DEW POINT PSYCHROMETER — ABSOLUTE HUMIDITY. 223 



perature.^ The tendency, therefore, is to bring the dry air to the sur- 

 face, and so far as it operates to promote evoporation,more particularly 

 at the warmer temperatures. 



Deio Foint — We may prove the presence of vapor in the atmosphere 

 by removing the pressure, as in the receiver of au air-pump, in which, 

 as the exhaustion goes on, the remaining air expands, and in expanding 

 cools, until presently a cloud of vapor appears within, and the surplus 

 moisture in the rarefied air, settles on the inside of the receiver as dew. 

 The temperature of the air, at the moment when this dew begins to ap- 

 pear, is called the dew point. It may be more easily determined by 

 noticing the degree of temperature at which dew begins to collect on 

 the outside of a bright metallic vessel in which water is placed, and 

 gradually cooled down. This process occurs in nature whenever the 

 radiation of warmth from objects on or near the earth's surface, cools 

 them down till the dew appears. Tbe presence of fog or of clouds, shows 

 that the air is there, saturated with moisture, and that the cooling point 

 has been reached, at which the atmosphere can hold the excess of vapor 

 in suspension no longer. No rain, snow, or dew can form, until the air 

 is cooled down to this point, and every cause which has a cooling tend- 

 ency, or that increases the amount of vapor in the atmosphere, favors, 

 if it does not actually produce, the formation of cloud and the precipita- 

 tion of the excess of moisture. 



It is a well-settled principle of physics, that no matter in any form, 

 whether solid, liquid, or gaseous, can pass from a rarer to a denser form 

 without emitting heat, or from a denser to a rarer form withoutab- 

 sorbing heat from all surrounding objects. Thus, when the air or vapor 

 is expanded from any cause, it becomes cooler, and when condensed 

 it becomes warmer. The heat absorbed when matter passes from one 

 state to another, as from water to ice, is termed latent Jieat. This heat is 

 given out again when the ice returns to water. The same thing occurs 

 when water is evaporated, oris condensed from vapor to a liquid form. 



Various instruments have been devised for measuring the amount of 

 moisture in the atmosphere ; the one commonly used being the Psy- 

 chrometcr. This consists of two similar thermometers, usually placed 

 but a few inches apart, so as to be exposed to the same conditions, ex- 

 cepting that one of them has its bulb covered with muslin, which is wet 

 a few minutes before observation. The evaporation from this covered 

 bulb soon reduces the temperature to a fixed point, and a reading of 

 both scales at that time will afford, by the aid of tables that have been 

 prepared for the purpose, the two important facts concerning the atmos- 

 phere with reference to its humidity, upon which many conditions of 

 climate and vegetation depend, — the absolute, and the relative, humidity. 



The Absolute Hum id it i/^ is shown by the elastic force of vapor at a given 

 time, in maintaining a column of mercury. This elastic force or tension, 

 at temperatures above the boiling point, affords our steam-power, and 

 at high degrees of heat it acquires destructive force. It exists at all 

 degrees of heat, down to the lowest, and at temperatures commonly 

 observed in the open air, it varies according to the differences of read- 

 ing between the wet and dry bulbs of the psychrometer, as is shown in 

 the following table : 



' At zero of Fahrenheit, the ilifference in weigh' in a cubic f Jot is but 0.44 grain troy ; 

 at 50° it is 2.44 grains, and at 90° it is 8.04 grains. 



