The age-herald., 1898-08-20
THE NATURE
 OF HUMIDITY

From the Philadelphia Inquirer.
 The humility is reported in a weather
 bulletin on certain day to be 95 per cent.
 Everyone gathers form this that the air
 is excessively charged with moisture, and
 everyone knows, too, that this circumstance
 has quite much to do with the oppres-
 siveness of the weather as does the temp
 erature as shown by the thermometer But
 how many persons can say precisely what
 is meant by humidity of 95 any other
 per cent? Here is, in substance weather
 mans interpretation of tie figures:
 The air always contains more or less
 moisture, since water evaporates and min
 gles with it in the form of invisible vapor
 even at the lowest temperature. The
 amount of vapor which the air can con-
 tain varies with its temperature. At a low
 temperature it can hold in suspension a
 smaller amount of water vapor than when
 1t is highly heated. When the air is chargL
 ed with the full amount of vapor, which
 at its temperature 1t can hold, it Is said to
 be 'saturated." Evaporation then stops,
 and If the temperature fails, however
 slightly, the air must necessarily give up
 portion of its charge of vapor, which
 may form into droplets and gather into
 clouds, or it may be deposited in the form
 of dew. Here is the explanation of the film
 of moisture which we sometimes see gather
 upon an ice-pitcher in a warm and close
 room. The layer of air in immediate con-
 tact with the pitcher has become too cold to retain all of it's moisture-has ben cooled
 below the "dew point'-and the surplus of
 vapor is deposited on the glass or metal. The figures which are given out by the weather bureau indicate only what is called
 relative humidity" That is to say, they indicate not the actual amount of water vapor in the air at any given time, but the degree of the airs approach to saturr ation. Complete saturation is represented by the figures 100 and air absolutely freq form vapor by 0, though the latter state of things never occurs a humidity as low as 10 being rarely found even in the most arid regions. In this way of reckoning, hu- midity of 50 per cent means that the air contains just one-half of the amount of vapor which would saturate it at the ex- isting temperature. While the humidity stands at this figure evaporation will still go on rapidly, and since the evaporation of moisture of our bodies tends to keep us cool, we experience no sense of oppressive heat, even though the thermometer stands high. With a humidity of 75 per cent which may be brought about either by a fall of temperature, while the amount Of moisture in the air remains the same, or by an increase in the amount of moisture, the temperature remaining unchanged, evaporation becomes less rapid, and when the figures 90 and upward are reached 1t nearly ceases; and if at the same time the temperature is fairly high, the moisture which oozes from our pores remains upon us and we swelter as in a steam bath.
 The instrument used for ascertaining the humidity of the air Is know as the hygro- meter There are several sorts of hygro- meters in use, the most common being the ''dry and wet bulb hygrometer." The es- sential parts of this instrument are two thermometers, of which one registers the temperature of the air in the ordinary way, while the other, which has its bulb covered with piece of muslin kept contin- ually moistened with water, indicates lower temperature caused by the cooling of its bulb by evaporation. The less moisture | there is in the air the Fnore rapid will be the evaporation from this wet bulb and the greater the fall of the mercury in its tube. l This instrument, therefore, indicates only l rapidity of evaporation; but this having been ascertained, the percentage of humid ity can be got at in a roundabout way by means of an algebraiac formula, or. as IS done in practice, from a table prepared TOT the purpose, which gives the dew point corresponding with the reading of the n.y. l prometer at different temperatures.