Your lucid bands condense with fingers chill The blue mist hovering round the gelid hill.

CANTO III. l. 19.

The surface of the earth consists of strata many of which were formed originally beneath the sea, the mountains were afterwards forced up by subterraneous fires, as appears from the fissures in the rocks of which they consist, the quantity of volcanic productions all over the world, and the numerous remains of craters of volcanos in mountainous countries. Hence the strata which compose the sides of mountains lie slanting downwards, and one or two or more of the external strata not reaching to the summit when the mountain was raised up, the second or third stratum or a more inferior one is there exposed to day; this may be well represented by forceably thrusting a blunt instrument through several sheets of paper, a bur will stand up with the lowermost sheet standing highest in the center of it. On this uppermost stratum, which is colder as it is more elevated, the dews are condensed in large quantities; and sliding down pass under the first or second or third stratum which compose the sides of the hill; and either form a morass below, or a weeping rock, by oozing out in numerous places, or many of these less currents meeting together burst out in a more copious rill.

The summits of mountains are much colder than the plains in their vicinity, owing to several causes; 1. Their being in a manner insulated or cut off from the common heat of the earth, which is always of 48 degrees, and perpetually counteracts the effects of external cold beneath that degree. 2. From their surfaces being larger in proportion to their solid contents, and hence their heat more expeditiously carried away by the ever-moving atmosphere. 3. The increasing rarity of the air as the mountain rises. All those bodies which conduct electricity well or ill, conduct the matter of heat likewise well or ill. See note VII. Atmospheric air is a bad conductor of electricity and thence confines it on the body where it is accumulated, but when it is made very rare, as in the exhausted receiver, the electric aura passes away immediately to any distance. The same circumstance probably happens in respect to heat, which is thus kept by the denser air on the plains from escaping, but is dissipated on the hills where the air is thinner. 4. As the currents of air rise up the sides of mountains they become mechanically rarefied, the pressure of the incumbent column lessening as they ascend. Hence the expanding air absorbs heat from the mountain as it ascends, as explained in note VII. 5. There is another, and perhaps more powerful cause, I suspect, which may occasion the great cold on mountains, and in the higher parts of the atmosphere, and which has not yet been attended to; I mean that the fluid matter of heat may probably gravitate round the earth, and form an atmosphere on its surface, mixed with the aerial atmosphere, which may diminish or become rarer, as it recedes from the earth's surface, in a greater proportion than the air diminishes.

6. The great condensation of moisture on the summits of hills has another cause, which is the dashing of moving clouds against them, in misty days this is often seen to have great effect on plains, where an eminent tree by obstructing the mist as it moves along shall have a much greater quantity of moisture drop from its leaves than falls at the same time on the ground in its vicinity. Mr. White, in his History of Selborne gives an account of a large tree so situated, from which a stream flowed during a moving mist so as to fill the cart-ruts in a lane otherwise not very moist, and ingeniously adds, that trees planted about ponds of stagnant water contribute much by these means to supply the reservoir. The spherules which constitute a mist or cloud are kept from uniting by so small a power that a little agitation against the leaves of a tree, or the greater attraction of a flat moist surface, condenses or precipitates them.

If a leaf has its surface moistened and particles of water separate from each other as in a mist be brought near the moistened surface of a leaf, each particle will be attracted more by that plain surface of water on the leaf than it can be by the surrounding particles of the mist, because globules only attract each other in one point, whereas a plain attracts a globule by a greater extent of its surface.

The common cold springs are thus formed on elevated grounds by the condensed vapours, and hence are stronger when the nights are cold after hot days in spring, than even in the wet days of winter. For the warm atmosphere during the day has dissolved much more water than it can support in solution during the cold of the night, which is thus deposited in large quantities on the hills, and yet so gradually as to soak in between the strata of them, rather than to slide off over their surfaces like showers of rain. The common heat of the internal parts of the earth is ascertained by springs which arise from strata of earth too deep to be affected by the heat of summer or the frosts of winter. Those in this country are of 48 degrees of heat, those about Philidelphia were said by Dr. Franklin to be 52; whether this variation is to be accounted for by the difference of the sun's heat on that country, according to the ingenious theory of Mr. Kirwan, or to the vicinity of subterranean fires is not yet, I think, decided. There are however subterraneous streams of water not exactly produced in this manner, as streams issuing from fissures in the earth, communicating with the craters of old volcanoes; in the Peak of Derbyshire are many hollows, called swallows, where the land floods sink into the earth, and come out at some miles distant, as at Ilam near Ashborne. See note on Fica, Vol. II.

Other streams of cold water arise from beneath the snow on the Alps and Andes, and other high mountains, which is perpetualy thawing at its under surface by the common heat of the earth, and gives rise to large rivers. For the origin of warm springs see note on Fucus, Vol. II.