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approaches more nearly the point of saturation, and from and the other gases and substances which are found in the the increased elasticity, the pressure rises to the evening air, will be afterwards adverted to. maximum. As the deposition of dew proceeds, and the fall of Besides these three constituents of air, there is a fourth, temperature and consequent downward movement of the air viz., the vapour of water, from which no air, even at the are arrested, the elasticity is again diminished, and pressure lowest temperatures yet observed, is wholly free, so that falls to the morning minimum. Since the view propounded absolutely dry air does not exist in the free atmosphere. some years ago, that if the elastic force of vapour be sub- The dry air of the atinosphere-oxygen (inclusive of ozone), tracted from the whole pressure, what remains will show nitrogen, and carbonic acid—is always a gas, and its quantity only one daily maximum and minimum, has not been con- is constant from year to year; but the vapour of water firmed by observation, it follows that the above explanation does not always remain in the gaseous state, and the is quite insufficient to account for the phenomena ; indeed, quantity present in the atmosphere is, by the processes of the view can be regarded in no other light than simply as evaporation and condensation, varying every instant. Water a tentative hypothesis.

evaporates at all temperatures, even the lowest, and rises Singularly enough, Lamont and Broun, a few years ago, into the air in the form of an invisible elastic gas called were led, independently of each other, to form an opinion aqueous vapour. The elasticity of vapour varies with the that the daily barometric oscillations were due to the temperature. At 0° Fahr. it is capable of sustaining a presmagneto-electric influence of the sun. It admits of no sure equal to 0·044 inch of the mercurial barometer, as calcudoubt, looking at the facts of the case so far as they have lated from Regnault's experiments; at 32° (freezing), 0.181 been disclosed, that the daily barometric oscillations inch; at 60°, 0.518 inch ; at 80°, 1.023 inch ; and at originate with the sun, and that more than the sun's 100°, 1.918 inch, being nearly 15 the average pressure of intluence as exerted on the diurnal march of the tempera- the atmosphere. ture and humidity of the atmosphere is concerned in In investigating the hygrometry of the atmosphere, the bringing them about. But from the facts adduced, it is chief points to be ascertained are— (1), the temperature of equally certain that, be the originating cause what it may, the air; (2), the dew-point; (3), the elastic force of vapour, its effects are enormously modified by the distribution of or the amount of barometric pressure due to the vapour land and water over the globe, by the wind, and by the present ; (4), the quantity of vapour in, say, a cubic foot of absolute and relative humidity of the atmosphere. The air; (5), the additional vapour required to saturate a cubic smallness of the amount of the summer oscillation from the foot of air; (6), the relative humidity; and (7), the weight forenoon maximum to the afternoon minimum over the of a cubic foot of air at the pressure at the time of obserNorth Atlantic as far south as lat. 30°, and its diminished vation. The vapour of the atmosphere is observed by means amount, as far south at least as the equator, will no doubt of the hygrometer (see HYGROMETER), of which it is only play an important part in the unravelling of this difficulty. necessary here to refer to Regnault's as the most exact, and

One of the most important steps that could be taken Augusts as the most convenient, and, consequently, the would be an extensive series of observations from such one in most general use. August's hygrometer consists of a countries as India, which offers such splendid contrasts of dry and a wet bulb, with which are observed the temperaclimate at all seasons, has a surface covered at one place ture of the air and the temperature of evaporation. Of with the richest vegetation, and at others with vast stretches these two observed data, the formula of reduction, as of sandy deserts, and presents extensive plateaus and deduced from Apjohn's investigations, is as follows -Let sharp ascending peaks—all which conditions are indis- | F be the elastic force of saturated vapour at the dew-point, pensable in collecting the data required for the solution of f the elastic force at the temperature of evaporation, d the this vital problem of atmospheric physics.

difference between the dry and wet bulb, and b the The ancients thought that air was one of the four elements barometric pressure, then from which all things originated, and this doctrine continued

d b to prevail till 1774, when Priestley discovered oxygen gas,

F=fand showed it to be a constituent part of air. Nitrogen, the other constituent of air, first called azote, was discovered when the reading of the wet bulb is above 32°; and soon after, and the marked differences between these two

d 6 gases could not fail to strike the most careless observer.

