12 WRITINGS OF JOSEPH HENRY. [1855- 



Though some of the heat of the sun is absorbed in its 

 passage through the atmosphere, yet by far the greater por- 

 tion (particularly at the equator) arrives at the surface of 

 the earth, is absorbed by the soil, and is imparted to the 

 stratum of air in contact with it. From various determi- 

 nations it is a well-established fact that the temperature of 

 celestial space beyond our atmosphere is at least 50° below 

 the zero of Fahrenheit's scale. The upper surface of the 

 atmosphere and the Arctic regions must therefore partake of 

 this low temperature, while that of the lower stratum at the 

 surface of the earth is at the equator about 80°. The air 

 therefore diminishes in temperature as we ascend, but the 

 rate of this diminution varies within certain limits in differ- 

 ent parts of the earth ; and to settle the law of diminution 

 definitely, a series of observations by means of ascents in 

 balloons will be required. For practical purposes however 

 we may assume in the temperate zone that the diminution 

 due to altitudes or mountains is about 1° of Fahrenheit for 

 300 feet. Furthermore, as we ascend and the pressure of 

 the superincumbent strata is thus reduced, the air becomes 

 lighter; and though the temperature of the several por- 

 tions diminishes very rapidly, yet the whole amount of heat 

 in each pound of air is very nearly the same. For ex- 

 ample, if a certain weight of air were carried from the sur- 

 face of the earth to such a height that it would expand into 

 double its volume, the heat wliich it contained would then 

 be distributed throughout twice the space, and the tempera- 

 ture would consequently be much diminished, though the 

 absolute amount of heat would be unchanged. If the same 

 air were returned to the earth whence it was taken, conden- 

 sation would ensue, and the temperature would be the same 

 as at first. 



2. On this principle a wind passing over a high mountain 

 is not necessarily cooled ; for the diminution of temperature 

 which is produced by the rarefaction of the ascent would be 

 just equivalent to the increase which is due to the conden- 

 sation in an equal descent. This would be the case if the 

 air were perfectly dry ; but if it contained moisture, para- 



