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readily impart moisture, by its condensation, to the surfaces of surrounding bodies; and to produce this effect, it is only necessary tha. those bodies should be a few degrees colder than the air with which vapour is united.

A simple experiment with a glass of cold water, illustrates this principle, on whose operation depends the formation of dew.

At night, when the great source of heat has withdrawn his energies, the earth rapidly cools, and a portion of the vapour in the air which lies next above its surface is deposited thereon. Hence the origin of dew, whose importance is better understood in regions where rain falls only three or four times in a year, than it is in our variable climate.

A reference to that well-known property of heat, termed radiation, is sufficient to indicate the means by which, in almost an incredibly short time after the sun has set, the earth acquires a temperature lower than that of the contiguous stratum of air. To what height this radiant heat ascends in the atmosphere, is, of course, dependent on a variety of circumstances, of which our knowledge is exceedingly limited. That the air at a considerable distance above the earth is much colder than in its immediate vicinity, is attested by the observations of those who have ascended in balloons, and traversed lofty mountains, the summits of which, even in tropical climates and beneath cloudless skies, are covered perpetually with snow.

The conditions which are known to favour radiation in bodies, generally exercise precisely the same influence in the formation of dew. For instance, if a plate of polished metal and a piece of dark-coloured cloth of equal sizes are exposed to the atmosphere, under precisely similar circumstances, on a clear night, the metal will maintain the temperature of the air next above it, but the cloth will be many degrees colder; and, whilst the former will remain perfectly dry, the latter will imbibe a profusion of moisture. At the surface of a smooth gravel-walk there will be scarcely ariy change of temperature during a whole night. On fresh-turned mould, or on grass, a thermometer will show a difference in the former case of 10°, and in the latter of 15°. Hence, it follows, that the most copious deposit of dew will be upon

the

grass. That heat passes by radiation from the earth towards a cold stratum of air, at a great elevation, is further illustrated by the effects often produced through the intervention of clouds on a previously clear night, when the deposition of dew will immediately cease, even in the most favourable situations; and not only so, but as will sometimes happen, that which had been deposited will speedily disappear, indicating that the temperature of the earth and of the air in contact with it, are very nearly equal. Whatever, therefore, prevents the escape of heat from the earth retards the deposition of dew; whatever accelerates the cooling process, promotes, in an equal degree, the condensation of aqueous vapour.

A great proportion of the moisture exhaled during the day is evidently designed to refresh vegetation at night. Who can fail to admire the arrangements by which the distribution of this moisture is effected? Vegetable substances, and especially blades of grass, radiate heat more rapidly than other bodies usually found at the surface of the earth-on

vegetables, dew is deposited in the greatest profusion. The fresh-turned mould, into which has been cast the seed, on whose speedy germination the hopes of a future harvest depend, receives also its allotted proportion. The gravelled road—the paved street—the barren mountainthe uncultivated plain—have no need of dew. From them it is, comparatively, withheld.

When air, which is heavily charged with vapour, has its temperature reduced below the point of saturation, either by admixture with air colder than itself, or by an enlargement of volume in its ascent to a greater altitude, a portion of the vapour present in it will be partially condensed. If this occurs to any great extent near the surface of the earth, the partially-condensed vapour, which is believed to consist of inconceivably thin vesicles of water, each separate vesicle enclosing a portion of air, constitutes a mist or fog. When this partial condensation takes place, as it most commonly does, at considerable elevations in the atmosphere, the accumulated masses of vapour are denominated clouds. The vapour of which clouds are composed, is perfectly condensed by a further reduction of temperature, and the water resulting from it unites in drops, which descend in the form of rain. If the condensation proceeds gradually, the rain will fall in a gentle shower; if it be sudden, as when lightning and thunder prevail, it constitutes a storm*.

Snow consists of vesicles of water, as mentioned above in reference to clouds, frozen in the upper regions of the atmosphere. Hail is perfectlycondensed vapour, like rain, formed into drops and frozen during its descent. Hoar-frost bears the same relation to dew that snow does to rain—the first being partially-condensed vapour, frozen at the surface of the earth; the latter, vapour in a similar state, frozen previous to its descent.