96 30 It is ’remarkable that Scheele independently discovered both oxygen and nitrogen, and was the first to enunciate when below it. From Regnault's values of the elastic force the opinion that air consists essentially of a mixture of of vapour, f is found, and d and b being observed, F is these two gases.

From experiments made by him to ascer- calculated. From F the dew-point is found. In calculattain their relative volumes he concluded that the propor- ing relative humidity, saturation is usually assumed to be tions are 27 volumes of oxygen and 73 volumes of nitrogen. 100, perfectly dry air 0. The humidity is found by dividIt was left to Cavendish to show from 500 analyses that ing the elastic force at the dew-point by the elastic force che relative proportions were practically constant, and that at the temperature of the air, and multiplying the quotient the proportion is 20-833 per cent. of oxygen. The results by 100. obtained by Cavendish, though not attended to for many The elastic force may be regarded as representing years after they were published, have been shown by recent approximately the absolute quantity of vapour suspended and more refined analyses to be wonderfully exact. The in the air. It may be termed the absolute humidity of most recent analyses of specimens of air collected under the atmosphere. Since the chief disturbing influences at circumstances which ensure that it is of average purity, work in the atmosphere are the forces called into play by give as a mean result the following :

its aqueous vapour, a knowledge of the geographical distri

bution of this constituent through the months of the year Oxygen......

20.96 per cent. is of the utmost possible importance. Hence every effort Nitrogen.....

79.00

ought to be made to place the observation of the hygrometry Carbonic acid

0:04

of the air, and the reduction of the observed data, on a

sounder basis than has yet been done. As regards geogra100.00

phical distribution, the elastic force is greatest within the The circumstances under which these proportions vary, | tropics, and diminishes towards the poles • it is greater over

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F=f

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Volume.

the ocean, and decreases on advancing inland; breater in local irregularities in the distribution of temperature in summer than in winter; and greater at midday than in the the atmosphere. The local expansion of the atmosphere by morning. It diminishes with the height generally; but in heat during the day is greatest over land, where the air is particular cases, different strata are superimposed on each clear, dry, and comparatively calm, and least over the other, differing widely as regards dryness and humidity, ocean, where the sky is clouded, and the air loaded with and the transitions from the one to the other are often moisture. On the other hand, the local contraction by sharp and sudden.

cold during night is greatest over land, where the air is The relative humidity of the air may be regarded as the clear, dry, and calm, or nearly so, and least over the degree of approach to saturation. It is greatest near the ocean, where the air is clouded, and loaded with moisture. surface of the earth during night, when the temperature, As familiar illustrations of atmospheric movements resultbeing at or near the daily minimum, approaches the dew- ing from local expansions by heat and contractions by cold, point; it is also great in the morning, when the sun's rays we may refer to the land and sea breezes, and what dehave evaporated the dew, and the vapour is as yet only pend upon exactly the same principle, the dry and rainy diffused a little way upwards; and it is least during the monsoons in different parts of the globe. But the illustragreatest heat of the day.

