268 WRITINGS OF JOSEPH HENRY. [1855- 



disregarded, and on the other, we think too much effect has 

 been attributed to this cause. It is customary in reducing 

 the observations made at European observatories to deduct 

 the elastic force of the vapor in the atmosphere at a given 

 time from the height of the barometer, and to consider the 

 remainder as the pressure of the dry air. This process would 

 give a correct estimate of the pressure of the dry air, pro- 

 vided the gaseous envelope of the earth were a perfect vacuum 

 to the vapor, and the latter were consequently regularly dif- 

 fused through the space in accordance with its diminution 

 of density due to a diminution of pressure and temperature 

 as we ascend ; but this we know to be far from the fact. In 

 the balloon ascent of Mr. Welsh, on the 21st of October, 1852, 

 the tension of vapor at the elevation of 800 feet was observed 

 to be greater than at the ground, and at a height of 3,000 

 feet it was still greater. In an ascent of the same observer 

 on the 17th of the previous August, the tension continued 

 to increase until an elevation of 8,400 feet was reached. 



To render this point more clear, we will for a moment con- 

 sider the relation of tension and pressure. By the tension of 

 vapor, (as has been seen,) we understand the elastic force or 

 repulsion of the atoms combined with the action of heat by 

 which they tend to enlarge the space in which they are en- 

 closed, and to force down the mercurial column in the experi- 

 ments by which table A (p. 217,) was constructed. At the tem- 

 perature of 60° F. this elastic force is just balanced by a 

 column of half an inch of mercury. Let us now consider the 

 nature of tension in regard to the atmosphere; for this pur- 

 pose let us suppose a piece of paper pasted over the mouth of 

 a glass tumbler so as to be air tight. This paper, though of a 

 very fragile texture, is not broken in by the superincum- 

 bent pressure of a column of air extending to the top of the 

 atmosphere and pressing with a force equal to nearly 15 pounds 

 on every square inch of the surface of the paper, because it 

 is counteracted on the lower surface by an upward pressure 

 due to the repulsive action or elastic force, that is to the ten- 

 sion of the inclosed air. The weight of the superincumbent 

 column on the upper side of the paper is known as the weight 



