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99
of wells. Some of this water out pours from faults in the rock stratum and is
available as a spring. Many towns and cities use water direct from rivers and
lakes. London uses the water of the Thames; Glasgow the water of Loch
Katrine; Manchester the water of Thirlmere. Under such conditions of supply,
very great-care has to be taken that pollution is avoided and that there, is suffi-
cient purification. Most local authorities, however, have not a, large lake or
river to draw upon. They find it necessary to set aside a suitable area in close
proximity to the town as a catchment’s area, or gathering ground, from which
the water can be collected and impounded in a reservoir – usually a valley hav-
ing a dam thrown across it. From here the water, is usually – but not invariably
– piped by means of a large sized conduit to a service reservoir on the edge of
the town. At this point any needful purification or softening is generally done.
The water from wells and borings is generally hard, i.e. it contains the salts of
lime, either calcium carbonate, causing "temporary" hardness which can be re-
moved by boiling, or calcium sulphate, causing "permanent" hardness which,
cannot be removed by boiling but only by a system of water softening; the base
exchange method is typical.
Heating and air conditioning
People are comfortable when they are neither too cold, nor too warm and
when the air about them is neither too dry, nor too damp and is not stuffy or dusty.
To bring about these desirable conditions the heating or air-conditioning appara-
tus must be capable of maintaining the following conditions inside the house,
whatever the conditions outside may be. For adequate heating when it is cooled
outside the heating plant, including the necessary ducts, registers, piping, radiators,
etc., must be able to keep all the rooms at 70° Fahrenheit, even if it is 20 or
30 degrees below zero outside. To avoid stuffiness, the air should be given a cer-
tain amount of motion. Under winter conditions this must be sufficient to distribute
the heat uniformly throughout the rooms. It must not be too cold at the floor, not
too hot at the ceiling. A stove causes the hot air around it to rise up toward the
ceiling and cooler air to flow toward the stove. A radiator acts in this respect
like a stove. Warm-air registers bring heated air into a room with a certain mo-
tion or velocity, which imparts movement to the air already in the room. An outlet
for this air should be provided in order to have good ventilation. In summer time
much greater air motion is needed, enough to change the air in a room completely
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