tion of the principle on the broadest scale is the system of Between the humidity, both absolute and relative, of the atmospheric circulation known as the equatorial and polar air and the temperature there is a vital and all-important currents of the atmosphere, which originate in the unequal connection. Observation shows that when the quantity of heating by the sun of the equatorial, temperate, and polar vapour in the air is great, and also when the relative regions. humidity is high, temperature falls little during the night, The other principal motive force in atmospheric circulaeven though the sky be perfectly clear; but when the tion depends on the aqueous vapour. The many ways quantity of vapour is small, or the relative humidity is low, in which this element acts as a motive force will be seen temperature rapidly falls. On the other hand, during the when it is considered that a large quantity of sensible day the temperature rises slowly, when the quantity of heat disappears in the process of evaporation, and reappears vapour is great, or relative humidity high, even though the in the process of condensation of the vapour into rain or sky be clear, but when the quantity of vapour is small, and cloud ; that saturated air is specifically lighter than dry humidity low, temperature rapidly rises. These facts are air; and that the olu and relative amount of the explained by the circumstance that perfectly dry air is vapour powerfully influences both solar and terrestrial diathermanous, that is, it allows radiant heat to pass through radiation. The question to be carefully considered here is, it without being sensibly warmed thereby. Add vapour how in these ways the vapour produces local irregularities to this air, and its diathermancy is diminished. The dia- in the distribution of atmospheric pressure, thus giving rise thermancy is also reduced if the temperature approach to aerial movements which set in to restore the equilinearer to the dew-point; in other words, if the relative brium that has thus been disturbed. humidity be increased, Hence, with an increase of It is from these local irregularities—using the word vapour or with increased humidity, the effects of both solar local in a very wide sense—in the distribution of atmosand terrestrial radiation are much less felt on the surface of pheric pressure, whether the irregularities originate in the the earth—the vapour screen performing, in truth, one of the temperature or aqueous vapour, that all winds, from the most important conservative functions of the atmosphere. lightest breeze to the most destructive hurricane, take their

Since ascending currents fall in temperature as they rise; for, as already stated, /wind is merely the flowing ascend, through diminished pressure and consequent dilata- away of the air from where there is a surplus of it to tion, they increase their relative humidity; and since where there is a deficiency. descending currents increase in temperature, and conse- In examining weather charts embracing a considerable quently reduce their relative humidity, it follows that, portion of the earth's surface, such, for instance, as those over a region from which ascending currents rise, solar and published in the Journal of the Scottish Meteorological terrestrial radiation is very considerably obstructed, but Society, vol. ii. p. 198, which include a large part of the over a region upon which currents descend, radiation is northern hemisphere, there are seen two different systems much less obstructed. Most of our exceptionally hot of pressure changing their forms and positions on the globe summer and cold winter weather is to be explained in this from day to day-one set being systems of low pressure way, on which occasions there is generally observed a high marked off by concentric isobarics enclosing pressures barometric pressure overspreading a comparatively limited successively lower as the central space is approached, and region, on which a slow downward movement of the air the other set being systems of high pressure marked off by proceeds.

roughly concentric isobarics bounding pressures succesOf the solar heat which reaches the surface of the sively higher towards their centres. These two systems globe, that part which falls on the land may be regarded are essentially distinct from each other, and without some as wholly absorbed by the thin superficial layer exposed knowledge of them the circulation of the atmosphere canto the heating rays; and since there is no mobility in not be understood. the particles of the land, the heat can be communicated 1. Areas of Low Pressure, or Cyclones. The annexed downwards only by conduction. On the other hand, the woodcut, fig. 1, is a good representation of a cyclone solar heat which falls on water is not, as in the case of which passed over north-western Europe on the morning land, arrested at the surface, but penetrates to a con- of 2d November 1863. The pressure in the central space siderable depth, the heating effect being in the case of clear is 28.9 inches, from which it rises successively, as shown water appreciably felt at a depth of from 500 to 600 feet. by the isobarics, to 29.1, 29.3, 29.5, 29.7, and 29.9 inches. Since the heat daily received by the ocean from the sun is The arrows show the direction and force of the wind, the diffused downwards through a very considerable depth, force rising with the number of feathers on the arrows. the surface of the ocean on which the atmosphere rests is The two chief points to be noted are the following :-(1.) much less heated during the day than is the surface of the The direction of the arrows shows a vorticose motion of land. Similarly it is also less cooled during the night by the air inwards upon the space of lowest pressure, the terrestrial radiation.

motion being contrary to that of the hands of a watch. It This points to a chief acting force on which the great will be observed that the winds blow in conformity with movements of the atmosphere depend, viz., simultaneous what is known as Buys-Ballot's “Law of the Winds,” already

