Uranus D 33 ... M 2,698'8 Venus

D 74... M

25.7

JANUARY 7. TWELFTH DAY was 12 days after Christmas, and is sometimes called Old Christmas Day. Many social rites have long been connected with Twelfth Day.

FEBRUARY 2. CANDLEMAS: Festival of the Purification of the Virgin. Consecration of the lighted candles to be used in the church during the year.

FEBRUARY 14. ST. VALENTINE'S DAY. The custom of sending a valentine to one's sweetheart was very general 50 years ago, but, since the introduction of Christmas Cards, has almost died out. February 15, Old Candlemas.

MARCH 25. LADY DAY: Annunciation of the Blessed Virgin Mary. April 7 is old Lady Day.

JUNE 24. MIDSUMMER DAY. Feast of the Nativity of St. John the Baptist. July 7 is old Midsummer Day.

AUGUST 1. LAMMAS DAY: Originally in England the festival of the wheat harvest. One

Stratus.

bodies following a zigzag path owing to varying conditions in the air through which it passes. This is highly dangerous. 2. Globular Lightning, in which the electric fluid seems to be condensed: the appearance is that of a luminous ball or globe of fire; it remains visible for several seconds, travels slowly with a strange hissing sound, and finally explodes with great violence. This is also dangerous. 3. Sheet Lightning, which covers a large surface of cloud and is, in fact, merely the reflection of distant unseen electrical discharges, or discharges of a feeble character. 4. The fourth kind of lightning is the luminous flash known as St. Elmo's fire. This is more common at sea than on land; it is quite harmless and shows itself during a thunderstorm at the extremities of pointed objects, such as yard-arms or mast-heads. Lightning travels, according to Wheatstone, about 290,000 miles a second-about half as quick again as light.

THUNDERSTORMS.-A thunderstorm is consequent on an electrical discharge in the clouds, and this discharge shows itself by a flash of lightning and a peal of thunder. When clouds become highly charged with atmospheric electricity a discharge of the electric force takes place, and any human being or animal which may happen to be in the path of the discharge receives a violent shock, often sufficient to cause death. The flash of this discharge is known to us as lightning. M. Arago has described four distinct kinds of lightning, two being dangerous and two harmless. 1. Zigzag, or Forked Lightning: a discharge between two oppositely electrified

RAIN.-Rain is the most important mode by which the moisture abstracted from the earth by evaporation is again returned to it in a liquid state.

Rain originates in the clouds, 'whence it is precipitated, by a sudden cooling of the air, in the form of minute globules, which, gathering in size as they fall, reach the earth in drops. The atmosphere which surrounds the earth is capable of holding only a limited amount of moisture in suspension; this varies according to its temperature, and a frequent cause of rain is the sudden mixing of hot and cold air, as, for instance, in the tropics, where currents of hot air ascend into colder regions and produce the great tropical summer rains, or when hot damp air is brought

into sudden contact with the slopes of a mountain or the cold earth, as on the west coast of continents in the temperate zone, which are more mountainous than the east coast.

Rainfall is measured by a gauge, the values given representing the depth of water which would have accumulated on a level piece of ground, if none of the rain which fell could escape. by drainage or evaporation. But although measurement in this manner seems an easy matter, considerable care is required to ensure accuracy. The size of the gauge is important, so, too, is the manner of exposure, both in regard to height from the ground and distance from walls and trees. Precautions must also be taken to prevent loss by evaporation or leakage, and other points materially affect results. The annual rainfall at sea level in the British Isles ranges from 20 inches on the east coast of England to 60 or 80 inches on the west coasts of Scotland and Ireland.

small aqueous particles produced by the con

densation of vapour descend from the clouds through atmosphere which has be

lower temperature, or with a cooled portion of the earth's surface such as a mountain side. Fog is very similar to mist, but the particles of moisture are somewhat smaller, and these often become coated or laden with unconsumed carbon or other suspended matter, and hence the brown or yellow appearance, and the disagreeable effects of a fog on the eyes and throat. Mists are most frequent in autumn after the earth has been heated during the summer, the air cooling faster than the earth; they are most dense after very hot summers. They also show that the air has become suddenly cooler, and frequently are a sign of snow.

come cooled down below the freezing point, they become frozen into tiny crystals; these gradually come into juxtaposition and form light feathery flakes which we term snow. The beauty of these snow crystals has often been noted. They have a form derived from the hexagonal figure, and it is said that there are over a thousand known varieties of them. A few are illustrated above.

