154 ANNUAL REPOET SMITHSONIAN INSTITUTION, 19 3 8 



Ascending through the cross-sections of the atmosphere, we find 

 there is a rapid decrease in the amount of atmospheric pressure. 

 Within the first 3 miles from the earth's surface, half the total amount 

 of oxygen and nitrogen, the principal atmospheric ingredients, are 

 included. The limiting height to which the thinning atmosphere 

 extends is somewhat difficult to fix. Perhaps we should place it at 

 400 or 500 miles, although recently Dr. Carl Stormer has observed 

 auroral streamers reaching to heights of 600 miles or more. Where 

 auroral streamers go, some of the thin atmosphere must extend. 



Through the courtesy of the Smithsonian Institution we have 

 provided for us here 2 a piece of apparatus from which the air can 

 be exhausted so that we can gradually simulate conditions that 

 would be encountered were we to board an imaginary balloon and 

 ascend upward. To apply such electric potentials as appear to 

 exist aloft, there are two electrodes entering this glass tube at either 

 end and a high voltage is applied across these terminals. Under the 

 standard atmospheric conditions of this room which is now the 

 condition inside the tube, you see there is no evidence of a passage 

 of electricity. Air under ordinary conditions is a relatively good 

 nonconductor. If, however, I now start the vacuum pump going 

 and begin to exhaust the air from this closed tube, the pressure will 

 be gradually reduced and we shall soon see the beginning of an illumi- 

 nation as an electric current passes through the rarefied air from one 

 end of the tube to the other. The color already simulates the red 

 tints of the aurorae. As the vacuum in this tube increases, we 

 imagine ourselves rising higher and higher through the stratosphere. 

 We note the changing form of the electrical discharge and the pale 

 blue color that now occurs as the ionization of the thinning air in the 

 tube becomes more complete. The glow is dimming now as we are 

 attaining the equivalent altitude of some 400 or 500 miles. At this 

 imaginary height the air molecules are becoming scarce. The artificial 

 auroral glow is dimming. Now it has ceased altogether for the 

 vacuum obtained is too nearly complete for current to pass longer. 

 We are at the very top. Now shutting off the pump we will throw 

 a little valve here, and gradually admit the air from the room again. 

 In our imaginary flight our gallant gondola is now descending. Watch 

 for the first appearances of the auroral glow. There it is — first pale 

 blue, gradually increasing in intensity as we rapidly descend to 

 greater densities of the air. Once again we see the pink striations. 

 We are coming rapidly into the stratosphere. Now come the irregular 

 discharges characteristic of the denser regions. The glow dims and 

 disappears once more. W T e have fallen rapidly down to full atmos- 

 pheric pressure again where the air becomes nonconducting and the 

 discharge has ceased. 



1 Experiment shown to the lecture audience. 