III. 5

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referred to, but which may be otherwise thus put :-Stand overspread the greater part of Europe at that time. Here with your back to the wind, and the lowest barometer, or the highest pressure is in the centre of the system, and, as centre of depression, will be to your left in the northern usually happens, the isobarics are less symmetrical than hemisphere (in the southern hemisphere to the right); this those near the centre of a cyclone. The winds, as rule holds universally.

usual in anticyclones, are light; this, however, is the (2.) The force of the wind

essential point of difference—the winds do not flow is proportional to the

inwards upon the centre, but outwards from the region of barometric gradient, or

high pressure;

and it will be observed that in many cases the quotient of the dis

they cut the isobarics at nearly right angles. Another tance between two places

important point of difference is in the air over the region stated in miles by the

covered by the anticyclone being, particularly in its central difference of pressure

portion, very dry, and either clear or nearly free from stated in inches of

clouds. mercury as observed at

Climatically, the significance of the anticyclone consists the two places. Hence,

in the space covered for the time by it being, on account in the Channel, where

of its dryness and clearness, more fully under the influence the isobarics are close

of both solar and terrestrial radiation ; and consequently together, winds are high,

in winter it is accompanied with great cold, and in summer but in the north of

with great heat. As shown by Buchan, in reviewing the Scotland, where the

weather of north-western Europe for 1868, the intense isobarics are far apart,

heat which prevailed in Great Britain during 2-4th August winds are light. This

of that year was due to the high barometric pressure rule also holds universally, though the Fig. 1.– Weather chart, showing cyclone of the atmosphere, the clearness of the sky, the dryness of

Fig. 1. – Weather chart, showing cyclone. accompanying this anticyclone, the comparative calmness exact relation requires still to be worked out by observa- the air, and the strong insolation which took place under tion. As regards the important climatic elements of tem- these circumstances. perature and moisture, the air in the S.S.E. half of the Thus, then, the tendency of the winds on the surface of cyclone is mild and humid, and much rain falls ; but in the earth is to blow round and in upon the space where the other half it is cold and dry, and little rain falls. A pressures are low and out of the space where pressures are succession of low pressures passing eastward, in a course high. Now, since vast volumes of air are in this way lying to northwards of Great Britain, is the characteristic poured into the space where pressure is low, without of an open winter in Great Britain ; on the other hand, if increasing that pressure, and, on the other hand, vast the cyclones follow a course lying to the southward, the volumes flow out of the space where pressure is high, winters are severe. This is a chief point of climatic impor- without diminishing that pressure, it necessarily follows tance connected with the propagation eastward of these that the air poured in is not allowed to accumulate over cyclonic areas.

this space, but must escape into other regions; and also 2. Areas of High Pressures, or Anticyclones—The accom- that the air which flows out from the anticyclonic region

must have its place supplied by fresh accessions from above. In other words, the central space of the cyclone is occupied by a vast ascending current, which after rising to a con

siderable height flows away as upper currents into sur297%

rounding regions; and the central space of the anticyclone

is filled by a slowly descending current, which is fed from 29.8

upper currents, blowing towards it from neighbouring regions. When the area of observation is made sufficiently wide, cyclones are seen to have one, or sometimes more, anticyclones in proximity to them, the better marked anticyclones having two, and sometimes more, cyclones in their vicinity. In fig. 2, a part of a cyclone in Iceland is seen, and another cyclone in the Crimea accompanied the anticyclone there figured. Hence the cyclone and the anticyclone are properly to be regarded as counterparts, belonging to one and the same great atmospheric disturbance.

From this it follows that observations of the winds cannot be conducted, and the results discussed, on the supposition that the general movement of the winds felt on the earth's surface is horizontal, it being evident that the circulation of the atmosphere is effected largely through systems of ascending and descending currents.

The only satisfactory way of discussing the winds, viewed especially in their climatic relations, is that recently proposed by

Köppen of St Petersburg, and applied by him with very 30-0

fruitful results in investigating the weather of that place during 1872 and 1873. In attempting an explanation of these phenomena, we are met with several as yet insuper

able obstacles (1.) An imperfect knowledge of the mode Fig. 2.- Weather chart, showing anticyclone. panying weather chart, fig. 2, for 2-4th August 1868,

1 Atlas Metéorologique de l'Observatoire Impérial, Année 1868. represents an anticylone or region of high pressure, which D. 39.

