SCIENTIFIC SUMMARY. 
285 
of copper conductor tliat would be suitable for the economical transmission 
of power by electricity to any stated distance ; and, taking Niagara as ex- 
ample, I pointed out that, under practically realizable conditions of intensity, 
a copper wire of half an inch diameter would suffice to take 26,250-horse 
power from water-wheels driven by the Fall, and (losing only twenty per 
cent on the way) to yield 21,000-horse power at a distance of 800 British 
statute miles ; the prime cost of the copper amounting to 60,00(K, or less 
than 3 1. per horse-power actually yielded at the distant station. 
‘ I remain, Sir, your obedient servant, 
1 William Thomson. 
‘ The University , Glasgow , June 9.’ 
1 Sir, — Prof. Osborne Reynolds’ letter in your issue of Saturday, the 11th 
inst., shows that the first idea that has occurred to him on reading SirWilliam 
Thomson’s letter on Faure’s “ electric store,” is probably what must have 
suggested itself to many engineers, viz., that so far from a million foot pounds 
being a surprisingly large amount of energy to be stored up in a mass of 
7 5 lbs., it is really extremely small ; and, indeed, while crossing over from 
Paris at the commencement of last week, I could not help thinking that the 
passengers were bringing to England literally in the smuts and blacks on 
their coats far more energy than had ever been imported into this country 
stored up in Faure’s secondary batteries. But although it is perfectly true, as 
Prof. Reynolds says, that l\ oz. of coal contains about one million foot pounds 
of work stored up in it, this is by no means all that has to be taken into 
account in considering this question ; for where is the engine for extracting 
this million foot pounds of work out of the 1^ oz. of coal P Indeed, as Prof. 
Reynolds would himself tell us, we cannot get much more than one-tenth of 
this amount of work out of the 1§ oz. of coal even in our largest steam- 
engines, which burn many pounds of coal per minute, and in which much heat 
is wasted in getting up steam. And if we come to the burning of one single 
1~ oz. of coal, 1 know of no engine that can obtain from this even one thou- 
sand foot pounds of work, or one thousandth of the energy contained in the 
coal, if no other coal be used in getting up steam, or in previously heating the 
engine. 
* But if a secondary battery be allowed to drive a magneto-electro-motor 
or a dynamo-electric machine with separate exciter only even for the short 
time necessary to develope, say, thirty foot pounds of mechanical work, I 
anticipate this can be done without using up in the whole process more than 
about thirty-five foot pounds of the electric energy stored up in the reservoir, 
since the experiments of Prof. Perry and myself have shown that, when the 
motor is running at high speeds with a light load, as much as ninety-three 
per cent of the electric energy put into a magneto-electro-motor is given out 
again as mechanical work measured by an absorption dynamometer. 
‘ It may be answered, however, that if a small bit of coal, although con- 
taining avast store of energy, is not of much practical use in producing work, 
in consequence of the absence, up to the present time, of a proper converter 
of the coal’s energy into mechanical work, at any rate a small galvanic 
battery (a little Daniell’s cell, for example) is not only a vast storehouse of 
power, but contains a store which we have the means of converting without 
