299 



copper and silver, iii all proportions, have the same conducting power* 

 with each of those metals separately ; while in alloys of tin and lead, 

 the heat evolved is a mean of that of the heats evolved from each 

 metal separately, taking into account the relative weights of each 

 metal present in the alloy. 



He observes, further, that very small quantities of alloy influence 

 materially the conducting power. Thus he found that wires, drawn 

 from foreign gold coins, reputed to be very pure, conducted much 

 worse than when drawn from the same gold refined. 



The substitution of a flattened for a cylindrical wire, or the sepa- 

 ration of the latter into four smaller wires, were found to have no 

 influence on the quantities of heat developed. 



On the Expediency of assigning Specific Names to all such Functions 

 of simple Elements as represent definite physical Properties ; with 

 the Suggestion of a new Term in Mechanics ; illustrated by an In- 

 vestigation of the Machine moved by recoil, and also by some Ob- 

 servations on the Steam Engine. By Davies Gilbert, Esq. M.P. 

 V.P.R.S. Sfc. Read January 25, 1827. [Phil. Trans. 1827, p. 25.] 



The author commences this paper by stating the necessity of dis- 

 tinguishing by separate appellations all such functions as measure 

 the intensity of physical properties, which he considers rendered ob- 

 vious by a reference to the controversy respecting motion. The 

 subject of this controversy, he observes, was the measure of motion 

 itself, it being contended on one hand that the motion of a body is 

 always proportional to its weight multiplied by its velocity ; this 

 opinion being supported by reference to the properties of the com- 

 mon centre of gravity of systems, &c. ; while on the other hand the 

 affections of elastic bodies in collision, and the general law of the 

 conservation of living or active forces, were adduced in support of 

 the latter measure. No sooner, however, were the terms " momen- 

 tum" and; " impetus" introduced into the science of mechanics, than 

 the opinions of the contending parties were reconciled by the re- 

 moval of every ground of dispute. 



In the Bakerian lecture on the force of percussion, read to this 

 Society in 1806, he observes, it is remarked, that neither impetus 

 nor momentum are usually correct measures of the effective action 

 of machines. The criterion of this is the force exerted, multiplied by 

 the space through which it acts, and this measure numerically ex- 

 pressed has been denominated DUTY by Mr. Watt ; and the raising 

 of one pound one foot high has been by him made the dynamic unit; 

 according to which estimate, the duty performed by one bushel of 

 coals, of 84 pounds, has been found to vary from 30 to 50 millions 

 of such units, according to the nature of the engine, and the mode 

 of combustion. To the measure or function represented by the force 

 applied, multiplied by the space through which it acts, the author, 

 however, proposes to give the name efficiency, retaining the word duty 

 for a similar function, indicative of the work performed ; and by a 