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of formation and propagation of low-pressure systumis; (2.) owing to the law of diffusion obtaining among elastic fluids Imperfect knowledge of the relations of the formation of mixed together. While the proportion of these gases

is in cloud and aqueous precipitation to barometric fluctuations; a general sense constant, there are, however, consistent (3.) A want of information with reference to the merely differences in the amounts of oxygen and nitrogen in the mechanical effects of ascending, descending, and horizontal air of unwholesome places, as first shown by Regnault. currents of air on the barometric pressure ; in other words, The following figures, showing the volume per cent. of we do not know how far the barometric pressure is an oxygen, rest on the authority of Dr Angus Smith, who has indication of the mass of air in the column vertically over given much attention to this subject :-Sea-shore of Scotit, when that column is traversed by air-currents ; (4.) | land and Atlantic (lat. 43° 5' N., long. 17° 12' W.), 20.99 ; An almost total absence of really good wind observations; tops of Scottish hills, 20-98; in sitting-room feeling close and (5.) Deficient information in nearly everything that but not excessively so, 20-89; backs of houses and closets

, respects aqueous vapour—its relation to radiant heat, 20:70; under shafts in metalliferous mines, 20:424 ; when both solar and terrestrial ; its mode of diffusion vertically candles go out, 18.50; when it is very difficult to remain in and horizontally in the free atmosphere, especially from an the air many minutes, 17.20. The variations in the amounts evaporating surface; the influence which its condensation of carbonic acid in different situations are great; thus—in into cloud and rain exerts on aerial currents,—in regard to the London parks it is '0301; on the Thames, .0343; where all which more satisfactory methods of observing this vital fields begin, 0369; in London streets in summer, 0380 ; element, and discussing the results of observation, are during fogs in Manchester, '0679; in workshops it rises to greatly to be desired.

There are here large important :3000, and in the worst parts of theatres to :3200; and fields of inquiry awaiting experimental and observational the largest amount, found in Cornwall mines, is 2:5000. physicists.

Great differences have been observed by Dr A. Smith The law of the dilatation of gases, known as the “Law of between country rain and town rain : country rain is Boyle” or “Law of Mariotte,” is this: The volume occupied neutral ; town rain, on the other hand, is acid, and corby a gas is in inverse ratio to the pressure under which it rodes metals and even stones and bricks, destroying mortar exists, if the temperature remains the same; or the density of rapidly, and readily spoiling many colours. Much infor& gas is proportioned to its pressure. Consequently, air mation has been obtained regarding impurities in the air of under a pressure equal to that of two atmospheres will occupy towns and other places by examining the rain collected in only half the volume it occupied under the pressure of one

different places. The air freest from impurities is that atmosphere; under the pressure of three atinospheres, one- collected at the sea-coast and at considerable heights. third of that volume, &c. By doubling the pressure we Again, ammonia is found to diminish, while nitric acid double the elasticity. If, however, the temperature be increases, in ascending to, at least, habitable heights. As increased, and the air occupy the same space, the pressure regards organic matter in the air, it corresponds to a conwill be increased ; but if the pressure is to remain the same, siderable extent with the density of the population. As the air must occupy a larger space. From Regnault's might have been supposed from the higher temperature, experiments, it is concluded that the co-efficient which more nitric acid is contained in rain collected on the denotes increase of elasticity for 1° Fahr. of air whose Continent than in the British Islands. This inquiry, which volume is constant equals .002036; and that the co- is only yet in its infancy, will doubtless continue to be efficient which denotes increase of volume for 1° Fahr. of vigorously prosecuted, particularly since we may hope air whose elasticity is constant equals •002039.