Hail is much denser than snow, and is, in fact, actual ice. True, there is a soft hail which falls chiefly in winter, when the air is dry, in the form of soft round pellets, but real hail is, as already stated, actualice. Hailstones are usually of a conical shape, and from an eighth to one inch in diameter.

DEW AND HOAR FROST.-The phenomenon of dew is consequent on the sunwarmed moisture-laden air being so rapidly cooled by contact with the cold earth that it cannot retain the same amount of moisture, and this is accordingly deposited on the surface of the earth in the form of very tiny liquid drops. Dew is seldom formed on cloudy nights, because the heat that has been communicated to the

earth during the day by the sun, instead of being radiated into space as it is on cloudless nights, is reflected back by the clouds and so retained. When the temperature of the earth falls below freezing point the dew-drops are congealed into ice, and the phenomenon of hoar frost is produced. The feathery forms of this are very beautiful, and should be carefully examined when opportunity offers. Subjoined are some of the ordinary forms.

THE ATMOSPHERE.-The gaseous air we breathe, and by which the air is surrounded, is composed chiefly of nitrogen and oxygen, with a small amount of aqueous vapour, carbonic

In the tropics this size is very much increased, and hailstones have fallen there measuring 3 or 4 inches in diameter, and weighing 10 or 12

ounces.

MISTS AND FOGS.-Mist is a heavy accumulation of watery vapour hanging over the surface of land or water, and rendered visible by a reduction in the température, either through coming in contact with a stratum of air of a

acid and ammonia. The proportions in 1000 volumes of air

are: oxygen 206'1, nitrogen 779'5, aqueous vapour, etc., 14 4. Air possesses weight; this, near the surface of the globe, is equal to about 15 lbs. to the square inch. The actual pressure is measured by a barometer, in which the weight of a column of air is indicated by the height of a column of mercury of equal sectional area. The temperature of the atmosphere is determined by a thermometer, whilst the amount of moisture in the air is measured by a hygrometer, or by comparing the readings of a wet bulb and a dry bulb thermometer.

THE BAROMETER.-Atmospherical Pressure is,as stated before, measured by the Barometer, which should be a mercurial one. The readings of the barometer are 'corrected" for instruinental errors and reduced to 32° and to sea-level. These are necessary, since the mercury expands when the temperature rises and the column is lengthened; and two similar, barometers cannot read alike unless at the same temperature. In the same way, since the height of the column of mercury measures the weight of the column of air of equal sectional area above it, if two similar barometers be placed one directly above the other it is evident that there will be a less quantity of air above the former than above the latter instrument, and it will read lower.

The following are conditions for a high barometer: (1) When the air is very cold, for then the lower strata are denser and more contracted than when it is warm. The contraction causes the upper layers to sink down, bringing a greater number of air particles, that is to say, a greater mass of air into a given vertical column of the atmosphere supposed unable to expand laterally, so that the pressure at its base is greater. (2) When the air is dry, for then it

The greatest depression of the barometer occurs daily about 4 a.m. and p.m., and its highest elevation about to a.m. and p.m.; in summer, these extreme points are reached from one to two hours earlier in the morning, and as much latef in the afternoon.

To convert English Barometrical readings into French (inches into millimètres) or vice versa the following table will be useful.

Mil. In. Mil. In. In. Mil. 1039

750=2539

0.3

7 6

2= '079

755-29.73

0.4

10 1

720-28 35 785-30.91 725 28 54 790 31 10

THE THERMOMETER.The determination of the temperature of the air is open to much uncertainty owing to the great difficulty of securing an unexceptionable exposure for the thermometers. An ordinary thermometer consists of a fine glass tube with a bulb (cylindrical or spherical) blown on one end, and partly filled with some liquid, mercury or spirits of wine, usually the former. This liquid

is denser than when it is moist. (3) When in any way an upper current sets in towards a given area, for this compresses the strata underneath.

