-1859] 



WRITINGS OF JOSEPH HENRY. 



47 



gion is so permeable to heat, even of low intensities, that a 

 very remarkable difference was observed on some occasions 

 when the camp-ground was chosen in a gorge between two 

 steep hills. The inter-radiation between the hills prevented 

 in a measure the usual diminution of temperature, and the 

 thermometer in such a position stood several degrees higher 

 than on the open plain. 



We shall next briefly consider the mechanical constitu- 

 tion of the atmosphere. The aerial ocean which surrounds 

 the earth consists of atoms of matter self-repellant, which in 

 proportion as the interior pressure is lessened, constantly tend 

 to separate from each other and produce an enlargement or 

 expansion of the whole mass. When the pressure is in- 

 creased the mass sinks into a less volume, the atoms are 

 brought nearer together, the force of repulsion is increased 

 with the diminution of distance between the atoms, and a 

 new equilibrium is attained. From this constitution of the 

 air it immediately follows that the density of the atmosphere 

 is greater near the surface of the earth than that at a higher 

 altitude, since the lower stratum bears the weight of all those 

 which are above it. The diminution in weight of equal 

 bulks of air as we ascend is in a greater ratio than the height, 

 since it diminishes on two accounts: first, because as we 

 ascend in the air the number of strata pressing on us is less; 

 and secondly, each succeeding stratum is lighter. From the 

 law of this diminution of density a table may be formed of 

 the pressure of the atmosphere at various heights, of which 

 the following is an example: 



Density of the air at increasing altitudes. 