thereby to arrive at the means of authoritatively defining Those portions of the atmosphere in contact with the the safe limits of the density of population, and the extent earth are pressed upon by all the air above them. The air to which manufactures may be carried on within a given at the top of a mountain is pressed upon by all the air The influence of atmospheric impurities on the above it, while all the portion below it, or lying between public health has received a good deal of attention. the top of the mountain and the surface of the sea, exerts The relation of weather to mortality is a very important no pressure whatever upon it. Thus the pressure of the inquiry, and though a good deal has been known regarding atmosphere constantly diminishes with the height. If, the question for some time, yet it has only recently been then, the pressure of the atmosphere at two heights be systematically inquired into by Dr Arthur Mitchell and observed, and if at the same time the mean temperature Mr Buchan, the results of the investigation which deals and humidity of the whole stratum of air lying between with the mortality of London being published in the the two levels were known, the difference in height between Journal of the Scottish Meteorological Society (New Series, the two places could be calculated. For the development Nos. 43 to 46). Considering the weather of the year as of this principle, see BAROMETRIC MEASUREMENTS OF made up of several distinct climates differing from each HEIGHTS.

other according to temperature and moisture and their The air thus diminishing in density as we ascend, if it relations to each other, it may be divided into six distinct consists of ultimate atoms, as is no doubt the case, it follows climates, characterised respectively by cold, cold with drythat the limit of the atmosphere will be reached at the ness, dryness with heat, heat, heat with moisture, and cold height where the force of gravity downwards upon a single with moisture. Each of these six periods has a peculiar particle is equal to the resisting force arising from the influence in increasing or diminishing the mortality, and repulsive force of the particles. It was long supposed, each has its own group of diseases which rise to the maxifrom the results of observations on the refraction of light, mum, or fall to the minimum mortality, or are subject to a that the height of the atmosphere did not exceed 45 miles; rapid increase or a rapid decrease. The mortality from all but from the observations of luminous meteors, whose true causes and at all ages shows a large excess above the character as cosmical bodies was established a few years average from the middle of November to the middle of ago, it is inferred that the height of the atmosphere is at April, from which it falls to the minimum in the end of least 120 miles, and that, in an extremely attenuated form, May; it then slowly rises, and on the third week of July it may even reach 200 miles.

suddenly shoots up almost as high as the winter maximum Though there are considerable differences in the specific of the year, at which it remains till the second week of gravities of the four constituent gases of the atmosphere, August, falling thence as rapidly as it rose to a secondary viz., oxygen, nitrogen, carbonic acid gas, and aqueous minimum in October.

ding the

ummer excess, vapour, there is yet no tendency to separation among them, which is so abrupt in its rise and fall

, it is almost altogether

area.

due to the enormous increase of the mortality among mere | together; the reason being that in Victoria deaths from infants under one year of age; and this increase is due bowel complaints are much greater, and those from diseases not only to deaths at one age, but to deaths from one of the respiratory organs much less than in London. class of diseases, viz., bowel complaints. If the deaths The curves show that the maximum annual morfrom bowel complaints be deducted from the deaths from tality from the different diseases groups around certaiu all causes, there remains an excess of deaths in the cold specific conditions of temperature and moisture combined. months, and a deficiency in the warm months. In other Thus, cold and moist weather is accompanied with a high words, the curve of mortality is regulated by the large death-rate from rheumatism, heart diseases, diphtheria, and number of deaths from diseases of the respiratory organs. measles ; cold weather, with a high death-rate from bronThe curve of mortality for London, if mere infants be chitis, pneumonia, &c.; cold and dry weather, with a high excepted, has thus an inverse relation to the temperature, death-rate from brain diseases, whooping-cough, convulrising as the temperature falls, and falling as the tempera- sions; warm and dry weather, with a high death-rate from ture rises. On the other hand, in Victoria, Australia, suicide and small-pox; hot weather, with a high death-rate where the summers are hotter and the winters milder, the from bowel complaints; and warm moist weather with a curves of mortality and temperature are directly related to high death-rate from scarlet and typhoid fevers. (See each other-mortality and temperature rising and falling CLIMATE.)

(A. B.)

