112 



THE POPTJLAB EDUCATOR 



British Islands the average is about 36 inches ; but even in our 

 small island the difference of the rain-fall in neighbouring locali- 

 ties is very remarkable : for instance, at Whitehaven, in Cum- 

 berland, there fell, in 1849, 32 inches ; while in the Vale of 

 Borrowdale, only 15 miles distant, the fall was 142 inches 

 almost four times the amount. This is due to the position of 

 the valley ; it is traversed by the prevailing winds, which come 

 loaded with moisture from the sea. The mountains condense 

 this moisture, and the valley receives the rain. 



The most remarkable instance of this combination of local 

 causes is perhaps found in the case of the Khasia Hills, which 

 form the southern side of the valley 

 of the Brahmapootra, just as it 

 enters the delta of the Ganges. 

 These mountains are some 4,000 or 

 5,000 feet high, and their south 

 flank looks over the delta towards 

 the Bay of Bengal. When the south 

 monsoon blows, it traverses the 

 river-flats, and arrives at the foot of 

 the Khasias loaded with moisture. 

 Impinging upon the mountains, it is 

 driven upwards ; it rises into colder 

 regions, and to a height at which 

 the air is greatly rarefied. Now, 

 when air is rarified, it acquires a 

 greater power of containing heat, and 

 consequently it absorbs its own sen- 

 sible heat, and thus its temperature 

 falls. From the combination of FIG. 7. BAIN-PRINTS AND WORM-TRACKS IN CARBONIFEROUS the excavation. 



these causes the monsoon delivers 

 up its moisture, and as much as 



600 inches of rain fall on the south flank of the Khasias yearly. 

 The denuding effect of this vast quantity of water is greatly 

 enhanced by the fact that it almost all falls in the six months 

 in which the monsoons blow. In many tropical countries the 

 fall averages 200 inches. Just as there are combinations of local 

 circumstances which produce unusual rain-falls, so there are 

 regions where the opposite effect is the result. Rainless regions 

 are found generally in the centres of vast continents. Ere the 

 winds reach these localities they have been dried, the moisture 

 they carried having been condensed by mountains which inter- 

 vened between the seas and the centre of the continent. There 

 is a strip of coast-line in Chili and Peru where no rain fall*, for 

 the prevailing winds deposit their moisture on the opposite 

 flank of the Andes, and when they reach the countries to the 

 loeward they are dry. No rain has fallen hero for years, as may 

 be proved by the fact that houses are built of " Chili saltpetre," 

 or sodium nitrate, which is a salt soluble in water ; hence the 

 existence of the structures is a testimony to the rainless cli- 

 mate. Considering that every drop of rain does a geological 

 work it either carries down a grain of sand to a lower level, or 

 Boaks into the soil, loosening it for the action of the next 

 ehower it may be conceived what a vast and universal work 

 is being carried on by drops of rain. If it required proof that 

 the surface of the earth had always been subject to the action 

 of rain, we should find it in 

 the frequent discovery of rain- 

 prints in some of the oldest sys- 

 tems. Many specimens of these 

 interesting remains have been 

 found in rocks of the Carbonife- 

 rous period. During the age in 

 which the coal-fields were depo- 

 sited, the atmosphere to encourage tho rapid growth of the 

 dense vegetation which characterised that epoch must have 

 been highly charged with moisture, and, as a consequence of this, 

 showers of rain must have been of constant occurrence. 



Pig. 7 shows a slab exhibiting rain-prints and worm-tracks, 

 discovered in the carboniferous green slate in Nova Scotia. 



Si?.- Charles Lyell has most satisfactorily explained the pre- 

 sence of these rain-prints. His observations were made on the 

 shores of the Bay of Fundy, where the tide rises higher than 

 any other place in the world more than 70 feet. The exten- 

 r>ive mud-flats are left dry for the period between the high tides. 

