37 



BARO'METER. 



BARO'METER. 



938 



The observers mounted to more than 13,000, 18,000, 19,000, and in 

 the last ascent to 22,370 feet, by which ascent the temperature fell 

 from 49 to nearly 10, and the dew-point fell from 37 to 12". The 

 temperature of the air decreased uniformly on ascending, but not con- 

 tinuously. At a certain height, varying on the different days, the 

 decrease was arrested, and for 2000 or 3000 feet, the temperature 

 decreased but little, or even increased, in ascending. Above this, the 

 diminution again went on at nearly the same rate as in the lower 

 region. This interruption was also accompanied by a sudden fall in 

 the dew-point, or by an actual condensation of vapour. This region is 

 the reijion of the clouds, and the increase of heat appears to arise from 

 the latent heat liberated when aqueous vapour is formed into clouds. 

 Dr. Miller, of King's College, analysed the samples of air collected at 

 the various heights, and found scarcely any sensible variation in its 

 composition with regard to the nitrogen and oxygen, but the samples 

 were too small to allow of a quantitative examination for carbonic 

 acid. 



The folluteiny rules are given by Mr. J. H. Belville, of the Royal 

 Observatory, Greenwich, in the third edition of his excellent little 

 ' Manual of the Barometer," London, 1858. He calls them " Phjeno- 

 mena of the Barometer." 



Strong winds in the winter from the west, with a steady high 

 pressure, invariably bring a high temperature and very little rain ; 

 with winds from the east, a low temperature and sharp frosts. 



If the mercury fall during a high wind from the south-west, south- 

 south-west, or west-south-west, an increasing storm is probable ; if the 

 fall be rapid, the windjwill be violent, but of short duration ; if the fall 

 be slow, the wind will be less violent, but of longer continuance ; the 

 disturbing cause is probably the same in each case, but its intensity 

 unequal ; nearly all our high winds from the south-west come with a 

 falling barometer. 



If the depression of the mercury be sudden and considerable with 

 the wind due west, a violent storm may be expected from the north- 

 west or north, during which the mercury will rise to its former height. 

 If the mercury fall with the wind at north-west or north, a great 

 reduction of temperature will follow ; in the winter severe frosts, in 

 the summer cold rains. 



A steady and considerable fall of the mercury during an east wind 

 denotes that the wind will soon go round to the south, unless a heavy 

 fall of rain or snow immediately follow ; in this case the upper clouds 

 usually come up from the south. The deep snow of the severe winter 

 of 1814 was a notable instance. 



The lowest depressions occur with the wind at south and south-east, 

 when much rain falls, and frequently, short and severe gales blow from 

 these points. In the winter months, sudden depressions of the mer- 

 cury with the wind in these quarters are attended with electrical 

 phenomena. 



A fall of the mercury with a south wind is invariably followed by 

 rain in greater or less quantities. 



A falling barometer with the wind at north, brings the worst 

 weather ; in the summer, rain and storm will follow ; in the winter 

 and spring, deep snows and severe frosts. This case is of rare occur- 

 rence. 



A great depression of the mercury during a frosty period brings on 

 a thaw ; if the wind be south or south-east, the thaw will continue ; if 

 the wind be south-west, the frost will be likely to return with a rising 

 barometer and northerly wind. 



In the winter season, a rapid rise of the mercury immediately after a 

 gale from the south-west with rain (the wind going round to north- 

 west or north) is usually attended with clear sky and sharp white 

 fronts. 



Great depressions in the summer months are attended with storms 

 of wind and rain with thunder and hail ; cold unseasonable weather 

 generally succeeds these depressions. 



During a period of broken cold weather in the whiter months with 

 the wind at north or north-north-west, a sudden rise of the mercury 

 denotes the approach of rain and a southerly wind ; thaws also com- 

 monly set in during the night. 



During a steady frost with the wind at north, north-east, or east, 

 a continued slow rising of the mercury indicates snow and cloudy 

 weather. 



If the mercury rise with the wind at south-west, south, or even 

 south-east, the temperature is generally high. 



Observation does not show that extremes of temperature are con- 

 temporaneous with 'the greatest elevations and least depression of the 

 mercurial column. 



Meteors are not prevalent during very low pressures ; the aurora 

 Iwealii has been noticed at all heights of the barometer. Small flashes 

 of lightning are of frequent occurrence during stormy weather in the 

 winter season when the mercury stands low. 



Great elevations in the summer are generally attended with dry 

 warm weather. 



Great depressions at all seasons are followed by change of wind and 

 by much rain. 



A 1-ining barometer with a southerly wind is usually followed by fine 

 weather. In the summer it is dry and warm ; in the winter, dry with 

 moderate frosts. This is of rare occurrence. 



