4 PlIILOSOrilY OF STORMS. 



phenomena, whose operations, as well as llie laws upon which they are 

 founded, were not, until recently, revealed to man. This circumstance 

 certainly gives peculiar force and character to his theory, as it indicates 

 a spirit of inquiry purely inductive. The experiments of Dalton prove 

 that heat is the true and only cause of the formation of vapor. He 

 found that the actual quantity of vapor, which can exist in a given 

 space, is dependent solely upon the temperature. This is a most beau- 

 tiful circumstance, that the thermometer, which was invented merely for 

 ascertaining the relative quantity of caloric in bodies, should now like- 

 wise be employed, not only in estimating the height of clouds, but in 

 determining the absolute quantity of vapor in the air ! As the quan- 

 tity of vapor is always proportionate to the temperature, so if we have 

 a low or high dew-point we have a small or larger quantity of vapor. 

 When the dew-point is given, therefore, we can always ascertain the ex- 

 act amount of vapor. In order, however, to understand this part of 

 the subject, it will be necessary first to refer to several propositions : 



1. All gases expand alike for equal increments of heat ; and all va- 

 pors, when remote from their condensing points, follow the same law. 



2. The rate of expansion is uniform for all degrees of heat. 



3. The rate of expansion is not altered by a change in the state of 

 compression, or elastic force of the gas itself 



4. The actual amount of expansion is equal to j|^th part of the 

 volume of the gas at 0°, for each degree of the same scale. 



Now to discover how much the volume of a gas or vapor would 

 be increased or diminished by a particular change of temperature, let it 

 be required, for example, to find tlie volume which 100 cubic inches of 

 gas at C0° would become on the temperature rising at 70°, 



The rate of expansion is ^J jlh part of the volume at 0° for each 

 degree; or 460 measures at 0° become 461 at 1°, 462 at 2% 460+60^= 

 520 at 60°, and 460+70= 530 at 70°. Hence 



Metis, at 60= Mcas. at 70' Mcas. at 60" Meas. at 70° 



520 : 530 : : 100 : 101.92 



Again : — If a barometer-tube filled with mercury be inverted, and a 

 few drops of water be passed up the tube into the vacuum above, the 

 mercury will be depressed to a small extent, and this depression will 

 increase with the increase of temperature. This depression depends 

 upon the vapor which instantaneously rises from the water into the 

 vacuum. Now the same power which forces the mercurial column 

 doW7i one inch against the pressure of the atmosphere outside the tube 

 wuuld of course clcvalc the column to the same height against a vacu- 



