The air surrounding us is important to everyone. Without air, we could not exist. Everyone understands that. But air is necessary in many other ways — ways that are not always so obvious or widely known.
If we did not have air, there would be no sound. Sound travels through air. Where there is no air, there is no sound. Without air there would be no fire. There would be no automobiles, since motors need air in order to operate.
Without air, there would be no wind or clouds. The night time would be very cold and the days very hot. We would be forced to seek shelter from the sun, as there would be no atmosphere to protect us from the sun’s deadly rays.
In this huge ocean of air surrounding the earth, there is more energy than in all the coal, oil and gas we have on the earth. Electrical energy is collected in the atmosphere as water is collected and stored in a dam (大坝). The existence of electricity in the air has been known for centuries. Men have gazed in wonder at the bright patterns of lightening in storm clouds. But thorough (彻底的，深入的) study of electricity in the air was not possible until the development of radio.
The existence of electricity in the air has been known ______.