microphone → receiver

transmitter → receiver

The forest and mountains respond to the scream with a booming echo.

the production of an echo

Experiment 2.
Can moving water carry soil?
Make a hill of dry soil and pour water over the top of the hill. The water runs down the hill in little streams, taking soil with it.
Where does the soil go?
Does the water make little ditches in the hill?
Pour more water on the hill and see if the ditches get bigger.

All you need to make your own snow are a book of safety matches, a silver coin, a drop of iodine and some cold air in an enclosed space. A black background and a flashlight help, however, by making the tiny flakes visible. The procedure is shown below. For cold air, try any of the following: (1) the inside of a home freezer or a frozen-food cabinet; (2) a big empty jar, chilled by packing dry ice around it in a bucket (but don’t touch the dry ice with bare hands!); (8) the inside of a closed car, or a similar space, on a cold winter night. Blowing into the ice-cube section of a refrigerator won’t work, because all the cold air spills out of it when the door is opened.

“How to make snow at home.”
Popular Science. June 1947.

Annie Leibovitz

a sensitive microphone