« ForrigeFortsett »
the smallest shot, while others are several inches in diameter. In August 1813, hailstones the size of eggs fell upon the
British army in the Pass of Maga in the Pyrenees; the storm lasted twenty minutes, and was not accompanied with thunder or lightning. On 4th June 1814, hail, from 13 to 15 inches in diameter, fell in Ohio. In the Orkney Islands, on the 24th July 1818, during thunder, a very remarkable shower of hail took place; the stones were as large as a goose's egg, and mixed with large masses of ice. In June 1835, hail fully three inches in circumference fell near Edinburgh from a dense cloud during a thunderstorm. On 8th May 1832, an immense mass of aggregated hailstones fell in Hungary, measuring about a yard in length, and nearly two feet in depth. A
mass nearly twenty feet in circumference, of an angular shape, and composed of lozenge-shaped pieces congealed together, is said to have fallen in Ross-shire, in August 1849. Masses of this sort are probably formed by regelation after the hailstones have fallen, by which their surfaces
are made to adhere together when Fig. 46.
rolled over each other by the
wind. 435. A hailstone (fig. 46) described by Captain Delcrosse · as having fallen at Baconnière in July 1819, was 15 inches in circumference, and had a beautifully radiated structure, showing it to be a single hailstone. On the 8th August 1857 Professor Tyndall saw hail fall among the Alps in the form of perfect spheres of ice, just as if the rain-drops had solidified in their descent. Further particulars regarding these and other meteors will be found in Thomson's Introduction to Meteorology,'—a work replete with exact, learned, and curious information of every sort in the different departments of the science.
436. No satisfactory theory of the origin of hail has yet been proposed which fully explains all the phenomena connected with hailstorms. Hail appears to be formed by a cold current of air forcing its way into a mass of air much warmer and nearly saturated, the temperature of the united mass being below the freezing-point. The warm moist air is easily accounted for, since hail generally falls in summer and during the day. The difficulty is to account for the intensely cold current which is sufficient to reduce the warm saturated mass below 32°.
437. In mountainous regions, cold currents from the fields of snow rushing down the sides of the mountains and mixing with the heated air of the valleys are no doubt frequent causes of hail ; and we have seen that such places are peculiarly subject to hailstorms.
438. The sudden ascent of moist warm air into the upper regions of the atmosphere, where a cold current prevails at the time, is, in all probability, a common cause of hail. This is confirmed by the circumstances generally attendant on hailstorms-viz., the sultry, close weather which precedes them, the slight but sudden barometric depression, the whirlwinds and ascending currents which accompany them, and the fall in the temperature which follows after the storm has passed.
439. WIND is air in motion. The speed of the wind varies, from the lightest breath that scarcely stirs the leaf on its branch, to the hurricane which, sweeping on in its fury, lashes the ocean into a tempest, strikes down the stateliest trees of the forest, and levels even substantially-built houses with the ground.
440. The force of the wind is measured by anemometers, of which there are different sorts—some measuring the velocity, others the pressure. Of the anemometers which measure the velocity of the wind, the simplest and best is the Hemispherical-Cup Anemometer, generally called Robinson's Anemometer, fig. 47. It consists of four hollow hemispheres or cups screwed on to the ends of two horizontal rods of iron crossing each other at right angles, and supported on a vertical axis which turns freely. When placed in the wind, the cups revolve; and the arms are of such a length that when a mile of wind has passed the anemometer, 500 revolutions are registered by the instrument. The accuracy of its construction may be tested by conveying it rapidly through the air on a perfectly calm day the distance of a mile and back again the same distance, and noting the number of revolutions made. The number of revolutions is registered by a system of indexwheels set in motion by an endless screw on the upright axis, which are read off in the same way as a gas-meter. The number of miles travelled by the wind during a day, an hour, or any other specified time, is found by multiplying the revolutions made in that time by 2, and dividing by 1000.
The rate per hour at which the wind blows at any time is found by observing the revolutions made in, say, two minutes; multiply by 30 and 2 or at once by 60, and divide by 1000. Thus, suppose 800 revolutions were made in two minutes, the velocity of the wind would be at the rate of 48 miles an hour.
441. In this form the anemometer only gives the whole velocity between two observations; it does not register the velocity at any moment. To effect continuous registration an elaborate machinery is required—too complicated to be here described—by which the result is transferred to paper by a pencil, or by photography.
442. The force of the wind is also ascertained by noting the pressure which it exerts on a plane surface of metal per pendicular to the direction of the wind. The pressure is generally given in pounds avoirdupois on the square foot.* The instrument is of simple construction, consisting of a plate a foot square acting on a spiral spring, to which an index showing the degree of pressure is attached. The plate is kept perpendicular to the wind by a vane. This is the principle of Osler's anemometer, which, by means of machinery, leaves a pencilling of the pressure of the wind for every instant.
443. The pressure is also measured by Lind's wind-gauge, fig. 48, which consists of a tube half an inch in diameter, in
the form of a siphon, one end of it being bent at right angles, so as to face the wind. It turns freely on a vertical axis, and a vane keeps the mouth of it directed to the wind. It is halffilled with water, and when the wind blows into the mouth of the instrument, it drives the water up the other leg, to which a scale showing the pressure is attached. The zero of the scale is the level at which the water stands when the air is calm. It may also be made to register maximum gusts of wind, by filling into
the tube a chemical solution which colours bits Fig. 48. of prepared paper, placed at different levels on
the scale-limb of the instrument. 444. Many observers who have no wind-gauge give the force of the wind by estimation. The scale generally adopted in this country is 0 to 6,40 representing a calm, and 6 a hurricane, or the greatest known force of the wind. Observations by the scale 0 to 6 are converted into pressure in pounds on the square foot by simple squaring. Sailors use the scale of 0 to 12. Observations on this scale are, of course, reduced to the preceding scale by dividing by 2. In Sweden,
* In most European countries the force of the wind is given in kilogrammes on the square metre; and since a kilogramme equals 2. 2046 lb, avoir., and a square metre 10.76 square feet, a pressure of one lb. on the square foot is equivalent to 4.88 kilogrammes on the square metre.