378 The N.Z. Journal of Science and Technology. [Dec. 
It is obvious from the foregoing that power for industrial purposes 
cannot be economically provided through long pipes, and that both 
economy and good regulation demand the shortest possible length of pipe 
consistent with the head. 
It is obvious also that, as a general rule, the two sources of water-supply 
and power-supply cannot be combined. Very rare cases occur, however, 
where the necessary bead is obtained in a short distance, and where the 
flow of water is sufficient to satisfy the power requirements, in which case 
the two services may with advantage be combined : that is to say, a power- 
station would be situated at the foot of the slope where electric power 
would be generated and transmitted to the town, which may be some 
miles off, and at the same time a domestic-water-supply pipe would be 
laid between the same limits. A combination of this kind was proposed 
at one time for Ohakune by Messrs. H. W. Climie and Sons, but was not 
carried out. A very interesting combination of electric-power supply, 
water-supply, and drainage has been carried out by the same firm for 
Raetihi. This consists of a hydro-electric-power station situated some 
five miles distant from Raetihi, at which electric power is generated and 
transmitted to Raetihi, and there employed for lighting and power, and, 
amongst other things, for pumping water from the Mangawhero River, 
in the vicinity of the town, to reservoirs built on an eminence. This, of 
course, does not involve the question of the common use of water-mains, 
but it is worth quoting as an example of a happy combination well thought 
out and of work well executed. 
Returning to the subject, cases arises where it is found advantageous 
to take power from town mains, as, for instance, when the object is to supply 
electric power during the night so as to economize fuel and labour in cases 
where an independent power plant is available for service during the day. 
A very satisfactory result is often obtained because the water-service is not 
otherwise in great demand during the night hours, and the water may be 
devoted wholly or in part to the supply of electric power. This is a very 
great convenience in small towns, where, for various reasons, an alter¬ 
nating-current system is deemed to be the most suitable for the require¬ 
ments, and where the expense entailed in maintaining the fuel power plant 
in operation during the night is not justified. Occasionally a battery of 
electric accumulators is employed under the circumstances mentioned, 
which is charged during the day by means of a direct-current generator 
driven by an alternating-current motor deriving its supply from a steam-, 
gas-, or oil-driven alternating-current power plant, as the case may be. 
Alternatively, when the conditions are suitable, the night load may be 
supplied most advantageously from the water-supply mains, by means 
of a turbine and a generator, which may be left without attendance, and 
the fuel plant shut down. 
In some recent instances attempts have been made to obtain a supply of 
electric power from town mains during the day as a temporary measure, 
pending the establishment of permanent works which have been postponed 
because of the war. 
Inasmuch as the device has been found useful on occasions, it may 
be worth while investigating the principles involved and illustrating the 
application of the same by a few examples. 
First as regards the maximum power which can be obtained from a 
given-sized pipe : Let h 0 be the static 'head in feet ; let v be the velocity 
of flow in feet per second, h the corresponding friction head, l the length 
of pipe, r the hydraulic mean depth. Then we have, according to Chezy’s 
