[_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 surface 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 sinks 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 atmosphere. 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 inflammable gas, or Hydrogene, exists over the aerial one.] [_Alarm with Comet-blaze_. l. 133. See additional notes, No. IV.] [_The Sun's phlogistic orb_. l. 136. See additional notes, No. V.] III. NYMPHS! YOUR fine forms with steps impassive mock Earth's vaulted roofs of adamantine rock; Round her still centre tread the burning soil, 140 And watch the billowy Lavas, as they boil; Where, in basaltic caves imprison'd deep, Reluctant fires in dread suspension sleep; Or sphere on sphere in widening waves expand, And glad with genial warmth the incumbent land. 145 So when the Mother-bird selects their food With curious bill, and feeds her callow brood; Warmth from her tender heart eternal springs, And pleased she clasps them with extended wings. [_Round the still centre_. l. 139. Many philosophers have believed that the central parts of the earth consist of a fluid mass of burning lava, which they have called a subterraneous sun; and have supposed, that it contributes to the production of metals, and to the growth of vegetables. See additional notes, No. VI.] [_Or sphere on sphere_. l. 143. See additional notes, No. VII.] "YOU from deep cauldrons and unmeasured caves 150 Blow flaming airs, or pour vitrescent waves; O'er shining oceans ray volcanic light, Or hurl innocuous embers to the night.-- While with loud shouts to Etna Heccla calls, And Andes answers from his beacon'd walls; 155 Sea-wilder'd crews the mountain-stars admire, And Beauty beams amid tremendous fire.