i • Royal Society. 469 



is made to ascend a gradient with increasing speed, there is work 

 done against friction, the resistance of the air, gravity, and inertia" 

 but not a hint is given that, if work can be properly said to be done 

 against inertia in any sense, it must be in a very different sense from 

 that in which it is said to be done against friction, and the rest. 

 Again, on page 147 we find the following paragraph: — "We infer 

 particularly from this law [i. e. Newton's Third Law of Motion], 

 that when a free mass m is receiving an acceleration (f>, it reacts with 

 a force opposite to the moving force m<p. When this reaction is 

 considered as a ?'esistance to acceleration, arising from the tendency 

 of the free body to persevere in its actual state of rest or motion, 

 it is called the force of inertia of the body." Now Newton's third 

 law can only apply to the mutual action of at least two bodies. This 

 is plain from Newton's own statement of the law as well as 

 from his illustrations, which Mr. Kerr in part translates. Sup- 

 pose, then, that in the above extract the acceleration <p origi- 

 nates in the attraction of a second body whose mass is m', then the 



reaction referred to is an acceleration of — , produced in the motion 



m' 



of the second body. We do not see how this can be regarded as a 

 " resistance to acceleration," nor with what propriety it can be 

 called the " force of inertia " of the former body. At the bottom 

 of the same page, in the statement of D'Alembert's principle, we find 

 the term force of inertia used as equivalent to "a force equal and 

 opposite to the effective force." In this use the " force of inertia " 

 is really a force and no longer the old vis inertice ; but then it must 

 be borne in mind that no such force really acts ; it is merely supposed 

 to act, in order to reduce certain questions of dynamics to corre- 

 sponding questions of statics, and thereby facilitate their solution. 



LXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 395.] 



Feb. 14, 1867. — Lieut. -General Sabine, President, in the Chair. 

 rpHE following communications were read : — 

 J- " On the Conversion of Dynamical into Electrical Force without 

 the aid of Permanent Magnetism." By C. W. Siemens, F.R.S. 



Since the great discovery of magnetic electricity by Faraday in 

 1830, electricians have had recourse to mechanical force for the 

 production of their most powerful effects ; but the power of the 

 magneto-electrical machine seems to depend in an equal measure 

 upon the force expended on the one hand, and upon permanent mag- 

 netism on the other. 



An experiment, however, has been lately suggested to me by my 

 brother, Dr. Werner Siemens of Berlin, which proves that permanent 

 magnetism is not requisite in order to convert mechanical into elec- 

 trical force ; and the result obtained by this experiment is remark- 

 able, not only because it demonstrates this hitherto unrecognized 



