evaporated depends, not on the quantity of water in the vessel, but
on the extent of evaporating surface exposed to the air. A quart of
water in a wide shallow pan will give more moisture than two gallons
with a small surface exposed to heat.

There is also no little wise economy in expense attained by keeping
a proper supply of moisture in the air. For it is found that the body
radiates its heat less in moist than in dry air, so that a person feels
as warm at a lower temperature when the air has a proper supply of
moisture, as in a much higher temperature of dry air. Of course, less
fuel is needed to warm a house when water is evaporated in stove and
furnace-heated rooms. It is said by those who have experimented, that
the saving in fuel is twenty per cent when the air is duly supplied
with moisture.

There is a very ingenious instrument, called the hygrodeik, which
indicates the exact amount of moisture in the air. It consists of two
thermometers side by side, one of which has its bulb surrounded by
floss-silk wrapping, which is kept constantly wet by communication
with a cup of water near it. The water around the bulb evaporates just
in proportion to the heat of the air around it. The changing of water
to vapor draws heat from the nearest object, and this being the bulb
of the thermometer, the mercury is cooled and sinks. Then the difference
between the two thermometers shows the amount of moisture in the air
by a pointer on a dial-plate constructed by simple mechanism for this
purpose.

There is one very important matter in regard to the use of furnaces,
which is thus stated by Professor Brewer: