150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



light upon the scale of the instrument. Now I warm the lamp as 

 before. See the spot of light move. To convince you that rays from 

 the warmed lamp are actually crossing the stage and warming the 

 electrical thermometer by being focused upon it, I insert this card 

 so as to hide the lamp from the focusing mirror. The spot of light 

 immediately falls back. I remove the card. It leaps forward. All 

 chis time the lamp, though warmed, has been too cool to glow. But 

 now I heat it to a glowing temperature, as you see. Note how very 

 much wider now is the swing of the recording light spot as again 

 I insert the card. 



RAYS MAY TRAVEL IN A VACUUM 



Thus we see that not only visible rays from very hot sources, but 

 invisible rays from bodies too cool to glow, are able to cross wide 

 spaces and to produce heat when they are absorbed by blackened 

 receiving surfaces like that of this electrical thermometer. In our 

 experiment the space traversed by the rays was filled with air. That 

 was a hindrance, not a help, to the passage of these rays. They will 

 cross a vacuum even more freely than they will pass through air. 

 When we say that a radio program is " on the air ", it is true that 

 the air fills the space between the broadcaster and the listener, but 

 it is not the air that carries the waves. Just as sunlight comes to 

 us across a 92,000,000-mile vacuum, radio rays can also travel in 

 vacuum. They do not em]Dloy the air for their conveyance. The 

 nature of the medium that carries light and radio rays is still 

 obscure, but we know that it is not the air. 



THE SUN IS THE EARTH'S HEAT SOURCE 



The sun's rays are almost the sole source of the heat that warms 

 our earth. They cross the void of space to us just as the rays from 

 the lamp crossed this stage. It requires 8 minutes for the sun's rays 

 to reach the earth, traveling 186,000 miles each second. We noted 

 that the electrical thermometer, when radiated upon by the hot lamp, 

 warmed up to a certain degree of temperature and then stayed con- 

 stant. The earth, wherever the sun's rays strike it, also tends to 

 warm up. Its temperature would stay constant as the thermopile 

 did when thus warmed, if conditions on the earth remained un- 

 changed. But clouds may form and obscure the sun, dust or smoke 

 frequently intervenes and, most important of all, the earth rotates 

 a complete turn every day. No spot on earth stays fixed underneath 

 the sun beam long enough to come to a constant temperature. The 

 earth is about 8,000 miles in diameter, and 25,000 miles in circum- 

 ference. Hence at the equator its surface passes under the solar 

 beam about 1,000 miles each hour. 



