268 UTILIZING THE SUn's ENERGY. 



The uncertainty which the engineer feels regarding this type of 

 motor is due largely to the difficulties arising from the fact that the 

 sun is not always available, even by day, and that it is entirel}^ out of 

 reach for power purposes for one-half the twent^^-four hours, and he 

 has as yet no idea of practical methods of storage, either of the heat 

 or the power, for use during cloudy periods, hours, days, and weeks 

 even, when the engine can not be kept in steady operation. It is, of 

 course, possible that much improvement may be effected in the elec- 

 tric storage battery, and it is even true that great improvements in 

 that precious device are apparently already in sight; but even the 

 ideal and perfect battery, could it be realized, would probably prove 

 so costly and so enormous, as a part of this system of sun-power 

 utilization, as to make its use practically out of the question in tem- 

 perate regions where the sky is overcast so often that not over one- 

 half the direct heat of the sun is each day, on the average, available, 

 or in the Tropics, where the rainy season makes it unavailable for 

 months together. Where, as ma}^ occasionally be practicable, storage 

 may be effected by raising water into extensive and elevated reservoirs 

 provided by nature, this difficulty may prove less serious; but such 

 exceptional advantages of location can not be relied upon for any 

 important aid in securing general utilization of the solar motor. 



For necessarily continuous use of power it is thus evident this sys- 

 tem gives little promise, and a cotton mill, for example, that must go 

 into operation only when the sun comes out from behind a cloud and 

 go out of action the instant it disappears again can hardl}^ be expected 

 to pay dividends. Water power must be its reliance when coal can 

 not be employed, rather than either sun power or wind power, and its 

 work must be done where a sufficient amount of fall and flow can be 

 had to meet its maximum requirements, even at the period of minimum 

 flow. 



The availability of sunlight and heat for the purposes of the engi- 

 neer difiers greatly in difierent places, and with every change of lati- 

 tude, as well as from season to season. This variability is an enor- 

 mous handicap where it is sought to employ this energy. The remark 

 is attributed to Professor Langley that all the coal deposits of Penn- 

 sylvania, if burned in a single second, would not liberate a thousandth 

 part as much heat as does the surface of the sun in that unit of time. 

 Yet it is evident that our coal deposits, so long as they last, are worth 

 more to us than all the available heat of the sun. 



In conclusion, we may thus make the following deductions: 



The rapid and rapidly increasing destruction of our stores of mineral 

 fuel must, sooner or later, bring us to a point at which it will be no 

 longer possible to derive the power required in the arts from that 

 source. 



That period is likel}^ to be ushered in before many generations, and 



