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Chairman-The Right Hon. LORD BROUGHAM, F.R.S., Memb. Nat. Inst. of France.
Treasurer-JOHN WOOD, Esq.
William Allen, Esq., F.R. & R.A.S.
The Right Hon. Lord Campbell.
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The Rt. Rev. the Bishop of St. David's, D.D.
Sir Henry De la Beche, F.R.S.
The Right Rev. the Bishop of Durham, D.D.
T. F. Ellis, Esq., A.M., F.R.A.S.
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John Forbes, M.D., F.R.S:
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Explanation of the columns headed "Remarks on the Weather."
THE whole of the results here recorded are the averages of observations relative to the pressure, temperature, and moisture of the atmosphere, made by Mr. Daniell, the present Professor of Chemistry at King's College, London. These observations were conducted with more than ordinary precaution; they were repeated three times each day, and continued for a period of three years, beginning with September, 1819, and ending August, 1822.
The barometer and thermometer by which the pressure and temperature are determiued are instruments too well known to need description. The pressure is measured by the height of the column of mercury which it is capable of supporting, and which is here expressed in inches and decimals of an inch. The temperature is expressed in degrees and decimals of a degree of Fahrenheit's thermometer.
The hygrometer, by which the state of the atmosphere is ascertained with reference to moisture, is not as generally known as the barometer and thermometer. Hygrometers are of various kinds, differing widely both in principle and construction, but most of them have been superseded by the very delicate instrument invented by Mr. Daniell above mentioned. Its principle is simply this:-a common thermometer is enclosed in a glass tube, which is then hermetically sealed. By a particular contrivance the temperature of the apparatus can be gradually reduced at pleasure. This reduction of temperature, unless the surrounding atmosphere is perfectly dry, will at length occasion a deposition of moisture in the form of dew upon the exterior surface of the glass; for it is known that, whenever aqueous vapour comes in contact with a substance whose temperature is less than its own, condensation immediately ensues; and this condensation is the more copious the greater the difference between their temperatures. Familiar instances of this are seen in summer, when a bottle of wine is brought from a cool cellar, or when a decanter is fresh filled from a well. As the reduction of temperature takes place slowly, the precise instant can be observed when the deposition of moisture commences. The indication of the enclosed thermometer at this instant is what is called the "dew-point." We see then that the dew-point is the temperature immediately below that of the vapour contained in the surrounding atmosphere; the difference, however, between the dew-point and that of the vapour is soslight that for all ordinary purposes they may be considered the same.
The term "dryness" can of course, in strictness, mean only the absence of moistureTM -drought and moisture being related to each other in precisely the same manner as heat and cold; it has, however, been applied to denote the difference between the temperature of the air and that of the vapour it contains:-thus in January the "mean dryness" is 3601 (the "mean temperature" of the air) diminished by 3403 (the mean temperature of the contained vapour, or "mean dew-point"); that is, 108. In like manner, the "greatest dryness" is the greatest difference observed during the month between the temperature of the air and the corresponding temperature of the contained vapour; and the "mean greatest do. of day" is the average of the greatest difference observed each day throughout the month. This use of the term " dryness," as synonymous with the difference of the temperatures of the air and contained vapour, would be less objectionable if the dryness of the atmosphere really depended solely upon this difference, which, however, is not the case; for this difference may be the same on two days when the atmosphere is charged very unequally with moisture. It is the temperatures themselves, and not their difference, which determines the quantity of moisture in the atmosphere at any time. When these temperatures are known the weight in grains of the aqueous vapour contained in a cubic foot of air will be given by the following
Rule. Find the number in column N of the annexed Table which is opposite to the dew-point nearest to that indicated by the hygrometer, and divide it by 448 increased by the temperature of the air. The application of this rule will be much facilitated by using the card of four-figure logarithms published by the Society; for this reason we give in a third column the logarithms of the numbers in the second.
