Hygrometer

“ On the contrary, if the hygrometer shows continuing or increasing dryness, a stronger wind is probable, without rain.
Domestic Uses.—Mason’s hygrometer is useful in regulating the moisture of the air of apartments; a difference in the thermometer readings of from 5° to 8° being considered healthy. Many complaints require that the temperature and humidity of the air which the invalid breathes should be carefully regulated. Hence it is a valuable household instrument. In a room, it should be placed away from the fire as much as possible, but not exposed to draughts of air. ”

“ The invisible steam is always ready to bathe the tender plants with dew, which would otherwise perish and be burnt up during a hot summer, if they did not radiate heat at night, and thus condense water upon themselves. The presence of watery vapour in the air becomes therefore a matter of great importance, and hence the construction of hygrometers or measurers of the moisture in the air.
Regnault's condenser hygrometer consists of a tube made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1.8 in depth by 8.10 in diameter. ”

“ Should the water become frozen on the ball, this hygrometer will still act; for evaporation goes on from the surface of ice in proportion to the dryness of the air. Leslie estimates, that when the ball is moist, air, at the temperature of the ball, will take up moisture equal to the sixteen-thousandth part of its weight, for each degree of his hygrometer; and as ice in melting requires one-seventh of the caloric consumed in converting water into vapour, when the ball is frozen, the hygrometer will sink more than when wet by 1° in 7° ”

“ The hair hygrometer gives directly the relative humidity or the ratio between the moisture in the air and that which it would contain if saturated. The very best forms perform very well for a time, and are strongly recommended by Pernter, and must be used in self-recording apparatus for balloons and kites; they are standardized by comparison with the ventilated psychrometer, which itself must be dependent on the standard dew-point apparatus. ”

“ It was one of those astonishing truths in science that come suddenly and unexpectedly upon mankind—like those connected with electricity—that take the reason captive, and are beyond the reach of human investigation. Men usually appeared incredulous when the theory was first broached to them; but when convinced of the fact, as proved indisputably by the hygrometer, they were compelled to acknowledge the truth, and forthwith looked upon it as a matter of course.
As the weather grew warmer—when the thermometer stood daily at about 86 degrees in a cool room—the nocturnal evaporation increased. ”

“ A reliable hygrometer should be as frequently used in a dwelling as a reliable thermometer. It is undoubtedly a very difficult thing to regulate the amount of moisture in the atmosphere of dwellings, but more could be done than is done. ”

“ It is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the Hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;—a most important item in the determination of the coming weather. ”

“ To determine the temperature of the dew point by Daniell’s dew point hygrometer and Regnault’s condensing hygrometer, and by the use of the dry and wet bulb thermometers, as ordinarily used, and their use when under the influence of the aspirator, causing considerable volumes of air to pass over both their bulbs, at different elevations, as high as possible, but particularly up to heights where man may be resident, or where troops may be located, as in the high table-lands and plains of India ”

“ For these reasons, I apprehend that a substance which, though capable of being distended by moisture and contracted by dryness, is so slow in receiving and parting with its humidity, that the frequent changes in the atmosphere have not time to affect it sensibly, and which therefore should gradually take nearly the medium of all those changes and preserve it constantly, would be the most proper substance of which to make such an hygrometer. ”

“ Rittenhouse exercised his ingenuity upon an object, which, though not of great importance, is nevertheless a matter of considerable utility and some curiosity; the invention of an Hygrometer, made wholly of wood. The simplicity of the construction of this instrument, renders it easily attainable by almost every one; and as it is found to answer, very well, the end for which more expensive instruments are often employed, it may be considered as being more generally useful. ”

“ The moment the air becomes rarefied in the receiver of the air-pump it becomes colder, as appears by the thermometer, and deposits its vapour; but the hair of Mr. Saussure's hygrometer is now warmer than the air in which it is immersed, and in consequence becomes dryer than before, by warming the air which immediately surrounds it, a part of its moisture evaporating along with its heat. ”

“ One set, the convex curves, is for the determination of relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the other, the concave curves, is derived from the vapor pressures and shows the amount of moisture per cubic foot at relative humidities and temperatures when read at the dew-point. The latter curves, therefore, are independent of all variables affecting the wet-bulb readings. They are proportional to vapor pressures, not to density, and, therefore, may be followed from one temperature to another with correctness. ”

“ The oldest meteorological instrument is, no doubt, the weather or wind-vane, which had its origin before the Christian era. The next oldest is the hygrometer, or instrument for measuring moisture in the air, the form which acts by absorption dating from the middle of the fifteenth century, and the condensation hygrometer being a century younger. ”

“ The second essay of the set, ‘On the Force of Steam,’ gave the first table of its varying elasticity at temperatures from 32° to 212°, and described the ‘dew-point hygrometer’ (p. 582) . The issue of some recent experiments was remarkably anticipated in the following sentence: ‘There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids of whatever kind into liquids; and we ought not to despair of effecting it in low temperatures, and by strong pressure exerted upon the unmixed gases’ (p. 550) . ”

“ The hair hygrometers commonly sold to incubator operators are known by scientists to be absolutely unreliable. The range between the wet and dry bulb thermometers was found in the Ontario experiments to give readings with and without fanning that varied 15 to 20 per cent. in relative humidity which, at the temperature of an egg chamber, would amount to a variation of three to four hundred of vapor pressure units, which, with the forced draught plan, would ruin a hatch of eggs in a few hours. ”

