THEIMPRESSOFLIGHT 95byexperimentandobservation,andthentotrace,one
by one,totheirsolar origin,the differentclasses of
energy which present themselves in terrestrial phe-
nomena—to show, for instance, how the power of
waterfallsisonlyatransformationofthesun'sheat;
andthatthesamethingistrue,alittlemoreremotely,
butjustas certainly,ofthepowerof steam,ofelec-
tricity, and even of animals. The idea is now so
familiarthatitishardlynecessaryto dwell uponit,
andyet, forsome ofourreadersatleast, itmaybe
worthwhiletoexamineitalittlemoreclosely.
"Wheneverworkis done, itis by theundoingof
somepreviouswork. Whenaclockmoves,itis the
unwinding of a spring or the falling of a weight
whichkeepsitgoing,andsomeonemusthavewound
it up to begin with. If the water of a river falls
yearafteryearovera cataract,and itis intercepted
to drive our mill wheels,the rivercontinuesto run
becausesomepoweriscontinuallyraisingandreturn-
ingtothehilltopsthewaterwhichhasflowedintothe
sea—a processprecisely equivalentto the daily re-
windingof theclock. If thepowder ina rifle ex-
plodesanddrives outthebullet,itsexplosiveenergy
depends upon the fact that some power has placed
thecomponent moleculesinsuch relationthat, when
thetriggeris pulled, and theexcitingspark has, so
tospeak,cutthebondswhichhold themapart,they
rush together just as suspended weights would fall
if free.
"Beforethesamesubstance,whichoncewasacharge
ofgunpowder,butnowisdustandgas,canagaindo