Example.-Suppose the corresponding observations of the thermometer and hygrometer give-Temp. of air 3601; dew-point 340-3 (we take the means for the month. of January). Here the number in the second column opposite to 340 (the dew-point
nearest to that indicated by the hygrometer) is 1211, which, divided by 484 1 (the temperature of the air increased by 448) gives 2 501 for the weight in grains of the moisture contained in a cubic foot of the atmosphere at the time and place of the observation. The logarithmic working of this example is given in the margin.
It only remains to notice the column headed "Radiation." The "solar radiation" is the excess of the indication of a thermometer with a blackened bulb when exposed to the direct rays of the sun, above that of a similar thermometer placed in the shade, the indications being observed simultaneously. The "terrestrial radiation" is the defect of the indication of a thermometer with a blackened bulb when exposed during the night upon a grass plat to the full aspect of the sky, below that of a similar thermometer placed under shelter.
It must be borne in mind that the results recorded at the head of the several months are the averages of observations made in by-gone years, with which it is extremely improbable that similar observations made in the corresponding month of any one subsequent year will entirely agree, but from which on the other hand it is improbable that the departure will be very considerable.
Explanation of the columns headed" Length of day," "Day's increase or decrease," "Day breaks," and "Twilight ends."
THE column headed "Length of day" contains the number of hours and minutes between sunrise and sunset. The column headed " Day's increase" expresses the number of hours and minutes which the day has increased since the shortest day; and, where the column is headed "Day's decrease," it expresses the number of hours and minutes which the day has decreased since the longest day; for example, the length of the longest day in 1844 is put down in the column "Length of day" at 16h. 34m.; and on the 6th of July following we find that the length of the day, or the number of hours and minutes between sunrise and sunset, is set down at 16h. 23m.; hence the day has decreased 11m. since the longest day, and, accordingly, in the column " Day's decrease," we find opposite July the 6th Oh. 11m.
Equation of Time.
In the Almanacs of the Society the calculations are all made for mean time (given by the clock), instead of apparent time (given by the sun-dial), which latter had been used up to the year 1833. It must be obvious that, for all practical purposes, mean time is the most useful; and to obtain it from apparent time, the columns in the Almanac headed "Equation of Time" should be used. The column Equation of Time' ought, for example, to be consulted when persons are desirous of setting their clock by a sun-dial. When clock after sun is written above the number of minutes and seconds opposite to the day, then the clock ought to be set so much slower than the sun-dial, and the contrary.
THE Moon's age is set down in days and the nearest tenths of days from the time of change. Thus it is New Moon on the 19th of March at 0h. 17m. morning, and therefore at noon on the 20th she is 1 day 11h. 43m. old, which is set down as 1 day and fivetenths. The fraction of the day of course continues the same throughout the lunation. LIST OF THE CORRESPONDENCE OF ERAS WITH THE YEAR 1844. [In those Eras which begin with the Christian year, the year alone is stated; in those which begin at a different season, the month in which the 1st of January, 1844, occurs is also given.]
Correspondence Abbrewith 1844.
24th Cohiac 2590
24th Cohiac 1560 Audynæus 2155 3rd month of 2167 Audynæus 1968 Audynæus 1892
Canun II. 4th month of
2d Kaghots 1293 12th Kaghots 1292 10th Dhulhajjah 1259 Poos or Margaly 4945
AUXILIARY TABLE FOR FINDING THE TIME OF SUNRISING AND SETTING.
The time of Sunrise and Sunset in the 'British Almanac' is adapted to the parallel of latitude in which London is situated-viz. 51° 30′. THE following table has been constructed to show the variations of time through the United Kingdom-namely, between the latitude of 58° and 50° 10′ N. The times of sun rising and sun-setting are computed for the instant that the sun's centre is even with the horizon of the sea. The number of minutes found in this table under the month-day, and in the required latitude, are to be applied to the time of sun-rising and setting found on that day in the Almanac; the result will be the time of his rising and setting at the place required.-Ex. At what time will the sun rise and set on May 21 at Edinburgh? The time of sunrise and sunset on that day in the Almanac is 4h. 2m. A.M., and 7h. 52m. P.M. In the tables in parallel of 56°, in which Edinburgh is found, and under May 21, are 23 minutes; which, subtracted from 4h. 2m., leaves 3h. 39m. for time of sunrise; and, added to 7h 52m., gives 8h. 15m. for time of sun-setting.
