■^7 



208 H. Helmholtz on the Interaction of Natural Forces. 



larly, the Avarm rarefied air ascends, wbile fresli cool air flows from 

 the sides to supply its place : in this way winds are generated. 

 This action is most powerful at the equator, the warm air of 

 which incessantly flows in the upper regions of the atmosphere 

 towards the poles; while just as persistently at the earth's surface, 

 the trade-wind carries new and cool air to the equator. Without 

 the heat of the sun, all winds must of necessity cease. Similar 

 currents are produced by the same cause in the waters of the sea. 

 Their power may be inferred from the influence which in some 

 cases they exert upon climate. By them, the warm water of the 

 Antilles is carried to the British Isles, and confers upon them a 

 mild imiform warmth^ and rich moisture; while, through similar 

 causes, the floating ice of the North Pole is carried to the coast 

 of Newfoundland and produces raw cold. Further, by the heat 

 of the sun a portion of the water is converted into vapor, which 

 rises in the atmosphere, is condensed to clouds, or falls in rain 

 and snow upon the earth, collects in the form of springs, brooks 

 and rivers, and finally reaches the sea again, after having eroded 

 the rocks, carried away loose earth, and thus performed its part 

 in the geologic changes of the globe; perhaps, besides all this, it 

 has driven our water-mill upon its way. If the heat of the sun 

 were withdrawn, there would remain only a single motion of 

 water, namely, the tides, which are produced by the attraction 

 of the sun and moon. 



How is it, now, with the motions and the work of organic 

 beings. To tlie builders of the automata of the last century, men 

 and animals appeared as clockwork which was never wound up, 

 and created the force which they exerted out of nothing. They 

 did not know how to establish a connexion between the nutri- 

 ment consumed and the work generated. Since, however, we 

 have learned to discern in the steam-engine this origin of mechan- 

 ical force, wc must inquire whether something similar does not 

 hold good with regard to men. Indeed, the continuation of life 

 is dependent on the consumption of nutritive materials : these are 

 combustible substances, which, after digestion and being passed 

 into the blood, actually undergo a slow^ combustion, and finally 

 enter into almost the same combinations with the oxygen of the 

 .atmosphere that are produced in an open fire. As the quantity 

 of heat generated by combustion is independent of the duration 



occurs 



late from the mass of the consumed material how much heat, or 

 its equivalent work, is thereby generated in an animal body. 

 Unfortunatcl}', the difficulty of the experiments is still very gi'cat; 

 but within those limits of accuracy which have been as }^et at- 

 tainable, the experiments show that the heat generated in the 

 animal body corresponds to the amount which would be genera- 

 ted by the chemical processes. The animal body therefore does 



