212 SECTION MECHANICS. 



these may be mentioned Professor Colladon's dynamometrical 

 apparatus, constructed in 1844; Richard's patent steam engine 

 indicator, an improvement on Watt's, and Mr. G. A. Hirn's flexion 

 and torsion pandynamometers. 



Eighth. The measurement of Electrical 'Units of electrical capacity 

 of potential and resistance, forms a subject of vast research, and 

 of practical importance, such as few men are capable of doing justice 

 to. It may be questioned, indeed, whether Electrical Measurement 

 belongs to the province of mechanical science, involving, as it does 

 problems in physical science of the highest order ; but it may be con- 

 tended on the other hand, that at least one branch of Applied Science, 

 that of Telegraphy, could not be carried on without its aid. I am happy 

 to say that this branch of the general subject will be brought before 

 you by my esteemed friend Sir William Thomson, than whom there is 

 no one more eminently qualified to deal with it. I may, therefore, pass 

 on to the next great branch of our general subject, the ninth : Thermal 

 Measurement. The principal instrument here employed is the ther- 

 mometer, based in its construction, either upon the difference of expan- 

 sion between two solids, or on the expansion of fluids such as mercury 

 or alcohol (the common thermometer) or upon gaseous expansion (the 

 air thermometer) ; or again, it may be based upon certain changes of 

 electrical resistance, which solids and liquids experience when sub- 

 jected to various intensities of heat. With reference to these, the air 

 thermometer represents most completely the molecular action of matter 

 which is the equivalent of the expansibility. I shall not speak of the 

 different scales that have been adopted by Rdaumur, Celsuis and 

 Fahrenheit, which are based upon no natural laws or zero points in 

 nature, and which are therefore equally objectionable upon theoretical 

 grounds. Would it not be possible to substitute for these a natural 

 thermometric scale ? One commencing from the absolute zero, of the 

 possible existence of which we have many irrefutable proofs, although 

 we may never be able to reach it by actual experiment. A scale com- 

 mencing in numeration from this hypothetical point would possess the 

 advantage of being in unison throughout with the physical effects due 

 to the nominal degree, and would aid us in appreciating correctly the 

 relative dynamical value of any two degrees of heat which could be 

 named. Such a scale would also fall in with the readings of an elec- 