When two or more currents of air, moving in opposite directions, meet, if each current is heavily charged with vapour, it not unfrequently happens that the sky becomes suddenly obscured by dense clouds. This oftener occurs in summer than at any other season, and as clouds formed under such circumstances but rarely yield rain, they prove a source of anxiety, or disappointment, to the husbandman. On some occasions it may be remarked, when two opposing currents of air meet, one of which is warm and charged with vapour, the other cold and comparatively dry, the clouds floating in the warmer current will, in a few minutes, change their appearance, and, finally, vanish away. This is an instance of par

, tially-condensed vapour accommodating itself to a change of circumstances, and uniting with air at a reduced temperature. It sometimes happens, when a shower of rain is falling, that a current of cold dry air will suddenly infringe upon the rain-clouds. In that case rain is changed into hail, an occurrence not uncommon on a warm day in Summer.

By ordinary observers, clouds are viewed only as confused masses of vapour, driven hither and thither at the mercy of the winds. We are assured, however, by those who have had opportunities of closely examining them, that their structure is singularly beautiful; the vesicles

* See Magazine of Popular Science, vol. i., pp. 23—26, for some interesting particulars respecting Rain.

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of which they are composed being arranged with as much regularity as the fibres of the most delicately-formed flower.

As, in evaporation, heat performs an important office, so in the diffusion of vapour through the atmosphere, its accumulation in the form of clouds, its dispersion, reunion, and final precipitation upon the earth, the agency of electricity is no less conspicuous. Our knowledge of the conditions which determine the developement of electricity is necessarily imperfect. A few of these conditions are tolerably distinct; bụt others are veiled in uncertainty, simply because our post of observation is remote from the scene of their operations.

The variable distances at which clouds float in the air, obviously implies a peculiarity of texture adapted to their respective situations and circumstances. Hence they admit of a classification which enables those who are skilled in this branch of Meteorology to distinguish by their form, colour, and relative distances, the several varieties, and to predict with considerable accuracy what kind of weather is likely to prevail.

THE BRITISH ASSOCIATION.

The Sixth Meeting of the Association had been last year fixed by the General Committee to be held at Bristol. A variety of considerations influenced the selection of this site from a number of others, whose claims were put forward. The Association had met once in the north of England (at York), twice in the midland counties (at Oxford and Cambridge); it had travelled into Scotland (to Edinburgh); and to hold its last meeting it had gone to Ireland. Thus might it be said to have perambulated every part of the United Kingdom, except the southern part of it: and thus it was that the claims of Bristol, the capital of the south, to the distinction-often and warmly preferred-were, after more of opposition, canvassing, and party feeling, than might have been expected, preferred to those of Liverpool, Manchester, Birmingham, and Newcastle, all of which great cities had, we believe, their deputations present.

The place of meeting had, moreover, advantages peculiar to itself. The north of Somerset is a county of great geological interest, it connects itself by an interesting chain of geological gradations with the muchdebated carbonaceous district of Devonshire, and its blue Lias quarries are in themselves a fossil museum. And, moreover, as lovers of science are not unfrequently idle men (with respect be it spoken), and lovers of pleasure, to their investigations were offered the magnificent scenery of the northern coast of Devon-Porlock, Linton, Ilfracombe, and Clovelly -names treasured amongst the tenderest recollections of travellers in search of the picturesque. Who, too, had not heard of that delightful excursion of a day, from Wells to Weston, if he had never been fortunate enough to make it-a day's journey which includes within it a visit to the remarkable cavern of Wokey, the cliffs of Cheddar, the bone-cave, more correctly the hyæna-den, of Banwell, and the bay of Weston. Again, northward, there were the Severn and the Wye, offering themselves to the use of the Association, for an excursion to Chepstow and to Tintern;

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and, last, but not least of objects of interest, was Clifton itself, with its vast limestone cliffs, or rather quarries, crowned with terrace upon terrace, and crescent upon crescent, of Bath-stone palaces.

