Thus, for want of proper instruction, most American housekeepers not only poison their families with carbonic acid and starve them for want of oxygen, but also diminish health and comfort for want of a due supply of moisture in the air. And often when a remedy is sought, by evaporating water in the furnace, it is without knowing that the amount 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: "I think it is a well-established fact that carbonic oxide will pass through iron. It is always formed in great abundance in any _anthracite_ fire, but especially in anthracite stoves and furnaces. Moreover, furnaces _always_ leak, more or less; how much they leak depending on the care and skill with which they are managed. Carbonic oxide is much more poisonous than carbonic acid. Doubtless some carbonic oxide finds its way into all furnace-heated houses, especially where anthracite is used; the amount varying with the kind of furnace and its management. As to how much escapes into a room, and its specific effect upon the health of its occupants, we have no accurate data, no analysis to show the quantity, and no observations to show the relation between the quantity inhaled and the health of those exposed; all is mere conjecture upon this point; but the inference is very strong that it has a very injurious effect, producing headaches, weariness, and other similar symptoms.