite decorations. Sometimes the top of one of these caves is nearly eroded away, leaving a part of its old roof standing as a natural bridge, such as the natural bridge of Virginia or of Utah. Sometimes the top falls in, leaving a sink-hole.
152.Geysers.
Experiment 128. -Fit a 250 cc. glass flask with a two-hole rubber stopper. Through one hole extend a glass tube (a) almost to the bottom of the flask and through the other hole a tube (b), 5 or 6 cm.