42 HERMANN VON HELMHOLTZ 



depends solely upon distance. The solvability of this problem 

 is the condition of the complete comprehensibility of nature. 

 In mechanical calculations this limitation of the idea of moving 

 force has not yet been assumed : a great number, however, of 

 general principles referring to the motion of compound systems 

 of bodies are only valid for the case that these bodies operate 

 upon each other by unchangeable attractive or repulsive forces ; 

 for example, the principle of virtual velocities ; the conservation 

 of the motion of the centre of gravity ; the conservation of the 

 principal plane of rotation ; of the moment of rotation of free 

 systems; and the conservation of vis viva. In terrestrial matters 

 application is made chiefly of the first and last of these princi- 

 ples, inasmuch as the others refer to systems which are supposed 

 to be completely free; we shall, however, show that the first 

 is only a special case of the last, which therefore must b^ 

 regarded as the most general and important consequence o. 

 the deduction which we have made. 



' Theoretical natural science therefore, if she does not rest 

 contented with half-views of things, must bring her notions 

 into harmony with the expressed requirements as to the 

 nature of simple forces, and with the consequences which 

 flow from them. Her vocation will be ended as soon as 

 the reduction of natural phenomena to simple forces is com- 

 plete, and the proof given that this is the only reduction of 

 which the phenomena are capable/ x 



Helmholtz accepted du Bois-Reymond's advice and wrote to 

 G. A. Reimer, who replied that he was only too glad, since 

 du Bois answered for the value of the treatise, to undertake 

 its publication. He brought it out in 1847, and presented 

 Helmholtz, to his great surprise, with an honorarium. 



The Law of the Conservation of Energy as put forth by 

 Helmholtz suffered the vicissitudes incident to the birth ol 

 all great thoughts. However much a generalization may be 

 foreshadowed by experiment in different directions, and sug- 

 gested and discussed by speculative thinkers, yet when it 

 finally appears in concrete form it is sure to encounter doubts 

 as to its accuracy, or, if the magnitude and worth of the dis- 

 covery are recognized, suspicions of its originality, and 



1 J. Tyndall, 'The Conservation of Force,' Scientific Memoirs [Natural 

 Philosophy], I, pp. ii4seqq. 



