with cloth, which is wet at the time the instrument is used. The thermometers are whirled for about a minute and the difference between the two noted. The relative humidity can then be easily calculated by means of a table furnished with the instrument.

At a temperature of 86° F. and a relative humidity of 80 per cent, a person begins to feel uncomfortable; there is a disinclination to work; the temperature of the body is raised, and the heart's action is accelerated. Under higher temperature there is frequently headache, nausea, vomiting, and considerable prostration. When the heat and relative humidity are excessive, the temperature of the body may rise to 104° F. or over, which, if not relieved, may be followed by unconsciousness and the symptoms of heat stroke. For a description of this condition see page 172. It is the combination of heat and

FIG. 8.-Sling psychrometer.

moisture which makes the air intolerable. A man can stand without discomfort a much higher room temperature if the air is dry. The high temperature of the air diminishes direct loss of heat from the body, and the presence of moisture decreases evaporation of perspiration. Both reduce the elimination of body heat and cause a sensation of uneasiness and illness, especially when the air is stagnant. These symptoms are ameliorated by setting the air in motion, either by a fan or by opening the doors and windows and allowing a current of air to blow through the room. Cool air replaces the envelope of stagnant, hot, moist air that surrounds the body, the blood in the vessels of the surface of the body is cooled, the temperature of the body falls, and a feeling of well-being ensues.

In winter the heated air of buildings is usually too dry; in fact, the relative humidity is often less than 20 per cent, which is drier than the air of a desert, the relative humidity of the driest climate of this continent being seldom less than 30 per cent. Harrington, in his book on Practical Hygiene,

says:

When outdoor air is heated so as to maintain an even temperature of 70° F., but with no addition of watery vapor, its capacity for absorbing moisture is very much increased, and it will take it up from all moist objects with "hich it comes in contact. It will take it from the skin, from the mucous membranes of the mouth, nose, and respiratory tract; from furniture made from wood which, in the process of kiln drying, was never brought to such dryness; from the leather binding of books, causing them to crack and fall to pieces; and from plants, which, in consequence, wither and die. It thus causes more or less dryness of the skin, irritation of the throat, and cough. It causes also need of a higher temperature to give the same sensation of warmth and comfort than is the case with air containing a normal amount of moisture. Air

at 25° F. saturated with moisture and then heated to 70° F., would need more than 0.5 pint of water in every 1,000 cubic feet to give it a humidity of 65 per cent.

The relative humidity in buildings in wintertime should be at least 50 per cent. The nearer it is to the temperature of the room the more comfortable the room will be; but if the weather outside is very cold and the amount of moisture in the room is great, drops of water will collect on the windowpanes, making it difficult to see through the windows.

Moisture may be imparted to the air by means of humidifiers, one of which is shown in figure 9. This apparatus consists of a container, which holds about a gallon of water and which has a trough at its lower portion. The trough extends lengthwise between the coils of the radiator. In the trough is placed a large felt pad which extends up between the coils. Water absorbed by the pad and evaporated by the heat of the radiator is replaced by water in the trough, which is kept filled by means of a float valve. Where there is sufficient moisture in the air, an indoor temperature of 62° to 68° F. will be found to be comfortable.

Good effects of cold air are well known and many persons sleep out of doors on porches. Wherever possible, in any climate, one should sleep with the windows of his bedroom open. These should be wide open so as to have a good circulation of air. Persons suffering from pneumonia are now treated in a room without heat, the cold air entering through open windows being one of the best remedies that can be employed for this disease. In certain cases prolonged exposure to cold, damp, air may be injurious. Old people, children, and persons suffering from kidney diseases or rheumatism should not expose themselves to it. Healthy persons, however, if well covered, will not be injured by it.

Drafts are only dangerous to robust persons when they cause a chilling of the body. Children and old persons, owing to their feeble resistance to constant changes, should not expose themselves to drafts. A draft may do harm to a strong, well person if he exposes himself to it when his body is in an overheated condition. It may increase the tendency to catch cold or to have pneumonia. When the body is hot there is a large quantity of blood in the vessels of the surface of the body. Cold air suddenly thrown upon the skin causes these vessels to contract, by which means the blood is driven inward, producing a congestion of the internal organs. It is not positively known that this internal congestion causes a person to catch cold, but it is one of the explanations that has been made to account for this condition.

Persons who live in poorly ventilated houses for a long period of time become pale. They are inclined to be thin and usually look undernourished. Their resistance to disease is lowered and they are liable to contract colds, pneumonia, consumption, and other diseases.

Natural Ventilation.

