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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1406, August 14, 1885
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Zeitschrift
The photographic news
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Band
Band 29.1885
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- Register Index III
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Band
Band 29.1885
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518 THE PHOTOGRAPHIC NEWS. LAvGUST 14, 1865. trict in all its most lonely and secluded sections. One of the many sketches then marie may here serve to show how much assistance even a single figure may render in giving effect to light and shade. Before I introduced the figure, the sunny effect I endeavoured to copy seemed very im perfectly rendered, but it brightened up wonderfully when I placed the shadowed flesh and dark dress of the lady in contrast with the strongest light. For this reason I have asked our Editor to reproduce it in black-and-white, instead of merely in lines. Another illustration which struck me as likely to be suggestively serviceable to the young photographic art student was obtained from “ Vietri,” a picture in the royal collection by Clarkson Stanfield, 1? A. The group ing and arrangement of the foreign fisher folk, with their fish and baskets, are admirably managed in the original to give effect and interest; but they are further aided by other figures—not given in my rough and hasty sketch —which figures by their relative sizes and tones, give wonderful expression to the feeling of air and space in this noble work of art. I remember in one of Witherington’s canvasses an excel lent result secured by the introduction of some figures working in a woodland gravel pit, but regret that the memory is not so strong that I could sketch it. In one of the best pictures Richard Redgrave, R.A., painted, a wood cut copy of which appeared in the 1859 volume of “ The Art Journal," there is an excellent example of figures in land scape, which increase the interest, give brilliancy to the light, and the perspective and the expression of air, while at the same time composing well pictorially. I shall resume this subject in my next paper, and add other sketches from the pictures of well-known painters. METEOROLOGY FOR PHOTOGRAPHERS. BY J. VINCENT ELSDEN, B.SC. (BOND.), F.C.S. Chapter IV.—Atmospheric Pressure—Barometers— Causes op Variation of Pressure—Sunshine and the Barometer.* We now come to speak of the more important corrections which must be made, in order that barometric readings, taken in different localities, may be compared with one another. One of these corrections, called the index error (due to an improper laying off of the scale), is given on the Kew certificate. If the barometer is a Fortin, this correction will be the same throughout the scale ; but if it is the Kew pattern, it will generally be variable, and as the corrections are only given for every half-inch between 27-5 and 31. it will be necessary to interpolate for inter mediate readings. The next correction is for temperature —i.e., to reduce all readings to a fixed temperature, 32° F. For this purpose, a thermometer should be attached to the case, as near as possible to the barometer tube, and • Continued from page 508. the temperature carefully taken before the height of the barometer is read. The necessity of applying this correction is obvious, since, owing to the expansion and contraction of the mercury at different temperatures, the correct height could not otherwise be ascertained. Tables are published* giving the correction for every half-inch, for each degree of the attached thermometer. These corrections are all subtractivef Since the height of the mercury varies with the elevation of the locality, it is also necessary to reduce all readings to one fixed level, viz., mean sea-level, which is taken to be the mean half-tide level at Liverpool. The amount to be added to each reading for this purpose is also given in the published tables. Thus, it is necessary that the height of the locality above sea-level should be known. This can be easily determined, with sufficient accuracy, in the following manner. Take the difference between the baro metric readings at the given locality, and at another locality, the height of which is known ; then multiply by 9 the difference taken in hundredths of an inch. The pro duct is the difference in height in feet.+ A common defect in barometers is the presence of air or moisture in the tube, which will cause the readings to be too low. The presence of moisture also tends to make the mercury sluggish, and adhere to the glass. To pre vent this, it is usual to boil the mercury in the tube. An improperly boiled tube can usually be detected by the appearance of small specks or air-bubbles in the mercury when examined by a lens. The access of air or moisture at any subsequent period can usually be provided against by the introduction of a pipette in the lower part of the tube. The almost constant variations in the height of the barometer are mainly due to two causes—viz., temperature, and aqueous vapour in the atmosphere. Air, like most other substances, expands when heated, and contracts on cooling. Cold air is, therefore, denser than warm air, so that cold air descends, and warm air ascends. The ascent of warm air causes a diminution of pressure, and the barometer therefore falls as the temperature rises. This influence of temperature is more especially seen in the dry interior of continents and in the tropics, where the daily variations of the barometer are so regular and striking, that it is said that the time can be told within twenty minutes by the height of the barometer, and any irregularity is an unfailing sign of a storm. In England, however, these diurnal variations are not more than -02 inch, and are nearly always concealed by the non-periodical oscillations, due to the passage of storms. But temperature is not so important an agent in affecting the pressure of the air as the influence of aqueous vapour. The more heavily laden with vapour the air becomes, the less is its pressure; for whereas a cubic foot of perfectly dry air weighs more than 500 grains, a cubic foot of water vapour only weighs about 5 grains under ordinary conditions. Supposing, then, that atmospheric pressure is due solely to the weight of the constituents of the air, we see why damp air should exert less pressure than dry air. We may, in fact, consider atmospheric pressure as made up of two constituent parts, dry air pressure, and the pressure of the vapour present, which latter is generally called vapour tension ; and it is • See “ Instructions in the Use of Meteorological Instruments,” by R. H. Scott, F.R.S. + The reduction may be made by the following formula h- 11 1+t(000015)* Where h=height of mercury at 0° 0., H its observed height, and t the temperature in centigrade degrees. t More accurately, the difference in elevation between two stations is found by the following formula of Laplace, which can also be used for re ductions to sea level, if the altitude is known : — 1=60150 10gA(l+ <+ ^) (1+ 00268 cos 21) (1+^^+^) Where/= the difference in height of the two stations, x the height of the lower station above sea-level, h and h1 are the barometric readings of tn two stations reduced to 32° F. ;,t, t 1 the temperatures, and I the latitude o the place. In the last factor, an approximate value is used for/.
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