Since about 1800, near the beginning of the Industrial Revolution, coal, petroleum, and gas have been used at a rapidly increasing rate. The supply of these     1    is limited, and electric power is very     2    in many places. Some scientists believe solar energy is the only     3    which can meet the world's enormous     4    for power.
    Great progress had been made in harnessing the sun. Not only is the sun now used in cooking,     5    it also supplies power for     6    things as beacon lights for ships and airplanes. It     7    telephone lines, portable radios, electric clocks, hearing aids, and even communication. Some homes and office buildings in the United States are now being     8    with solar energy.
    The three most significant types of solar equipment     9    are the furnace, the still, and the cell. Solar furnaces heat water or air,     10    is then circulated through a building to make it comfortable.
    Solar stills are     11    important because they can provide     12    water at a relatively cheap rate. Salt can easily be     13    from sea water with a solar still.
    The most highly developed type of solar     14    equipment is the solar cell. Some cells are so     15    that they can turn 16 percent of the energy they receive from the sun     16    electric energy. One of the cell's biggest advantages is     17    it can be made either small enough to carry     18    large enough to produce a current that can run an automobile. Besides, it has a very long life. It is still too expensive for the     19    consumer but when ways are     20    to produce it more cheaply, we can expect its use to be wide-spread.