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In the preceding Section, we learned that many houses heated by hot water are supplied with fresh-air pipes which admit fresh air into separate rooms or into suites of rooms. In some cases the amount which enters is so great that the air in a room is changed three or four times an hour. The constant inflow of cold air and exit of warm air necessitates larger radiators and more hot water and hence more coal to heat the larger quantity of water, but the additional expense is more than compensated by the gain in health.

Owing to the shape of the earth and its position with respect to the sun, different portions of the earth are unequally heated. In those portions where the earth is greatly heated, the air likewise will be heated; there will be a tendency for the air to rise, and for the cold air from surrounding regions to rush in to fill its place. In this way winds are produced. There are many circumstances which modify winds and currents, and it is not always easy to explain their direction and velocity, but one very definite cause is the unequal heating of the surface of the earth.

The invention of the "Fireless Cooker" depended in part upon the principle of non-conduction. Two vessels, one inside the other, are separated by sawdust, asbestos, or other poor conducting material (Fig. 18). Foods are heated in the usual way to the boiling point or to a high temperature, and are then placed in the inner vessel. The heat of the food cannot escape through the non-conducting material which surrounds it, and hence remains in the food and slowly cooks it

The Unit of Heat. It is necessary to have a unit of heat just as we have a unit of length, or a unit of mass, or a unit of time. One unit of heat is called a _calorie_, and is the amount of heat which will change the temperature of 1 gram of water 1 deg. C. It is the amount of heat given out by 1 gram of water when its temperature falls 1 deg. C., or the amount of heat absorbed by 1 gram of water when its temperature rises 1 deg. C. If 400 grams of water are heated from 0 deg. to 5 deg. C., the amount of heat which has entered the water is equivalent to 5 x 400 or 2000 calories; if 200 grams of water cool from 25 deg. to 20 deg. C., the heat given out by the water is equivalent to 5 x 200 or 1000 calories.

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