Conversely, the barometer stands low when (1) the lower strata are heated, causing the surfaces of equal pressure to rise, and the upper layers to slide off as already described, for by this means the mass of air pressing on each unit of area below is reduced. (2) When the air is damp, for as the density of aqueous vapour, at the temperature of 60° and pressure of 30 ins., iso 622, air being 1, the mixture is lighter the more vapour it contains, and consequently damp air does not press so heavily as dry on the unit of area below. When the air from any causes has an upward movement, for this of course acts in the same manner as (1). From these principles it follows that a fall in the barometric reading usually betokens rain and wind, or an increase in both; but a rise the reverse. The direction of the wind, and the temperature, must be noted, however, for bad weather may accompany a rise, if the wind be from a cold quarter, the north-east winds tending to raise the barometer the most.

ters

are

expands on being heated, and contracts again on being cooled. By the amount of the expan

sion, the temperature is ineasured by means of a scale marked off on the tube. Selfregistering ther mom einstruments

furnished with some contrivances to mark the highest or the lowest

REAUMUR

CENTIGRADE

FAHRENHEIT

into Reauniur, deduct 32, multiply by 4, and divide by 9. To convert degrees Centigrade into Fahrenheit, multiply by 9, divide by 5, nd add 32. To convert Reaumur into Fahrenheit, multiply by 2, divide by 4, and add 32. The diagram on previous page shows corresponding degrees.

WINDS AND STORMS.

The motion of the air, both in direction and velocity, is regulated by the distribution of atmospherical pressure at the surface of the earth, which is shown by the distribution of the readings of the barometer in the weather chart.

66

The force of the wind, as distinguished from its direction, is related to the amount of difference of barometrica! pressure over a given distance, and this is defined as the gradient." Where the lines of equal barometric pressure (the "isobars") are close together, we have a steep gradient, and may hence expect strong winds to restore the atmospheric equilibrium. The force of the wind therefore does not depend on the absolute height of the barometer at any given station, but on its height as compared with that for the surrounding districts.

Storms were formerly divided into two great classes, circular storms (hurricanes and typhoons) and straight line storms. The former are almost the only class of storms which occur within the tropics, and are known under the general name of cyclones. Straight line

Anemometer.

storms were

formerly supposed to be the usual type in temperate latit:des, inasmuch as the wind will blow hard for many days together

from the same point. Later observations have shown, however, that these storms are almost without exception cyclonic in their nature, although they are not so characteristically developed as in the tropics.

CYCLONES.-There are two great classes of atmospheric systems, anti-cyclonic and cyclonic. Anti-cyclonic systems are characterised by very slow circulation of the air (light winds), by low temperature in winter, great "absolute" dryness of the air, at least at their centres, and consequent absence of rain, though fog may be very prevalent.

Cyclonic systems on the other hand are characterised by rapid circulation of the air causing strong winds-which appear to flow towards the centre, so that the air is supplied from below and ascends in the centre-a comparatively high temperature, much moisture, and therefore heavy rain. At the rear of the disturbance it may be very dry.

These are the winter conditions. In summer they are exactly reversed, at least in temperature.

One great distinction between cyclones and anti-cyclones is that the former move over more or less large areas, whilst the latter are usually stationary. The actual force of the wind is

The Cone point downwards means that gales or strong winds are to be expected, at first from the Southward, viz., from S.E. round by S. to N.W. The South Cone is hoisted if it appears probable that a gale will begin from between E. and S.E., and also that it is likely to veer towards S. or S.W.

The Cone point upwards means that Northerly gales or strong winds are probable, viz., from N.W. round by N. to S.E. If it is probable that a gale will begin from between W. and N.W., and also that it is likely to veer towards N. or N.E., the North Cone is hoistea.

The Signal is kept hoisted until dusk, and then lowered; hoisted again the following morning at daylight, and so on for 48 hours from the time at which the message was issued from the Meteorological Office, unless otherwise ordered. At dusk, when a Signal ought to be flying, the Night Signal should be hoisted in place of the Cone-point downwards for South Cone, point upwards for North Cone.

The object of these warnings is only to make known the greater and more general disturbances of the atmosphere, and the hoisting of the Signals is a sign that an atmospherical disturbance is in existence, which will probably cause a gale, frem the quarter indicated by the Signal used, within a distance of about 50 miles of the place whe; e the Signal is hoisted.

A Southerly wind is more likely to veer rapidly to a point North of West than a Northerly wind is to veer to a point South of East; a gale from the Eastward is more likely to back to the Northward than to veer to the Southward.

VELOCITY OF WIND.