. The principles of physics. he air is said to be dry. If the tem-perature of the air be little abovedew-point, the air is said to behumid, which means that itcan hold but little more sensation of dryness experi-enced, especially in rooms heatedartificially, does not dependupon the absolute quantity ofwater vapor present per cubicfoot. The heat of a stove, for in-stance, dries the air of a roomwithout destroying any of itswater vapor. In such a room,the lips, tongue, throat, and skinexperience a disagreeable sensartion of dryness, owing to therapid evaporation which takesplace from t
. The principles of physics. he air is said to be dry. If the tem-perature of the air be little abovedew-point, the air is said to behumid, which means that itcan hold but little more sensation of dryness experi-enced, especially in rooms heatedartificially, does not dependupon the absolute quantity ofwater vapor present per cubicfoot. The heat of a stove, for in-stance, dries the air of a roomwithout destroying any of itswater vapor. In such a room,the lips, tongue, throat, and skinexperience a disagreeable sensartion of dryness, owing to therapid evaporation which takesplace from their surfaces. Thisshould be takMi as natures ad-monition to keep water in the stove urns, and in tanks con-nected with furnaces. The quantity of water vapor present in the air is expressedeither (1) by the mass of vapor per unit of volume ; or (2) bythe ratio between the quantity actually present and that whichwould be present if the air were saturated at the temperatureof observation. The latter is the more common and more. Pig. 214. WET AND DRY BULB THEEMOMETER. 293 useful method, and this ratio is called the relative humidity,or simply humidity of the air. It is expressed in per-centages. Thus, relative humidity = 76 per cent., or ,denotes that the air contains three-fourths the quantity ofwater vapor required to saturate it at the present tempera-ture.^ 256. Wet and dry hulh thermometer. The relative humidity of air is measured in various ways and byvarious devices. The instrument most commonly used is Masonswet and dry bulb thermometer, or as it is frequently called, psy-chrometer. It consists (Fig. 214) of two thermometers mounted sideby side a short distance apart, one having a dry bulb and the othera bulb covered with muslin, kept moist by capillary action through ^ The student may be profited by a perusal of the following, copied from the dailymeteorological report in the London Times. [Speed the time when our own govern-ment, in consideration of its educational v
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1895
