68 Mr William Petrie on the Application of 



be a creation of dynamic force, the evolving of an unlimited 

 force from a limited source. The tendency to an J opposing 

 induced current in the primary wire must therefore be in- 

 volved in the very principle of the system, so that no inge- 

 nuity can ever get rid of the retarding influence of the in- 

 duced action ; the only way to overcome its power, so as to 

 maintain the primary current from falling below a given rate 

 or quantity, when the machine is allowed to attain rapid 

 motion, is to increase the electro-motive power of the battery^ 

 the intensity (not the quantity) of the current, so that it shall 

 be less afi^ected by the opposing induction. 



The practical importance of these not altogether unknown 

 truths may justify this particular notice of them.^'For want 

 of a clearer apprehension of them, inventors of considerable 

 attainments have misapprehended the direction in which 

 improvements were to be made, and much ingenuity and 

 means have been wasted. 



Some of the best electro-magnetic machines of other in- 

 ventors, which have been properly tested by the author and 

 others, on a practically useful scale, have only given a power 

 at the rate of 50 or 60 lb. of zinc, per horse power, per 

 hour. The smallness of this power, in comparison with 

 the absolute value of the current (1*56 lb. of zinc, per horse 

 power, per hour) should not cause surprise, if we consider 

 the present case of steam, after many years of improvement. 



According to the determinations of Joule and Rankine on 

 heat, 1 lb. of water raised 1° of temperature, is equivalent to 

 700 lb. weight raised 1 foot ; and assuming, from the best 

 experiments on the subject, that the combustion of 1 lb. of 

 carbon will afi'ord as much heat as 1° of temperature in 

 15,000 lb. of water, we find that one horse power is the theo- 

 retic or absolute dynamic force possessed by the heat due to 

 the combustion of ^ of a pound of carbon per hour. This is 

 only jV of the consumption of the most perfect steam-engines 

 yet constructed, and less than j^^ part of the consumption 

 of many railway locomotives. This, by the way, is an in- 

 structive fact, shewing how far we yet are from perfection 

 in our means of obtaining moving power from heat, and what 

 great rewards may yet await the exercise of inventive genius 



