1866. | Wilde’s Magneto-electrie Machine. 509 
required to be consumed in the production of this amount of electric 
force. Mr. Wilde says that a 7-horse power engine is required to 
drive the machine. One-horse power is equal to 1,980,000 foot- 
pounds per hour; that multiplied by seven is 18,860,000 foot- 
pounds per hour, which therefore represents the actual power 
required to drive the machine. Now, by multiplying the British 
Fahrenheit units of heat, produced by the combustion of one pound 
of coal, by Joule’s equivalent, or 772 foot-pounds, the result will be 
the total heat of combustion expressed in foot-pounds. In the best 
coal this is as high as 12,000,000 foot-pounds. We arrive therefore 
at the startling conclusion, that, to overcome the friction of the 
different parts of the machine; to whirl a mass of metal, weighing 
several hundred-weights, round with a velocity of 1,500 revolutions 
per minute; to generate a current of electric force far surpassing 
anything ever before produced; and, after allowing for the waste 
inherent in its passage through the conducting wires and electric 
lamp, to cause it to blaze forth with an intensity of light, before 
which the rays of the sun himself appear pale and feeble ; to keep 
up this mtense development of energy for one hour, requires an 
expenditure of force represented by the combustion of less than 
184 ounces of coal ! 
This is the theoretical calculation ; but if reduced to actual 
practice, the results are scarcely less astonishing. The economy of 
the power actually employed by Mr. Wilde cannot be calculated, as 
the engine is a 60-horse one, and is used for driving the very heavy 
machinery of a wire mill, as well as performing the various opera- 
tions required in an engineer’s workshop; but the efficiency of an 
engine, 7. e. the ratio of the work actually performed to the mecha- 
nical equivalent of the heat expended, is well known, and varies in 
extreme cases between the limits 0:02 and 0:2. Taking an average 
efficiency as 0-1, or one-tenth, we find that the ordinary consumption 
of coal required to work a 7-horse power engine, midway between 
excessive wastefulness on the one hand and rigid economy on the 
other, is 10 x 184 ounces or 114 lbs. of coal per hour, worth about 
one halfpenny. 
The above expense of one halfpenny per hour for coal is of 
course only one item in the cost,—to it must be added the expense 
of carbon rods for the lamp, which will be about ten inches per hour, 
worth perhaps a penny; there must also be added interest of the 
cost of purchase of machines, expense of maintenance and repairs, 
which will perhaps bring up the total expense per hour to sixpence 
or eightpence. Comparing this with the hourly expense of the 
electric lights already in existence we find, according to the Abbé 
Moigno, that the French machine costs altogether sixpence per 
hour for a light equal to 900 wax candles, whilst the actual work- 
ing expenses of maintaining the electric light at Cape La Heve 
2 MZ 