ATMOSPHERIC RAILWAY, a milway in which the / and Kingstown Railway to adopt Clegg & Samuda's scheme pressure of air is used directly or indirectly to propel car- upon an extension of their line from Kingstown to Dalkey, riages, as a substitute for steam. It was devised at a time where it was in operation in 1844. Later on, the same when the principles of propulsion were not so well under- system was adopted on a part of the South Devon line and stood as they are now, and when the dangers and inconveni- in several other places, and during the years 1844-1846 ences attendant on the use of locomotives were very much the English and French patent records show a very large exaggerated. It had been long known that small objects number of more or less practicable and ingenious schemes could be propelled for great distances through tubes by air for the tubes, valves, and driving gear of atmospheric pressure, but a Mr Vallance, of Brighton, seems to have been railways. The atmospheric system was nowhere perthe first to propose the application of this system to passenger manently successful, but in all cases was eventually traffic. He projected (about 1825) an atmospheric railway, superseded by locomotives, the last atmospheric line being consisting of a wooden tube about 6 feet 6 inches in diameter, probably that at St Germains, which was worked until with a carriage running inside it. A diaphragm fitting the 1862. Apart from difficulties in connection with the tube, approximately air-tight, was attached to the carriage, working of the valve, the maintenance of the vacuum, &c., and the air exhausted from the front of it by a stationary other great practical difficulties, which had not been engine, so that the atmospheric pressure behind drove the indicated by the experiments, soon made themselves known carriage forward. Later inventors, commencing with Henry in the working of the lines. Above all, it was found that Pinkus (1835), for the most part kept the carriages stationary engines, whether hauling a rope or exhausting a altogether outside the tube, and connected them by a bar tube, could never work a railway with anything like the with a piston working inside it, this piston being moved economy or the convenience of locomotives, a point which by atmospheric pressure in the way just mentioned. The is now regarded as settled by engineers, but which was not tube was generally provided with a slot upon its upper so thoroughly understood thirty years ago. Lately, the side, closed by a continuous valve or its equivalent, and principle of the atmospheric railway has been applied on a arrangements were made by which this valve should be very large scale in London and elsewhere, under the name opened to allow the passage of the driving bar without of " PNEUMATIC DESPATCH(q.v.), to the transmission of permitting great leakage of air. About 1840, Messrs small parcels in connection with postal and telegraph work, Clegg & Samuda made various experiments with an for which purpose it has proved admirably adapted. (See atmospheric tube constructed on this principle upon a paper by Prof. Sternberg of Carlsruhe in Hensinger von portion of the West London Railway, near Wormwood Waldegg's Handbuch für specielle Eisenbahntechnik, vol. i. Scrubs. The apparent success of these induced the Dublin | pt. 2, cap. xvii.

ATOM

A

The atomic theory is a theory of the constitution of body has parts, and that whatever has parts may be bodies, which asserts that they are made up of atoms. The divided. opposite theory is that of the homogeneity and continuity In ancient times Democritus was the founder of the of bodies, and asserts, at least in the case of bodies having atomic theory, while Anaxagoras propounded that of conno apparent organisation, such, for instance, as water, that tinuity, under the name of the doctrine of homæomeria as we can divide a drop of water into two parts which are (Ouolouépia), or of the similarity of the parts of a body to each of them drops of water, so we have reason to believe the whole. The arguments of the atomists, and their that these smaller drops can be divided again, and the replies to the objections of Anaxagoras, are to be found in theory goes on to assert that there is nothing in the nature Lucretius. of things to hinder this process of division from being In modern times the study of nature has brought to repeated over and over again, times without end. This is light many properties of bodies which appear to depend on the doctrine of the infinite divisibility of bodies, and it is the magnitude and motions of their ultimate constituents, in direct contradiction with the theory of atoms.

and the question of the existence of atoms has once more The atomists assert that after a certain number of such become conspicuous among scientific inquiries. divisions the parts would be no longer divisible, because We shall begin by stating the opposing doctrines of atoms each of them would be an atom. The advocates of the land of continuity before giving an outline of the state of

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