When the mercury is very unsteady during calm, rainy weather, 



it denotes that the air is in an electrical state, and that thunder will 

 follow. 



In the summer months, if a depression of two- or three-tenths of 

 the mercury occur in a hot period, it is attended with rain and thunder, 

 and succeeded by a cool atmosphere. Sometimes heavy thunderstorms 

 take place overhead without any fall of the mercury; in this case 

 reduction of temperature does not usually follow. 



Rain in some quantity may fall with a high pressure, provided the 

 wind be in any of the northerly points ; and when much rain falls 

 with a steady rising barometer and the mercury attains a great 

 elevation, a long period of fine weather usually succeeds. 



If, after a storm of wind and rain, the mercury remain steady at the 

 point to which it had fallen, serene weather may follow without a 

 change of wind ; but on the rising of the mercury, rain and a change 

 of wind may be expected. During a series of stormy weather the 

 mercury is in constant agitation, falling and rising twice or thrice in 

 the space of twenty-four hours, the wind changing alternately from 

 south to west, and backing again to the south; this alternation of 

 winds continues until the mercury rises to a bold elevation, when it 

 ceases and the weather becomes settled. 



Storms of wind, especially when accompanied with much rain, 

 produce the greatest depressions of the mercury. No storm of wind 

 on record has blown without some rain falling, although the time of 

 its falling and its amount have been variable ; sometimes the rain has 

 increased with the increasing storm and sinking mercury ; at other 

 times the rain has fallen suddenly at the close of the storm, or at the 

 time of the minimum pressure. 



No great storm ever sets in with a steady rising barometer. 



As far as regards the locality of Greenwich, the most violent gusts 

 of wind come from due south, and those next in violence from due 

 north ; in both instances, the mercury is stationary at its minimum 

 point during the greatest horizontal pressure ; the winds from these 

 quarters are of short duration, and limited in their extent. The 

 ordinary south-west gales will blow unremittingly for twenty-four 

 hours, and will sweep over the whole of the British Isles. 



Note. Although a rising mercury attends a northerly wind, great 

 depressions occur previously to a great storm coming from that 

 quarter. 



In England, the winds which blow for the greatest number of days 

 together without intermission, are the west and west-south-west : they 

 blow chiefly during the winter months, and are the principal cause of 

 our mild winters. 



The east, and east-north-east, are the winds the next most prevalent. 

 The great antagonist winds, the north and south, are the origin of 

 our most violent storms. 



The westerly winds surge mostly by night, and their average force 

 is twice that of the easterly winds. 



The easterly winds are generally calni at night, but blow with some 

 power during the day. 



On an average, sunrise and sunset are the periods of the twenty-four 

 hours in which there is the least wind. An hour or two after noon is 

 the period when the wind is the highest. 



As a general rule, when the wind turns against the sun, or retrogrades 

 from west to south, it is attended with a falling mercury ; when it 

 goes in the same direction as the sun, or turns direct from west 

 to north, the mercury rises, and there is a probability of fine 

 weather. 



It never hails in calm weather. When hail .falls, it is during 

 sudden gusts of wind, and the mercury rises while the hail is actually 

 falling.- 



If the weather during harvest time has been generally fine, and a 

 fall of the mercury with a shower occur if the wind turn a few points 

 to the north, and the barometer rises above 30 inches, the weather 

 may be expected to be fair for some days. 



The finest and most beneficial state of the atmosphere, more espe- 

 cially as regards the health of man, is with a uniform pressure at a 

 mean height of the climate varying from 29'SO to 30'00. 



When there is only one current of air subsisting in the atmosphere, 

 there is seldom much variation in the height of the mercurial column. 

 It is when two or more strata of the air are in motion in different 

 directions at the same time, that great fluctuations of the mercury 

 occur. 



In high pressures, the upper current usually sets from the north- 

 ward ; in low pressures it sets from the south and south-west. 



The variations of the barometer are always greater in the winter 

 than in the summer. 



In accounting for the different cm-rents of the atmosphere, it must 

 be remarked that the great heat of the torrid zone causes a constant 

 ascent of air over it, which passes northward and southward ; while an 

 under current of cold air flows from the poles to supply its place ; 

 the diurnal rotation of the earth, combined with these currents, causes 

 the trade-winds, whose direction is from east to west : these currents 

 would from the same causes become in the north temperate zone 

 south-east and north-west winds ; but the great irregularities of the 

 temperature from the seasons, the large tracts of ocean, and the dif- 

 ferent geographical formations of the land, subject them to inter- 

 ruptions, and give to every country its prevailing winds, derived 

 from local causes. In England the south-south-west, south-west, and 