“ But the vague prognostics of old times are not sufficient for the guidance of the skilful and provident farmer of our day. The barometer, the thermometer, and even the hygrometer, should be his companions and guides, or occasional counsellors. To the description and useful indications of these instruments, therefore, a sufficient space is devoted in the book before us. We do not know any other source from which the practical farmer can draw so much meteorological matter specially adapted to his own walk of life, as from this chapter upon the weather. ”

“ The air was so dry this afternoon that I failed to procure a deposit of dew upon Daniell's hygrometer, although the internal temperature was lowered from 61° to 37°. One of Jones's portable hygrometers was also tried, and the temperature was lowered to 31°1⁄2 without a deposit; so that, the difference being more than thirty degrees, the expansive force of the air must have been less than 212, the dryness, on the thermometric scale, less than 367, and the weight of vapour, in a cubic foot of air, less than 2,355 grains.
[47] Fires made to attract attention, and invite strangers to land. ”

“ They sometimes have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or a clock, &c., singly or combined. The frame admits of much variety of style and decoration. It may be carved or inlaid. The usual adjustment of scale is suited for localities [Pg 30] at no considerable elevation above the sea. Accordingly, being commercial articles, they have been found frequently quite out of place. When intended for use at high elevations, they should have a special adjustment of scale. As household instruments they are serviceable, and ornamental. ”

“ Fig. 82 shows the instrument arranged on brass tripod stand, with folding legs and metal cover, to render it portable.
[Pg 108] 107. Self-Registering Hygrometer.—A maximum thermometer and a minimum thermometer, each fitted up as a wet-bulb thermometer, record the highest and lowest temperature of evaporation during the interval of observation. Negretti’s mercurial maximum, and an alcohol minimum, answer best.
108. Causes of Dew.—“The aqueous vapour of our atmosphere is a powerful radiant; but it is diffused through air which usually exceeds its own mass more than one hundred times. ”

“ The decline of temperature was very regular. A siphon barometer, dry and wet bulb thermometers, aspirated and free, and a Regnault hygrometer were taken up. Some air collected at a considerable height was found on analysis not to differ appreciably in its composition from air collected near the ground. For the original observations see Phil. Trans., 1853, pp. 311-346.
At the meeting of the British Association for the Advancement of Science held at Aberdeen in 1859, a committee was appointed for the purpose of making observations in the higher strata of the atmosphere by means of the balloon. ”

“ Jeffries paid one hundred guineas to accompany Blanchard on his fifth ascent, which was made from London November 30, 1784. He took with him a thermometer, a barometer, a hygrometer, an electrometer, and a mariner's compass, also several numbered bottles, filled with water and provided with glass stoppers, which were to be emptied and corked up at different heights in the atmosphere. It was arranged to record the observations on ruled paper with a silver pen, because the doctor would not trust a common pen or pencil as liable to accident. ”

“ Each sample was liquefied by slow heat in a closed vessel (to prevent the moisture evaporating) placed in a water bath. It was then reduced to a temperature of about 60° Fahr., poured into a test-glass and the hydrometer inserted (see Fig. 2) . The hydrometer will gradually sink until it finally registers the specific gravity. In the case of honey being too dense to be treated in this manner weigh up, say, 8 oz. of honey, then add the same weight of warm water, and thoroughly mix; when reduced to 60° Fahr. it will be ready for testing. ”

“ Hilgard believes that the hygroscopic moisture can be of considerable help in carrying plants through rainless summers, and further, that its presence prevents the heating of the soil particles to a point dangerous to plant roots. Other authorities maintain earnestly that the hygroscopic soil-water is practically useless to plants. Considering the fact that wilting occurs long before the hygroscopic water contained in the soil is reached, it is very unlikely that water so held is of any real benefit to plant growth. ”

“ Clinging to every tree, stone, or animal tissue is a small quantity of moisture varying with the temperature, the amount of water in the air, and with other well-known factors. It is impossible to rid any natural substance wholly of water without heating it to a high temperature. This water which, apparently, belongs to all natural objects is commonly called hygroscopic water. Hilgard states that the soils of the arid regions contain, under a temperature of 15 deg C. and an atmosphere saturated with water, approximately 5-1/2 per cent of hygroscopic water. ”

“ A better hygroscope is the toy chalet from which a woman emerges in dry weather, while a man with an umbrella stands in the door during damp weather. The manikins are suspended by a short piece of catgut which, twisting in the one case and untwisting in the other, because of changing moisture content, indicates roughly the changes in the humidity of the air. ”

“ Summary.—The significance of hygroscopic moisture in connection with plant growth may then be thus summarized:
1. Soils of high hygroscopic power can withdraw from moist air enough [Pg 201] moisture to be of material help in sustaining the life of vegetation in rainless summers, or in time of drought. It cannot, however, maintain normal growth, save in the case of some desert plants.
2. High moisture-absorption prevents the rapid and undue heating of the surface soil to the danger point, and thus often saves crops that are lost in soils of low hygroscopic power. ”

“ Testing With Hydrometer The estimation of the gravity of the boiling pulp by the use of a hydrometer is not in very general use, and it has been stated by some scientific men that a hydrometer is no good for tomato pulp or catsup except where the sample on which the test is made is previously filtered so as to get a clear liquid. A test which involves filtering, is, in my opinion, entirely unsuited for everyday use on the cooking platform. ”

“ The rate of movement of the granules is accelerated by moderate warmth and retarded by cold. It is in viewing the circulation in water plants that the warm stage of the microscope is brought into use. Borne along with the protoplasmic stream are a number of solid particles consisting of starch granules and other matters. The method of viewing the circulation is by cutting sections off a portion of the plant with a very sharp knife, and arranging them in a growing cell with a few drops of water, and covering over with a thin cover-glass. ”