Words like down, in are not always prepositions. Compare:
I ran down the road. He's in his office.
Please sit down You can go in
In the expressions down the road and in his office, down and in are prepositions: they have objects (the road, his office).
In Please sit down and You can go in, down and in have no objects. They are not prepositions, but adverbs of place, which modify the verbs sit and go.
Small adverbs like this are usually called 'adverb particles' or 'adverbial particles'. They include in, out, up, down, on, off, through, past, away, back, across, over, under. Adverb particles often join together with verbs to make two-word verbs, sometimes with completely new meanings. Examples: break down = 'stop working'; put off = 'delay', 'postpone'; work out = 'calculate'; give up = 'stop trying'. For information about these verbs, see the next section.
The Unit of Heat
Temperature and 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° C. It is the amount of heat given out by 1 gram of water when its temperature falls 1° C., or the amount of heat absorbed by 1 gram of water when its temperature rises 1° C. If 400 grams of water are heated from 0° to 5° C., the amount of heat which has entered the water is equivalent to 5 × 400 or 2000 calories; if 200 grams of water cool from 25° to 20° C., the heat given out by the water is equivalent to 5 × 200 or 1000 calories.