Royal Society. 149 



accompanies a certain amount of condensation, whatever be the 

 neighbouring condensations, and whether the waves be long or 

 short, and the excursions of the particles be swift or slow, and 

 that it is exactly proportional to the condensation ; (2) that this 

 increment of temperature is acquired simultaneously with the 

 condensation; (3) that the increment of temperature is quam 

 proxime the same in unconfined air as when the air suffers the 

 same amount of condensation in a confined space. Since these 

 laws are not deduced from acknowledged principles, they can 

 have no other basis than experiment to rest upon. 1 am not, 

 however, aware of any experiments which lend them support, 

 unless the new experiments of Prof. Tyndall justify the third 

 hypothesis. But I confess I do not yet see in what way they do 

 this, or, if they give countenance to this hypothesis, that the 

 theory is made good, so long as the other two hypotheses remain 

 unsupported. I cannot but think that the most probable account 

 of the matter is, that when the same portion of air is continually 

 undergoing condensations and rarefactions in rapid succession, 

 the respective caloric effects are continually neutralizing each 

 other, and thus the rate of the velocity of propagation remains 

 unaltered. 



Cambridge, July 9, 1862. 



XXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 73.] 



June 20, 1861. — Major- General Sabine, It. A., Treasurer and Vice- 

 President, in the Chair. 



rgiHE following communication was read: — 

 -*- "On the Measurement of Electric Resistance." By Professor 

 W. Thomson, F.R.S. 



Part I. New Electrodynamic Balance for resistances of short 



bars or wires. 



In measuring the resistances of short lengths of wire by Wheat- 

 stone's Balance *, I have often experienced considerable difficulty in 

 consequence of the resistances presented by the contacts between the 

 ends of the several connected branches or arcs. This difficulty may 

 generally be overcome by soldering or amalgamating the contacts, 

 when allowable ; but even with soldered connexions there is some 

 uncertainty relating to the dimensions of the solder itself, when the 

 wires tested are very short. When soldering was not admissible, I 



* I have given this name to the beautiful arrangement first invented by Pro- 

 fessor Wheatstone, and called by himself a " differential resistance measurer." 

 It is frequently called " Wheatstone's Bridge," especially by German writers. 

 It is sometimes also, but most falsely, called " Wheatstone's Parallelogram." 