 If a shower fall while the mud is yet soft, an impression is 

 made, which the hot sun bakes into an enduring cast, so that 



GREEN SLATE. 



effaced, but a layer of mud is deposited upon them. In process 

 of time this becomes rock, and when split the rain-prints are 

 exhibited, faithfully registering the fact of the shower, and 

 frequently showing the direction of the wind, by the cavity 

 formed by the drop being deeper on that side to which the drop 

 was driven. 



Springs. Not satisfied with the work done as individuals, the 

 rain-drops seek to combine their power, and collecting in cavi- 

 ties in the hills, and in the more porous rocks, form springs 

 which permanently feed rivers. Although the degrading action 

 of springs cannot be compared to that of rivers, yet it is by no 

 means despicable; their action is 

 continuous, and every spring wears 

 for itself a valley, of greater or less 

 size, according to the length of time 

 it has flowed, the quantity of water 

 it discharges, and the nature of the 

 rock from which it issues. 



The origin of one class of springs 

 will be at once conceived by watch- 

 ing a child dig a hole on the sea- 

 beach. The retiring tide has left 

 the sand saturated with water, which 

 drains into the hole and soon fills it. 

 This is precisely tho case with porous 

 rocks soaked with rain-water. If a 

 well be sunk down to tho base of such 

 a rock, where a less porous stratum 

 underlies it, water will soon rise in 

 Now suppose there 



St. PauPs. 



Fia. 8. IDEAL SECTION OF LONDON BASIN. AA, CHALK ; BB, CLAY 

 c, GRAVEL ; a, SURFACE WELL ; H, ARTESIAN WELL. 



exist some fissure in the rock, 

 we have at once a natural well ; 



and as soon as an outlet is found, which is at a lower level 

 than the surface of the water, a spring will be the result. 

 Springs will be plentiful at the outcrop of a layer of clay, or 

 some other deposit impervious to moisture, above which lies a 

 porous rock, such as chalk or sandstone, which will retain tho 

 rain. Tho reason why there is not an issue of water from tho 

 whole line of the outcrop of the impermeable strata is that tho 

 water follows the valleys formed by the inequalities of the sur- 

 face of the strata, so that where a spring does occur we may 

 conclude that that is the end of a valley of which the spring 

 may be conceived to be the river. In this instance tho valley 

 made the river, and not the river the valley. 



Artesian wells have thrown some interesting light on the 

 underground system of waterworks. Those wells or borings 

 are made by an auger generally about four inches in diameter. 

 When rock is reached, it is triturated by an iron rod, and tho 

 debris removed by the auger. The sides of tho "boro are pro- 

 tected by sinking iron pipes. The boring is continued until a 

 porous stratum is reached. The Artesian wells in the neigh- 

 bourhood of London are sunk to reach the chalk, and are about 

 320 feet deep, and they yield some 15,000,000 gallons a day ; 

 but the water does not rise so high in the well as it used, 

 proving that the chalk reservoir is not inexhaustible. 

 There is a well at Grenelle, near Paris, 1,800 feet deep. 



Frequently, in the course of 

 the boring, a subterranean cavity 

 is tapped, from which the water 

 rises with great force. At 

 Tours, when the depth of 364 

 feet was reached, there was r, 

 sudden rise of water, whicii 

 brought up with it a quantity of 

 sand, shells, branches of thorns, seeds, etc., and there was rea- 

 son to believe that these came from some of the valleys of Au- 

 vergne, 150 miles distant, proving a cavernous connection 

 between the two places. 



Fig. 8 represents an ideal section of the London basin: 

 a is a " surface well ;" H, an Artesian well. The water from a 

 looks sparkling, and is more refreshing than from H ; and is dele- 

 terious just in proportion as these qualities recommend it, for 

 they are due to organic matter and nitrates, which the rain 

 gathers as it comes into contact with decomposing animal matter 

 on the surface; whereas the water from H enters the chalk 

 strata, A A, far out in the country. 

 Artesian wells derive their name from Artois, in France, 



when the next high tide covers the flat the rain-prints are not j where they were first sunk. 