The places which follow the different parallels are situate within 15 miles of latitude, either north or south of it.
58° 0'-Dornoch, Tain, Dunrobin, Portenleik, Dunclaim.
57° 30-Peterhead, Fraserburg, Banff, Elgin, Cromarty, Inverness, Applecross.
57° 0'-Aberdeen, Bervie, Braemar, Laggan, Cornock.
56° 30'-Forfar, Dundee, Perth, Comrie, Ardchattan.
56° 0'-Berwick, Haddington, Edinburgh, Linlithgow, Kinross, Stirling, Dumbarton, Glasgow.
55° 30'-Embleton, Jedburgh, Selkirk, Sanquhar, Lanark, Irvine, Ayr.
55° 0'-Newcastle, Morpeth, Carlisle, Annan, Dumfries, New Galloway, Wigtown. 54° 30'-Scarborough, Whitby, Hartlepool, Stockton, Richmond, Appleby, Cockermouth, Whitehaven, North part of Isle of Man.
54° 0'-New Malton, York, Aldborough, Clitheroe, Lancaster, Preston.
53° 30'-Grimsby, Kingston-upon-Hull, Pontefract, Manchester, Wigan, Liverpool, Beaumaris, Holyhead.
53° 0'-Lynn Regis, Boston, Lincoln, Nottingham, Derby, Stafford, Flint, Chester, Denbigh, Caernarvon, Harlech.
52° 30′-Yarmouth, Norwich, Thetford, Ely, Peterborough, Leicester, Coventry, Lichfield, Shrewsbury, Ludlow, Montgomery, Aberystwith.
52° 0'-Ipswich, Colchester, Cambridge, Hertford, Bedford, Buckingham, Oxford, Gloucester, Worcester, Hereford, Monmouth, Brecon, Caermarthen, Cardigan, St. David's.
51° 30'-LONDON, Ramsgate, Canterbury, Rochester, Chelmsford, Windsor, Wallingford, Marlborough, Malmesbury, Bath, Bristol, Newport, Cardiff, Llandaff, Pembroke. 51° 0'-Dover, Winchelsea, Brighton, Guildford, Chichester, Winchester, Portsmouth, Southampton, Salisbury, Shaftesbury, Wells, Ilchester, Taunton, Bridgewater, Minehead, Barnstaple.
50° 30'-Newport (I. W.), Poole, Weymouth, Exeter, Ashburton, Totnes, Plymouth, Tavistock, Launceston, Bodmin, Camelford, Padstow.
50° 10′-Truro, Falmouth, Helstone, Penzance.
55° 0'-Carrickfergus, Antrim, Coleraine, Londonderry, Lifford, St. Johnstown. 54° 30'-Belfast, Killyleagh, Downpatrick, Armagh, Charlemont, Dungannon, Augher, Donegal, Ballyshannon, Enniskillen, Sligo.
54° 0'-Carlingford, Newry, Dundalk, Drogheda, Kells, Cavan, Belturbet, Carrick, Boyle, Castlebar, Killala.
53° 30′-Dublin, Swords, Naas, Athboy, Mullingar, Philipstown, Kilbeggan, Athlone,
Roscommon, Lanesboro', Tulsk, Tuam, Ballinrobe.
53° 0'-Wicklow, Blessington, Baltinglass, Carlow, Athy, Kildare, Portarlington, Maryboro', Ballynakill, Banagher, Galway, Ennis.
52° 30′-Newborough, Enniscorthy, Wexford, Kilkenny, Cullen, Clonmell, Cashell, Killmallock, Limerick, Askeyton.
52° 0'-Waterford, Dungarvon, Youghal, Tallagh, Lismore, Rathcormack, Cork, Marlow, Killarney, Tralee, Ardfast, Dingle.
51° 30'-Kinsale, Bandor, Clonekelty, Baltimore.