236 



NATURE 



[November 21, 1918 



rocks have been subject to very considerable mass- 

 movements ami deformation, the displacements 

 unting in extreme cases to as much as ten miles 

 in the vertical and one hundred miles in the hori- 

 ontal direction. The ultimate cause of these move- 

 ments is unknown; thej can onlj I"' directly ob- 

 served in the outermost skin, and are probably taken 

 up in a different form in the dei pi r layers, but require 

 that beneath the outer solid layer- which for con- 

 venience, and because some name is required, is com- 

 monly called the crust there must be material which 



has some of the properties of a fluid, but not neo 



saril) more than the powei of change of form when 

 exposed to stress of sufficient magnitude and dura- 

 tion. The thickness of the outer crust has been 

 estimated by several distinct lines of deduction, all of 

 which agree in giving a figure of about twenty-five 

 miles, and this ma\ be taken as indicating the order 

 of its magnitude. The only means of arriving at anv 

 idea of the nature of the transition from the crust 

 to the underlying material is in the reflection of earth- 

 quake waves; this is ordinarily treated as taking place 

 at the surface of the earth, but there are grave diffi- 

 culties in the way of accepting this interpretation. A 

 more probable one is that reflection takes place at the 

 undei-surface of the crust, indicating a somewhat 

 abrupt transition from the solid and rigid crust to 

 the more yielding layer below. Whether this is a 

 separate layer or merely the outermost part of the 

 shell capable of transmitting both forms of elastic 

 waves is still unknown. 



The general result is that three distinct divisions 

 can be recognised in the interior of the earth : — (i) The 

 outer crust of solid matter possessing a high degree 

 of rigidity, whether against permanent or temporary 

 stress, of comparatively small thickness amounting 

 to about J per cent., and not more than i per cent., 

 of the radius; (2) 'a shell of material of thickness 

 about one-half of the radius which has a high rigidity 

 as against stress of the duration involved in the pro- 

 duction of the tides, or of shorter duration, but, in the 

 outer part at least, a comparatively low power of 

 resistance to stress of secular duration ; and (3) a 

 central nucleus of material which has a very low- 

 degree of rigidity, even against stress of onlv a few- 

 seconds' duration. The transition from the first In 

 the second of these three divisions is somewhat abrupt, 

 sufficiently so to give rise to reflection ; between the 

 econd and third the passage is more gradual, and 

 lies at about four-tenths or five-tenths of the radius 

 from the centre of the earth. These three divisions 

 may be further reduced to two — the outer layer, which 

 in geology is known as the crust, not from any im- 

 plication of the nature of the rest: of the earth, but 

 merely in recognition of a difference in character; and 

 the ci ntral core, consisting of the rest of the earth. 



HYDRO-ELECTRIC POWER SUPPLY.^ 

 T ARGE works have been established for supplying 

 ■*-* Bombay with water-power for its numerous mills 

 and factories, which have hitherto used steam-power, 

 to the extent of more than 100,000 h.p. Coal in most 

 of India is too expensive to allow competition with 

 ountries for many products, though the raw 

 materials are grown or found in India, and labour 

 is cheap and docile, while highly educated Indians 

 abound. To Bombay coal has mostly to be carried 

 1 200 miles. 



The water-power now provided is very much cheapcr 

 than power from coal or oil, gives a better "drive," 



1 Abstract of a paper on "The Tata Hydro-electric Power-supply Works 

 B >mbay," by Mr. K. ]'.. Joyner, read at the Institution of Civil Ensinccrs 

 on November iq 



NO. 2560, VOL. 102") 



and frees Bombay from the clouds of deleterious 

 which the poor Indian coa! gives. 

 The works take advantage of the verj heavy rain- 

 fall on the precipitous edge of the Western Ghats, 



about 2000 ft. above, and about i from, 



Bombay. \- the rain falls only during three or four 

 months of the year and the watercourses are dn all 

 the rest ol the year, it was necessary to store water 

 sufficient to give about 100,000 h.p. for ten or twelve 

 hours a daj during about nine months ol thi 



Three lakes are formed by four masonry dam-, 

 ranging from nearlj , mile long and 34! ft. high to 

 nearly r£ miles long and 96 ft. high. ' Two of 1 

 form a "monsoon" lake of sufficient capacit 

 provide power during the longest "breaks" in the 

 monsoon, and thus give an uninterrupted supply of 

 power for three mouth-, and more. The other 

 are for storage, and maintain the power during the 

 eight or nine months in the year when no rain falls. 



The monsoon rain on the Western Ghats, though 

 always heavy, is very variable in amount. The leasl 

 annual amount during the last forty-eight years was 

 82 in. on the edge of the Deccan plain, and thi 

 greatest amount during the past eleven years, in which 

 special gauges have been fixed, on hilltops as will as 

 in plains, is 546 in., which fell in a little more than 

 three months, 4<>o in. falling in about two months. 

 The minimum fall of 82 in. is very exceptional, and 

 the maximum given may be equally so. The com- 

 bined available capacities of the two storage lakes is 

 about 10,100,000,000 cubic ft., whilst the water required 

 to give 100,000 h.p. ex turbines for nine months, 

 allowing for the great loss by evaporation and bv 

 soakage and for friction in the pipes and turbines, 

 is 6,700,000,000 cubic ft. The excess capacity is given 

 ow-ing to the verv variable amounts of the monsoon 

 rains, so as to carry on the balances in years of exces- 

 sive rainfall to make up for the occasional short mon- 

 soons. It was arrived at by assuming the works had 

 been completed forty years ago, there being one rain- 

 gauge record covering that period — which includes four 

 minimum years' fall — and deducting from each year's 

 supply the amount which would have been used, lust 

 by evaporation, run to waste, or carried on to the next 

 year, which gives the excess capacity required for a 

 Suffii i< nt number of v tars. 



The amount of 546 in. measured at one hill station 

 in the lakes catchment is not more than has been 

 measured in two or three out of the past fifty 

 odd vears at Cherrapunii, in the Assam Hills, which 

 has the heaviest rainfall hitherto known ; but therf* 

 rain falls during seven months of the year, so that 

 the amount measured for this work for that particular 

 year mav claim to be the heaviest rainfall ever yet 

 measured. 



The work-s are notable for the following reasons: — 

 Thev are the largest of the many similar hvdro-electric 

 works which have been constructed during the past 

 ten or twenty years, taking into consideration the 

 great head used, combined with the large discharge of 

 water. The first is equal to about five times the 

 height of St. Paul's Cross, and the tatter is greater 

 than the summer flow of the River Thames during 

 five months. Thev are also (he first works to store 

 water for power for use during about three-fourths of 

 the year. One of the masonry dams, taking the 1 x- 

 posed face area, is probably the largest vet constructed. 

 The works are probably unique, considering the very 

 heavy rainfall and the very steep rocky slopes, giving 

 the greatest discharge perhaps ever recorded. The 

 catch'rmnt area of the two lakes is onlv m 1 square 

 miles, while of this the full lakes area is about 

 |uare miles. 



The water is led from the monsoon lake and from 



