22 Prime Movers. 



that it must be more expensive to maintain an electro-magnetic 

 engine in action, than a steam-engine capable of doing the 

 same work. One grain of zinc was found to raise only eighty 

 pounds one foot high ; but one grain of coal, in the furnace of 

 a Cornish engine, will raise one hundred and forty -three 'pounds, 

 through the same distance. The cost of one hundred-weight 

 of coal is nine pence, that of one hundred-weight of zinc about 

 two hundred and sixteen pence. Hence, electro-magnetic power 

 is nearly fifty times as expensive as that obtained from steam. 

 It has been asserted by Liebig, that the zinc of the battery 

 cannot give out more power than the coal required to smelt it ; 

 and that the heating power of a galvanic battery is the equiva- 

 lent of its mechanical power ; so that, if applied to the vaporiza- 

 tion of water, the steam produced by the heating power would 

 do as much work as can be expected from its application to 

 an electro-magnetic machine. Favre has shown that the heat 

 liberated from a galvanic battery is proportional to its che- 

 mical action ; and that the mechanical work performed by the 

 current always incurs an expense of the heat, borrowed from 

 that which is evolved by the battery. It is worthy of notice that 

 when an effect opposite to the magnetic attraction is produced 

 — as, for instance, when magnetized bodies are forcibly drawn 

 asunder — the heat is augmented. It has been ascertained that, 

 using " Joule's unit," one pound of zinc consumed in a Grove's 

 battery would, if the heat were utilized, raise 1,698,000 pounds 

 one foot high; and one pound consumed in a DanielFs battery, 

 1,019,000 pounds. But 1,698,000 pounds raised one foot 

 high, are equivalent to only one-horse power, during fifty-one 

 minutes. These, besides, are the maximum theoretical effects ; 

 but, from the imperfect nature of the electro-magnetic machine, 

 nothing like them are really attainable. If, as Joule supposes, 

 heat is changed into mechanical effect during the action of the 

 engine, the maximum power of even a perfect electro-magnetic 

 machine must be far less than is produced with the same ex- 

 penditure, by steam. Liebig observes that, according to the 

 experiments of Despretz, six pounds of zinc combining with 

 oxygen give no more heat than one pound of coal ; and that, 

 therefore, the coal should produce six times as much power as 

 the zinc. We may remark that the heat given out by the zinc 

 was that with which it combined during smelting, and was ob- 

 tained from the fuel. It would follow that the power of the 

 electro-magnetic machine is derived from the same source as 

 that of the steam-engine, but is not obtained so directly. 



It has been replied to some of these facts that electricity, 

 and not heat, is required for electro-magnetism. But that 

 galvanic battery which produces most heat — a single cell of 

 large size — produces also most electro-magnetism. And in- 