Natural ventilation is that which takes place through openings, such as doors, windows, and cracks, in buildings. It also takes place, to some extent, through the materials of which the building is constructed. This form of ventilation depends upon changes in

FIG. 13.—Diagrammatic sketch of various provisions for ven

tilation. A, Sash window with Hinckes-Bird's arrange

temperature. Cold air entering a room falls to the bottom. As it becomes warm it expands and either rises to the top of the room or goes out through the fireplace or stoves, where it assists in the combustion of fuel. A constant circulation of air is thus maintained. There are various ways of assisting this process. Air ducts

ment. B, Hopper sash light falling inward. C, Louvred may be placed in outlets. D, McKinnell's ventilator. E, Sheringham's valve. the inner wall, one F, Tobin's tube (showing valve open). G, Ellison's, conat the top for an ical bricks. H and I, Grid ventilators below floor joists. (From "Hygiene and Public Health," by Drs. L. C. Parkes inlet and another and H. R. Kenwood, London; H. K. Lewis, Philadelphia, at the bottom for an outlet. A win

Blakiston, 1911.)

dow may be slightly raised for an inlet, a glass or wooden screen being used to deflect the air upward (fig. 10), and an outlet pipe may be placed over the fireplace, or a ventilator may surround the stovepipe. Figure 13 is a diagrammatic sketch of various provisions for ventilation. In large buildings in closely built-up cities, mines, big passenger vessels, and the like, some form of mechanical ventilation is necessary, as it would be impossible otherwise to remain in them. Ventilation is accomplished in these structures by three methods, one in which the air is forced into the room, another in which air is drawn out of the room, the third being a combination of the first two methods.

The stream of air that is constantly passing through a well ventilated room not only reduces the temperature but sweeps away harmful gases resulting from the combustion of candles, coal oil, and illuminating gases. It also blows out dust, bacteria, and foul odors. There are sound reasons for the belief that diseases like consumption, pneumonia, and colds are transmitted by contact with persons suffering with or carrying the germs of the disease, which probably are not borne for any great distance through the air. The transmission may occur when the carrier of the germ coughs, sneezes, or otherwise sprays the secretion of mouth, nose, or throat over the faces of persons near him in street cars, theaters, or other places where persons collect in crowds.

Dust.

Dust is especially dangerous in certain occupations. It may be called "The greatest enemy of the workman." It may be of organic or inorganic origin. It is usually considered that dust of organic origin, such as cotton, wool, wood, coal, and the like is not so harmful as dust of inorganic origin, such as granite dust, which, owing to the hard sharp angles of its particles, is more irritating. In addition to these the air may contain metallic poisons and toxic gases, fumes, and vapors. Sommerfeld, in the table shown below, gives the death rate per thousand from consumption of persons engaged in various trades where dust is a prominent factor:

Wool sorters' disease, or anthrax pneumonia, is an example of infection caused by the inhalation of animal dust. The germs of anthrax are breathed in with the dust from the wool and cause pneumonia. Dust containing sharp, gritty particles, such as are present during the cutting of hard rock, sets up a chronic irritation of the air passages, which then become favorable lodging places for the germs of consumption. The germs thrive in the weakened tissues and disease ultimately results. Soft-coal dust is less dangerous than hard-coal dust; miners working in bituminous-coal fields are not especially liable to tuberculosis. The bad effects of dusty working places may be obviated by the use of respirators. These are, however, rather uncomfortable to wear, so that the better way is to re

move thè dust at its source by mechanical devices or to prevent its accumulation in the air by the use of water.

Heating.

The proper heating of a building depends to a large extent upon ventilation. Most houses are imperfectly ventilated, with the result that as the air is abnormally dry, they are overheated, for dry air causes excessive evaporation and gives a sense of chilliness. Air at a temperature of 65° F. and a relative humidity of 70 per cent has a greater feeling of warmth and is more comfortable than air at 2 temperature of 73° F. with only 50 per cent moisture. As many heated rooms have a relative humidity of less than 50 per cent, the temperature of the air has to be maintained at a much higher degree than would be necessary if the proper amount of moisture were present. On account of this chilly feeling due to dry air, many persons wear too much clothing while indoors, with the result that a layer of moisture covers the skin, rendering the person susceptible to drafts and to the catching of cold. If the proper humidity is maintained indoors, a person therein need not wear warmer clothes in winter than in summer. When going outdoors the body may be protected from cold by heavy wraps.

Buildings are heated by fireplaces, stoves, hot-air furnaces, hot water, or steam, and to a small extent by electricity. An open fire is cheerful, and the hot chimney acts as a good ventilator for the room. It has been estimated that a coal fire burning briskly in a fireplace of the usual size will cause 18,000 cubic feet of air to pass up the chimney in an hour, but as seven-eighths of the heat of the fuel is carried up the chimney and lost, this method of heating is wasteful and inadequate for cold places, as many parts of the room in which the fireplace is located are insufficiently warmed, the heat reaching only those persons near the grate. It is similar in some respects to a fire outdoors, where the portion of the body turned toward the fire becomes too warm, while the opposite side is cold.

A stove is better than a fireplace in that it radiates heat in all directions if it is set out in a room. The air also coming in contact with the hot stove ascends and mixes with the rest of the air in the room, thereby giving a more even temperature. The hot fire and the stovepipe act as a ventilator, but as the amount of air passing through a stove is much smaller than that which ascends the chimney from an open fire, air conditions in a room heated by a stove are not as good as when a fireplace is used. An extremely poisonous gas, known as carbon monoxide (p. 22), may be produced in stoves in which combustion is incomplete and may pass out of cracks, if the dampers are closed, and there is not a free circulation of air to