TEMPERATURE OF SCHOOL-ROOMS IN MICHIGAN.

Seven years ago I made an investigation into the physical condition of many of the school-rooms in this State, giving special attention to the state of the school-room when filled with scholars. Among other observations, I noted the temperature of the air of the room, both at the level of the desk, and at the floor level. In this way, I carefully examined forty-one different school-rooms during the session of the school. The average temperature of these forty-one school-rooms, at the desk level, was 66°.92 Fah.; at the floor level, 61°.8. The range of temperature in the rooms at the desk level was 63° to 73° F; at the floor level 42° to 71°, omitting one exceptional case, where the floor was heated to 77°, on account of being over the top of the furnace.

Here we find an average difference of 5th degrees in the temperature at the level of the chest and of the feet, in a child sitting in our school-rooms. If we compare the temperature at the floor, and six feet above the floor, where the room thermometer is usually hung, we shall find a still larger difference where good ventilation is wanting, and the lake of cold air on the floor is left undisturbed.

In the temperature for living-rooms given by DeChaumont and Morin, we are not told at what height the temperature of the room should be taken. This becomes a matter of importance, when we consider that the difference in temperature between the top and bottom of a room is often 15° to 20° Fah. If the temperature is to be taken six feet from the floor, in unventilated rooms, their 59° to 61°, at six feet from the floor, may easily become 51° to 53° at the floor. No child, with the usual amount of clothing, could sit still in air at such temperature for any length of time, without such discomfort as would prevent all desirable mental activity; if active at all, it must be in the line of mischief.

SHALL WE WARM BY RADIATION? OR BY CONVECTION?

In this criticism of the temperature of living-rooms, recommended by DeChaumont and Morin, I have assumed that the method of heating is the same as that used in this State, and that the physical condition of the room, in all other respects, is the same as ours. But if their method of heating differs essentially from ours, such criticism may be manifestly unjust.

The cost of fuel is greater in Europe than in America, which may explain, in part, the lower temperature to which Europeans accustom themselves. They find it cheaper to save animal heat by an increased amount of clothing, rather than use fuel to heat their rooms to a comfortable temperature. An artist, for example, will wear thick woollen clothes, thrust his feet into fur muffs, use a small brazier of coals to warm his fingers when they become too numb to hold his brush, and will thus work all day in some cathedral or picture-gallery, whose temperature would be torture to an American, if deprived of active exercise. But I question whether this saving of animal heat by excessive clothing, with a low temperature in the room, is conducive to better health. It is too much like keeping our rooms warm by shutting off all ventilation-for excessive clothing diminishes. personal ventilation-retaining the cutaneous excretions, which demand removal as imperatively as the pulmonary excretions. A certain amount of clothing to guard the body against sudden changes of temperature, is the demand alike of decency and comfort; but to secure animal heat by smothering the body with clothing, is a questionable economy.

The difference in temperature of air in living-rooms in Europe and in America, may also in part be explained by the difference in method of heating; in Europe they aim to warm the person without heating the air to any great degree; in this country, we warm the person by heating the air surrounding him. In Europe they use radiant heat; in America we use what I shall call convectent heat; that is, heat supplied to the person by convection, or air warmed by movement over some highly heated body. If the person is warmed by the air surrounding him, then the temperature of such air must manifestly more nearly approximate blood-heat than when the person is warmed by radiant heat without warming the air. Like light, radiant heat passes in straight lines from any heated surface, and may pass through a transparent medium without heating in the least, and it exhibits the properties of heat only when it is absorbed and retained by some body which will not transmit it.

I was walking along the streets of Lansing last summer, when a merchant, who was sunning himself before his door, hailed me: "Doctor, from what source do we derive our heat?" "Ultimately from the sun." "Then what warms me now is heat from the sun?" "Certainly." "How can that be when

the heat coming from the sun must pass for millions of miles through space colder than anything we ever find on the earth? Why is not the sun's heat used up in warming space and lost long before it reaches the earth?" "Because space is transparent to heat, arrests none of the heat rays any more than it does the rays of light, and it is only when radiant heat is arrested that it becomes that form of heat which is capable of manifesting changes of temperature. The sun's rays, containing light and heat associated together, may fall upon a block of clear ice and pass through it without heating it a particle; cut that block into the form of a large convex lens, to bring the rays to a focus, and you can ignite wood or melt gold with the radiant heat which has passed through ice without heating it enough to melt a drop. This is because clear ice is transparent to solar heat; but if charcoal dust is scattered through the ice, it will arrest the heat rays and the ice will melt around such opaque particles."

