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THE SOCIETY FOR THE DIFFUSION OF USEFUL
FOR THE YEAR OF OUR LORD
Chairman-The Right Hon. LORD BROUGHAM, F.R.S., Memb. Nat. Inst. of France.
Treasurer-JOHN WOOD, Esq.
William Coulson, Esq.
R. D. Craig, Esq.
The Rt. Rev. the Bishop of St. David's, D.D.
J. F. Davis, Esq., F.R.S.
H.T. De la Beche, Esq., F.R.S.
The Right Hon. Lord Denman.
Samuel Duckworth, Esq.
The Right Rev. the Bishop of Durham, D.D.
Sir Henry Ellis, Prin. Lib. Brit. Mus.
John Elliotson, M.D., F.R.S.
Thomas Falconer, Esq.
Sir I. L. Goldsmid, Bart., F.R. and R.A.S.
B. Gompertz, Esq., F.R. and R.A.S.
J. T. Graves, Esq., A.M., F.R.S.
G. B. Greenough, Esq., F.R. and L.S.
M. D. Hill, Esq., Q.C.
Rowland Hill, Esq., F.R.A.S.
Right Hon. Sir J. C. Hobhouse, Bart., M.P.
Thos. Hodgkin, M.D.
Thos. Hewitt Ker, Esq., A.M.
R. I. Murchison, Esq., F. R.S., F.G.S.
P. M. Roget, M.D., Sec. R.S., F.R.A.S.
Sir Martin Archer Shee, P.R.A., F.R.S.
Sir George T. Staunton, Bart., M.P.
A. T. Thomson, M.D., F.L.S.
Thomas Vardon, Esq.
Jas. Walker, Esq., F.R.S., Fr. Inst. Civ. Eng. H. Waymouth, Esq.
Thomas Webster, Esq., A.M.
Right Hon. Lord Wrottesley, A.M., P.R.A.S. J.A. Yates, Esq.
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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 determined 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 Hygrometers are of various kinds, differing widely both in principle and construction, but most of them have been superseded by the very delicate instrument invented by Mr.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 so slight that for all ordinary purposes they may be considered the same.
The term " dryness" can of course, in strictness, mean only the absence of moisture -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 3403 (we take the means for the month of January). Here the number in the second column opposite to 340 (the dew-point B 2