[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.
[Hurl innocuous embers. l. 152. The immediate cause of volcanic eruptions is believed to be owing to the water of the sea, or from lakes, or inundations, finding itself a passage into the subterraneous fires, which may lie at great depths. This must first produce by its coldness a condensation of the vapour there existing, or a vacuum, and thus occasion parts of the earth's crust or shell to be forced down by the pressure of the incumbent atmosphere. Afterwards the water being suddenly raised into steam produces all the explosive effects of earthquakes. And by new accessions of water during the intervals of the explosions the repetition of the shocks is caused. These circumstances were hourly illustrated by the fountains of boiling water in Iceland, in which the surface of the water in the boiling wells sunk down low before every new ebullition.
Besides these eruptions occasioned by the steam of water, there seems to be a perpetual effusion of other vapours, more noxious and (as far as it is yet known) perhaps greatly more expansile than water from the Volcanos in various parts of the world. As these Volcanos are supposed to be spiracula or breathing holes to the great subterraneous fires, it is probable that the escape of elastic vapours from them is the cause, that the earthquakes of modern days are of such small extent compared to those of antient times, of which vestiges remain in every part of the world, and on this account may be said not only to be innocuous, but useful.]
"Thus when of old, as mystic bards presume,
Huge CYCLOPS dwelt in Etna's rocky womb,
On thundering anvils rung their loud alarms,
160 And leagued with VULCAN forged immortal arms;
Descending VENUS sought the dark abode,
And sooth'd the labours of the grisly God.—
While frowning Loves the threatening falchion wield,
And tittering Graces peep behind the shield,
165 With jointed mail their fairy limbs o'erwhelm,
Or nod with pausing step the plumed helm;
With radiant eye She view'd the boiling ore,
Heard undismay'd the breathing bellows roar,
Admired their sinewy arms, and shoulders bare,
170 And ponderous hammers lifted high in air,
With smiles celestial bless'd their dazzled sight,
And Beauty blazed amid infernal night.