194 NINTH ANNUAL REPORT OF 



the difficulty we mentioned as to its capability of existing in the atmo- 

 sphere at all temperatures. In accordance with this law, the atoms of 

 water, as well as those of the several gases which compose the atmo- 

 sphere, are as far separate from the atoms of their own kind as they 

 would be if each alone occupied the space. The pressure of the com- 

 pound atmosphere is therefore made up of the joint pressures of the 

 individual gases. 



Strictly speaking, vapor is not condensed by the pressure of air, but 

 by diminution of temperature ; and^he amount of moisture which can 

 exist at any time in the atmosphere depends upon the amount of heat. 

 Air saturated with moisture may be likened to a vessel filled with 

 round shot or sand. The spaces between the atoms of the permanent 

 gases are occupied by the atoms of steam, in the same way as water 

 fills up the vacant spaces between the shot or sand. The atoms of 

 water are at a distance when steam at any temperature is by itself, and 

 they can possess no greater repulsive force than is due to the tempera- 

 ture of the evaporating surface. Pressure applied to the atoms will 

 cause condensation, if the heat aiising from this act is lost. This is 

 the same in the moistest atmosphere — the atoms of water are at a 

 distance, and their repulsive or diffusive force is the cause of their re- 

 maining invisible. Evaporation, therefore, in the atmosphere at a 

 given temperature, can only take place when the atoms of water are 

 removed as far from those on the evaporating surface as they would 

 be in pure steam at the same given temperature. 



Air is said to be saturated with moisture when as many atoms of 

 vapor are contained in it as would exist in a vacuum at the same tem- 

 perature. The atoms of air merely fill up the interstices of the elastic 

 vapor of water, or the converse. If the temperature be diminished, 

 the repulsion of the atoms will not be sufficient to support so great a 

 pressure, and a part of the vapor will be condensed into water. The 

 reduced temperature at which this condensation begins to take place is 

 called the dew point, or point of precipitation. 



The elastic force of vapor would ultimately cause it to rise to the 

 top of the atmosphere by virtue of this repulsive or diffusive property, 

 if it were not checked by other causes. At one time meteorologists 

 attributed many atmospheric phenomena to the great activity of the ' 

 diffiisive power of the vapor of water ; but we are indebted to Profes- ■'■ 

 sor Espy for showing that this has been very much overrated, and i; 

 other agents are the more active causes. 



The air has its temperature raised during the day by the direct heat 

 from the sun and contact with the earth, and loses the greater part of !|i 

 this heat during the night by radiation into space. The ground radiat- 

 ing heat much faster than air in a clear, calm night, is generally cooled 

 down below the point of precipitation of the vapor, and thus conden- 

 sation of moisture is produced and dew formed. This effect of radia- 

 tion has a most beneficial influence on vegetation. The formation of 

 dew is regulated by a very simple law, viz : that the dewed surface is 

 always colder than the air in contact with it. In the same way the dew- 

 point of the air is ascertained by cooling down any bod}^ until moisture 

 is formed upon it. Several hygrometers have been formed on this 

 principle. 