We see the same principle at work in a variety of ways. I have often seen snow melting on the side of a stump exposed to the direct rays of the sun while the temperature of the air was but little above zero F. On a sunny June morning, when the temperature of the air is 60° F., you feel warm and comfortable in the sunshine because you are warmed by the sunshine rather than by the air; sit down in the shade for an hour and you will "catch cold." Part of the invigorating influence of a bright spring morning, and one reason why it is a joy to be in the sunshine, is that we are warmed by radiant heat while we are in a cool atmosphere. We are warmed and invigorated instead of being heated and debilitated. Every thorough scholar must have noticed how much more energetically the mind works in a cool atmosphere, provided the body is comfortably warm, and how sluggishly the mental operations go on during hot and sultry weather. But to secure bodily warmth in cool air we want radiant heat. To live radiant lives we require radiant heat.

HEATING BY RADIATION.

In Europe house-warming is mainly by radiant heat. In Great Britain the open grate, with its glowing fire of coals or peat, warms and irradiates the British house. In Germany and France the porcelain stove warms mostly by radiation of non

luminous heat, but in part also by convection. The radiant heat that warms the person upon which it falls instead of the air through which it passes will not require so high a temperature of the air in order to maintain bodily comfort as will that mode of heating where the air itself is the chief heating material.

HEATING BY CONVECTION.

In this country we once had the blazing open fire, but this has given place to "that sullen gnome, the air-tight stove," and this is fast giving place to hot-air furnaces or coils of steam-pipe. To a large extent we have banished radiant heat from our houses, and depend almost entirely upon convectent heat. Even when we retain the stove which radiates some heat the air of the room is warmed mostly by convection; in hot-air furnaces of every kind the warming is entirely by convection. It is obvious that if we heat a person by hot air we must have such air hotter than when we send the heat through the air by radiation from some highly heated surface. Convectent heat requires a hotter atmosphere than radiant heat. The radiation from steam coils is deficient in penetrating quality as compared with radiation from highly heated, and especially from luminous bodies. It is correspondingly feeble in its influence on animal temperature. If our rooms are warmed by the method of convection of heat by means of hot-air furnaces of any kind, or even by radiation from surfaces of low intensity, such as ordinary steam coils, I think the air must be heated some ten degrees hotter than when we are warmed by radiant luminant heat. I confess that I cannot sit in comfort in a furnace-heated room when the usual room thermometer marks less than 70° F.

HEATING BY CONVECTION REQUIRES VENTILATION AT THE FLOORLEVEL.

One necessary condition of properly warming a room by convection is too often neglected, namely, that the whole body of air in the room must be in motion in order to warm the air at the floor-level. The primary currents in convection of heat are all upward; the hot air pouring into a room from the furnace through a register in the floor, passes directly to the top of the room. If the ventilator is near the top of the room this hot air may escape by it, having traversed the room without any marked

influence on the temperature of the lower stratum of air. The hot air at the top of the room cannot warm the lower portion of the room, except by replacing the cold air below. No amount of heat at the top will be conducted to the bottom, because air is almost an absolute non-conductor of heat. A test-tube filled with ice-cold water and a bit of ice frozen in the bottom may be boiled at the top without melting the ice at the bottom. The boiling water is lighter than ice-water, and will not descend to take its place. The same is true of air. To demonstrate this I took a combination tube of nearly infusible glass, twenty-four inches long and one-half inch internal diameter; the tube was closed at bottom to prevent currents of air, a thermometer was placed in the tube, the tube placed in a nearly vertical position, and a strong gas flame applied to the tube six inches from the upper end and eighteen inches from the bulb of the thermometer. The thermometer so nearly filled the tube that the bulb of the thermometer was nearly screened from radiant heat of the gas flame, and could only be warmed by heat conducted to it by the glass tube or by the air contained in it. The upper part of the tube was heated to the melting point of zinc (773° F.), and this temperature maintained for an hour, yet the thermometer indicated an increase of temperature of less than one-half a degree. Here I had a room with only eighteen inches between floor and ceiling, and a difference of temperature of more than 700° F., yet the floor is warmed in one hour less than half a degree. I then ventilated this room by opening the bottom of the tube, and rapidly withdrawing the air from the floor-level by an aspirator, when the thermometer in one minute rose 275° F. (from 65° F. to 340° F.). These experiments give some idea of the difficulty of warming the bottom of a room by convection of heat, unless there is good ventilation from the floor-level.

Radiant heat is not influenced by gravity, and may penetrate and warm the bottom as well as the top. In nature we find the hot air at the bottom of the atmosphere and colder air as we ascend, because the earth is warmed by radiant heat, and the air is mainly warmed by contact with the earth.

COMBINATIONS OF CONVECTION AND RADIATION.

Every person will concede the superiority of heating by radiation over that by convection; also that the open fire secures ventilation; but they object that it is a wasteful way of house