MISCELLANEOUS. LESSON XLVIII.—AVALANCHES. THIS lesson might follow one on the geography of Switzerland. I. Formation and Localities. Consist of masses of snow or ice; descend from elevated regions; frequent upon all lofty mountains-e.g., Alps, Pyrenees, Dofrine, &c. II. Description of the various Kinds. Four kinds-(a) drift, (b) sliding, (c) creeping, (d) ice or glacier. a. Drift.-Formed of loose or drift snow; originate at a great elevation. "The avalanche-the thunder-bolt of snow." Generally happen in winter; descend unexpectedly without apparent cause - probably started by the wind; increase on the way. Air compressed by their velocity-destroys houses, trees, &c. kind rarest of the four, but most destructive. Some descend a distance of ten miles. b. Sliding. More frequent; less destructive, yet, from their number, cause greater damage. Sheets of frozen snow; formed on steep declivities and middle regions of mountains; fall in spring; cover meadows and fields; several summers before they melt. Very common in Norway. c. Creeping.-Called so from their slow, creeping approach; formed in a similar manner to (b) sliding on more gentle slopes; small in volume; very frequent; little damage. Occur in spring. d. Ice or glacier.-Masses of glacier detached in their descent; occur in summer; generally fall upon uninhabited places; less fatal effects. "Mountains have fallen, Leaving a gap in the clouds, and filling up III. Instances of Effects. Notice a few instances of their effects; e.g. :— 1. In 1749: whole village of Bueras (Switzerland) buried or removed by a sliding avalanche, in the night; noiseless, the inhabitants unable to account for the continued darkness. One hundred persons dug out—sixty alive. 2. In 1806: a forest transplanted to the opposite side of the valley; a fir-tree was removed to the roof of the parsonage. 3. Sixty deer were found dead in a narrow valley in Norway; how were they killed? 4. At Radna, in 1819: a glacier avalanche carried beams of houses a mile away; raised and removed millstones; broke down the church spire, which was of stone. "Oft rushing sudden from the loaded cliffs, Mountains of snow their gathering terrors roll And herds, and flocks, and travellers, and swains, Are deep beneath the smothering ruin whelm'd." LESSON XLIX.—THE BAROMETER. I. Its Principle. A COLUMN of air is equal to one of water thirty-three feet high-or another of quick-silver of thirty inches. The pressure of the atmosphere equals the pressure of a layer of water thirty-three feet deep, or a layer of mercury thirty inches thick— hence they counter-balance; change in the one will produce a proportionate change in the other. A barometer might be constructed of either liquidmercury the more convenient. Name derived from baros, "weight," and metron, "a measure; a measure of the weight of the atmosphere—sometimes called a weather-glass. II. (1) Description and (2) Construction. 1. A glass tube (r*) filled with mercury; the open end placed in a vessel of the same fluid; a scale, graduated in inches and tenths of inches, placed behind to measure the height of the mercury. 2. Take a glass tube thirty-four inches long; fill with quicksilver; cover the open end with a finger; plunge into a vessel (v); withdraw the finger without admitting the air; the metal descends, vibrates, and finally rests at about twenty-nine or thirty inches above the surface (s). If to be moved about, the cup (v) must be closed, the tube fixed, and the whole placed in a wooden or metal case. Other forms of the instrument known by different * Draw a diagram on the black-board. names, as (a) the wheel barometer and (b) the marine barometer. a. The wheel barometer similar to the last. A tube (T*) with a short leg (L) filled with mercury; a piece of glass or an iron ball floats at G, attached to a silken cord passing over a pulley (P), to which is fixed an index or hand working around a circular plate graduated with "Rain," "Fair," "Dry," "Changeable," &c. Any movement of the glass at G indicated by a movement of the hand; thus all changes in the weight of the air are measured. b. In the marine instrument-the tube contracted to a very narrow bore in one part, to prevent the sudden rising and falling of the mercury which the motion of the ship would occasion. III. Uses, &c. The space A is a vacuum (the Torricellian vacuum, first observed by Torricelli)-a space entirely empty of every substance; obtained by filling the tube with mercury, thereby excluding all air; the mercury sustained in the tube by atmospheric pressure.† Medium altitude on the surface of the earth, twenty-nine inches; affected by the constantly fluctuating state of the atmosphere; therefore useful as a (1) Weather-glass to farmers, travellers, and sailors; the air being heaviest in clear weather, the mercury rises; in dull damp weather, air, lighter, mercury falls.. A low glass indicates rain and foul weather. * Draw a diagram on the black-board. If a barometer be a bad one, on swinging it in a dark room you see flashes of light, which denote the presence of vapour in the tube-not a perfect vacuum. In a good instrument the mercury beats heavily against the tube, proving the space a vacuum. "The hollow winds begin to blow, Compare this with the common error, that in foggy weather the air is heaviest, and that the heavy air presses down the smoke, vapours, &c. Disproved by the principle that the lighter a fluid is, the deeper any heavy substance will sink in it. (2.) To scientific men, it supplies a simple and accurate method of measuring heights, mountains, elevation of balloons, &c. Mercury falls in rising above the surface-e.g., in carrying a barometer from the level of the Thames to the top of St. Paul's, the mercury falls half an inch, showing an ascent of about five hundred feet-on the summit of Mont Blanc (fifteen thousand eight hundred feet) it sinks to sixteen inches. M. du Lac, the highest aeronaut, found his barometer stand at twelve inches, when he reached an elevation of twentytwo thousand feet. (3.) Indicates the approach of storms; greatly depressed before tempests, winds, hurricanes, &c. ; known to sink a hundred degrees in a few hours before a violent storm. During the Lisbon earthquake the mercury fell so low in the barometer, even in Great Britain, as to disappear from the uncovered portion. (4.) Of great use in coal mines; by it the presence of fire-damp is discovered; in a heavy atmosphere the fire-damp is pressed into the crevices and cracks, and the place is safe-in a lighter, it expands and explodes when it comes in contact with flame. Note. "It was in a southern latitude, the sun |