But, to the actual business of the Association. The following is a list of the General Officers, as advertised for the occasion :Trustees (Permanent)–C. Babbage, Esq.,-F.R.S.; R. J. Murchison, F.R.S. ;

John Taylor, Esq., F. R. S. President,—The Most Noble the Marquis of Lansdowne. Vice-Presidents,- The Most Noble the Marquis of Northampton, F.R.S.;

Rev. W. D. Conybeare, F.R.S. ; James C. Prichard, M.D., F.R.S. General Secretaries,—Francis Baily, F.R.S. ; Rev. William V. Harcourt, F.R.S. Assistant General Secretary,–Professor Phillips, F.R.S.

Treasurer,—John Taylor, F.R.S. LOCAL OFFICERS. — Treasurer, George Bengough, Esq. Secretaries, C. Daubeny, M.D., F.R.S.; V. F. Hovenden, Esq. At each of its meetings, the first step is the assembling of its General Committee, composed of those of its members who have contributed papers to the published transactions of philosophical societies, or who have been appointed deputations from such societies; and this committee met at Bristol, at twelve o'clock on Saturday the 20th, in the Chapter-House of the Cathedral. Among the more distinguished of the members present, we observed Professor Sedgwick, Professor Babbage, Sir David Brewster, Mr. Baily, Mr. Whewell, Sir W. Hamilton, the Provost of Trinity College, Dublin, (Dr. Lloyd,) Dr. Dalton, Professor Moseley, Professor Lloyd, Dr. Daubeny, Dr. Lardner, and Mr. Vernon Harcourt. The chair was taken by Mr. Whewell, the minutes of the proceedings of the committee at its last meeting were read, and the arrangements for its present meeting detailed by the honorary secretary, Mr. Vernon Harcourt, and the assistant secretary, Professor Phillips,-the latter referred principally to the officers recommended to be appointed in the different sections,-their presidents, vice-presidents, secretaries, and committees: they will be found detailed below. SECTION A.-MATHEMATICAL AND Secretaries-Dr. Apjohn. Dr. C. Henry, PHYSICAL SCIENCE.

W. Herapath, Esq.
President-Rev. W. Whewell.

Committee Vice Pr idents—Sir D. Brewster, Sir Dr. Barker Dr. R. D. Thomson es W. R. Hamilton.

Professor Daubeny Dr. Turner

C.T. Coathupe, Esq. Dr. T. Thompson Secretaries-Professor Forbes, W. S.

Rev. Wm. Vernon T. Thomson, Jun., Harris, Esq., F, W. Jerrard, Esq.

Harcourt

Esq.
Committee

Professor Hare H. H. Watson, Esq. C. Babbage, Esq., Rev. Dr. Lloyd, Pro- Professor Johnston William West F.R.S.

vost of Trin, Coll. G. Lowe, F.R.S. Rev. W. Whewell F. Baily, Esq. Professor Moll Professor Miller Dr. Yellowley Professor J. Challis Rev. G. Peacock R. Phillips. Esq. Colonel Yorke. Mr. Chatfield Professor Rigaud Dr. Roget Profess. Mc Cullagh Professor Ritchie R. W. Fox, Esq. J. Robinson, Esq.

SECTION C. GEOLOGY AND GEOWm. Frend, Esq. Professor Stevelly

GRAPHY.
G. Gerrard. Esq. H. F. Talbot, Esq.

President-Rev. Dr. Buckland.
Professor Lloyd Profes. Wheatstone
J. W. Lubbock, Esq.

Vice Presidents—R. Griffith Esq., G. R.

Greenough, Esq. SECTION B.-CHEMISTRY AND

(For Geography) R. I. Murchison, Esq. MINERALOGY.

Secretaries—W. Sanders, Esq., S. President-Rev. Professor Cumming. Stutchbury, Esq., T. J. Torrie, Esq. Vice Presidents-Dr. Dalton, Dr. Henry. (For Geography) F. Harrison Rankin, Esq.

Committee

Bracey Clarke, Esq. Dr. James Johnson
H. T. De la Beche, Rev. T. T. Lewis E. Cock, Esq. R. Keate, Esq.
Esq.

