Moulton, Forest Ray, 1872-1952 · New York, The Macmillan company · leaf 96
of 50 miles above the earthssurface in quantities sufficient to produce twilight. The results obtained by the various methods for determin-ing the height of the atmosphere disagree because its densitydecreases with altitude, as is found by ascending in balloons,and different densities are required… … FlG. 20. — Determination of the height ofatmosphere from the duration of twilight. the 68 AN INTRODUCTION TO ASTRONOMY [ch. ii, 31 face. At this altitude its density is of the order of one four-millionth of that at the surface. When the whole earth isconsidered it is found that the atmosphere forms…
Astronomy
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