NORTHERN LIGHTS 



By A. S. Eve, C. B. E., D. Sc, F. R. S. 

 Emeritus Professor of Physics, McOill University 



[With 5 plates] 

 THE LOWER AIR 



Before considering the upper atmosphere it may be well to recall 

 the remarkable unseen events taking place within this lecture theater 

 of the Roj^al Institution, so pregnant with famous memories. You 

 probably did not notice that, as the lecturer entered, a large number 

 of an essential part of this gathering went out as quickly as he came 

 in. This number may be estimated at about a million million million 

 million, a number almost sufficient to impress even Sir James Jeans. 

 The calculation is not difficult, for my weight somewhat exceeds that 

 of 2 cubic feet of water. The late Sir Arthur Shipley pointed out 

 that all men, including even the Archbishop of Canterbury, are about 

 90 percent water, and it is clear that my volume is, more or less as 

 the lawyers say, 2 cubic feet or 5,000 cubic centimeters; but the num- 

 ber of air molecules in a cubic centimeter down here is roughly 2.T by 

 10 ^^, a vast number equivalent to 27 followed by 18 noughts. Multi- 

 plying these figures together, you obtain more than the million million 

 million million, or 10 ", molecules that left the room as each one of 

 you came into the hall. The molecules of air are not at rest but are 

 moving more rapidly than a rifle bullet. They are frequently collid- 

 ing with one another, each one about 5,000 million times a second, 

 and providing everywhere, by their bombardment, a pressure of 15 

 pounds to the square inch. If the air in this room, about half a ton 

 of it, could be suddenly removed, my voice could not reach the audi- 

 ence, who would all be dead within 2 minutes, if they had not already 

 exploded outward from their internal pressure of 15 pounds to the 

 square inch. The air molecules are small ; about 100 million of them, 

 side by side, would stretch for half an inch or so, but they are by no 

 means crowded, for their average distance apart is 300 times their 



^A lecture delivered before the Royal Institution of Great Britain at the Weekly Eve- 

 ning Meeting, Friday, Feb. 7, 1936. Reprinted by permission, with slight alterations, 

 from the pamphlet of the Rsyal Institution, and including additions to the text and illus- 

 trations published in Nature, vol. 137, no. 3472, May 16, 1936. 



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