J. Macadam, Esq. J. W. Cusack, Esq. 0. King, Esq.
M. Van Breda Sir G. Mackenzie H. Daniel, Esq. Dr. Prichard
Joseph Carne, Esq. M. Van der Melen J. B. Estlin, Esq. 0. Rees, Esq.
Penzance
Lord Northampton

Dr. Evanson Dr. Riley
Edw. Charlesworth, Professor Parigot W. Hetling, Esq. Rich. Smith, Esq.
Esq.

Professor Phillips Dr. Hodgkin J. C. Swayne, Esq. Major Clerke

Professor Sedgwick Dr. Houston N. Vye, Esq.
Lord Cole
W. Smith, Esq. Dr. Howell

Dr. Yellowley
Rev. W. Conybeare John Taylor, Esq.
R. Griffith, Esq. Dr. William West

SECTION F.-STATISTICS.
Rev. W. Hopkins Sam. Worsley, Esq.

President-Sir Charles Lemon, Bart. R. Hutton, Esq. Rev. James Yates B. Ibbotson, Esq.

Vice PresidentsH. Hallam, Esq., Dr.

Jerrard. SECTION D.-ZOOLOGY AND Seoretaries-Rev. J. E. Bromby, C. B. BOTANY.

Fripp, James Heywood, Esq.

Committee
President-Professor Henslow.

J. W. Cowell, Esq. M. Von Raumer Vice President-Rev. F. W. Hope, Dr. I.

M. Dupin

Right Hon. T. S. Richardson, Professor Royle.

Lord King

Rice
Secretaries—John Curtis, Esq., Professor Professor Babbage Major Clerk
Don, Dr., Riley, S. Rootsey, Esq.

Dr. Bowring M. P. Porter, Esq.
Committee

T. Wyse, M.B. Professor Mounier Wm. Yarrell, Esq. W.C. Hewitson

Rev, E. Stanley

Lord Sandon Rev. Mr. Jenyns Professor Scouler

Col. Sykes

Lord Nugent T. Mackay, Esq. Dr. Jacob

Dr. W. C. Taylor Carpenter Rowe, C. Babington, Esq. Rev. Mr. Ellecombe Henry Woolcombe, Esq. Professor Nilsson G. J. Jeffrys, Esq. Esq.

Thomas Moore, Esq Hon. Charles Harris R. M. Ball, Esq. J. Simpson, Esq. Rev. W. L. Bowles Rev. Mr. Phelps

Colonel Sykes
Rich. Taylor, Esq. J. L. Knapp, Esq. SECTION G.-MECHANICAL
T. C. Eyton, Esq. · Vigors, Esq.

SCIENCE. ,J. E. Bowman, Esq. E. Forster, Esq.

President-Davies Gilbert, Esq. SECTION E.-ANATOMY AND ME- Vice President-M. I. Brunel, Esq., DICINE.

John Robison Esq.
President-Dr. Roget.

Secretaries—T. G. Bunt, Esq., G. T. Vice Presidents Dr. Bright, Dr.

Clark, Esq., William West, Esq.
Macartney

Committee
Secretaries-Dr. Symonds, G. D. Fripp, Captain Chapman Professor Moseley
Esq.

G. Cubitt, Esq. M. le Playe
Committee

J. S. Enys, Esq.

Sir John Rennie Dr. O'Beirne S.D.Broughton, Esq. Wm. Hawkes, Esq. Geo. Rennie, Esq. Dr. Bernard R. Carmichael, Esq. E. Hodgkinson, Esq. John Taylor, Esq. Dr. James Bernard Dr. Carson

Dr. Lardner Rev. W. Taylor

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The scheme of the proceedings of the Association was to remain as heretofore; as, however, some of our readers may not be acquainted with it, and as some knowledge of it will perhaps assist them in following what we may have hereafter to say, we shall here, in a few words, describe it.

The subjects, the discussion of which is considered as properly belonging to the province of this Association, are embraced under seven distinct heads, and the members attaching themselves to the discussion of them, are supposed to divide themselves into seven distinct sections, which are, in their collective capacity, the Association; and which are distinguished from one another, and designated by seven successive letters of the alphabet.

To each of these sections are assigned officers a president, vicepresidents, secretaries, a committee, and a place of meeting. At Bristol,

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