1918.] The N.Z. Journal of Science and Technology. 53 
cost of coal required to supply his own generating plant, so that he makes 
a direct saving of the cost of labour and maintenance, and if he has not 
already installed a power plant he saves interest and depreciation on the 
cost of the power plant as well. 
It is also found that the motive power is more uniform, resulting in 
a greater yield from the same plant, the amount of which depends upon 
the nature of the product. Experiments made, for instance, with certain 
fireclay-mills showed an increase of 30 per cent, from the same machinery 
with, of course, a greater energy-consumption. Certain products, like cotton 
and cloth, are more uniform in texture, have fewer breakages, and in 
consequence show a larger output, due to fewer stoppages. In the case 
of Hour-milling, the facility with which the power can be measured and 
recorded enables a blockage to be detected at once and the yield maintained. 
These advantages may, of course, be gained by adopting electricity as 
motive power, and generating electricity on the premises; but experience 
shows that this is seldom done, the reason being that a change of the kind 
is seldom effected except as the result of some immediate tangible advantage 
which will show on the profit and loss account. The advantages mentioned 
are not self-evident, and are often problematical, and cannot be demon¬ 
strated without considerable expense. In consequence, unless such induce¬ 
ment as can only be offered by a power-supply authority is forthcoming, a 
change to electric driving is rare. 
Utilization of Surplus Power from Industrial Operations. 
A number of power-sources exist which are intermittent in character 
or are more or less lacking in continuity, and are therefore unsuitable for 
the purpose of meeting unaided the exigencies of a power-supply system, 
while taken individually they are generally inadequate for such a purpose. 
Such sources, however, may readily be utilized as auxiliary sources, pro¬ 
vided there is already established a power system deriving its supply from 
a generating-station or from a station which is devoted to the purpose 
of supply, and from which the supply can be controlled and regulated. 
Examples of such sources are combustible gases from coke-ovens, of which 
the surplus over and above the requirements of the furnaces amount to 
some 80 horse-power per ton of coke ; blast-furnace gases, the available 
surplus of which amounts to some 375 horse-power hours per ton of pig 
iron when burnt under boilers, or say twice the amount if expended in 
gas-engines. A notable example of the utilization of waste gases as auxiliary 
sources of power is found on the north-east coast of England, where the 
power company operating there has connected to its system eleven waste- 
heat stations, which supply electric energy equivalent to that which would 
be produced from 150,000 tons of coal per annum.* 
Auxiliary power may similarly be obtained from inferior grades of 
coal or from screenings and waste-heaps of the coal-mines, either by 
pulverizing and burning under boilers, or, preferably, by distilling the coal 
and utilizing the waste gases and at the same time recovering the tar- 
products and ammonia. 
Peat deposits may be treated in the same way for a number of valuable 
organic compounds by well-known processes, and the waste gases utilized 
for generating power, which may be absorbed and utilized by a general 
power-supply system. 
* R. P. Sloan, loc. cit. 
