LIMITS TO ECONOMY. 43 



quantity (a degree of static electricity) propagated through 

 the length of one foot in one hour of time." 



Joule uses these units in the magnetic experiments 

 recorded in this letter, in which he determines the maximum 

 power of magnets when fully saturated, and gives rules for 

 their construction, which are still adopted. He had, how- 

 ever, reached an epoch in his research, and although he 

 continued his magnetic investigation till 30th April, 

 1 84 1, without interruption, and subsequently, at intervals, 

 throughout his life, he clearly considered that he had 

 exhausted the subject as far as determining the limit 

 to the economy of the electro - magnetic engine. This 

 is shown by the following interesting paragraph which 

 occurs in his last paper, that in which he attributes 

 to Jacobi the discovery of the internal resistance of the 

 electro-magnetic engine, and which contains the substance of 

 a lecture at the Victoria Gallery, Manchester, February 16th, 

 1 84 1, the first of the very few public lectures Joule ever gave. 

 " With my apparatus every pound of zinc consumed in 

 a Grove's battery produced a mechanical force (friction 

 included) equal to raise a weight of 331,400 lbs. to the 

 height of 1 foot, when the revolving magnets were moving 

 at 8 feet per second. Now the duty of the best Cornish 

 steam engine is about 1,500,000 lbs. raised to the height of 

 1 foot by the combustion of one pound of coal, which is 

 nearly equal to 5 times the extreme duty that I was able 

 to obtain from my electro-magnetic engine by the consump- 

 tion of a lb. of zinc. This consumption is so unfavourable 

 that I confess I almost despair of the success of electro- 

 magnetic attractions as an economical source of power, for 

 although my machine is by no means perfect, I do not see 

 how the arrangement of its parts could be improved so as 



