* And bend the twilight. l. 126. The crepuscular atmosphere, or the region where the light of the sun ceases to be refracted to us, is estimated by philosophers to be between 40 and 50 miles high, at which time the sun is about 18 degrees below the horizon; and the rarity of the air is supposed to be from 4,000 to 10,000 times greater than at the sur∣face of the earth. Cotes's Hydrost. p. 123. The duration of twilight differs in different seasons and in different latitudes; in England the shortest twilight is about the beginning of October and of March; in more northern latitudes, where the sun never finks more than 18 degrees, below the horizon, the twilight continues the whole night. The time of its duration may also be occasionally affected by the varying height of the atmo∣sphere. A number of observations on the duration of twilight in different latitudes might afford considerable information concerning the aerial strata in the higher regions of the atmosphere, and might assist in determining whether an exterior atmosphere of inflam∣mable gas, or Hydrogene, exists over the aerial one.
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