178 



REPORT 1865. 



§ 6. Variation of the Hygrometric Condition op the Air "with Elevation. 

 All the adopted readings of the temperature of the dew-point in Section 4 

 were laid down on diagrams, and joined by lines drawn from one to the other. 

 In the case of the temperature of the air, when thus joined, a curved line can 

 be drawn through them, giving equal weight to every observation ; but this 

 cannot be done with respect to the temperature of the dew-point, it being far 

 more variable than the temperature of the air ; and the numbers in the fol- 

 lowing Table are those read at every 200 feet, from the diagram formed 

 simply by joining the points of observation. 



Table VII. — Showing the Variation of the Hygrometric condition of the 

 Air at every 200 feet of Height. 



Twenty-third Ascent. 



December 1. — The temperature of the dew-point on the ground before start- 

 ing was 40°, or 8° below that of the air ; this difference slightly increased to 8|° 

 at 1600 feet, when the air became more moist, the temperature of the air at 

 2200 feet being 43°, and that of the dew-point 37|° ; these temperatures then 

 separated suddenly, the difference between them at 2600 feet being rather 

 more than 11°; this difference remained about the same until 3600 feet were 

 passed, when it began to increase quickly up to 5000 feet, where it was 1 9°, 

 the temperatures of the air and dew-point being 31°-1 and 12°-1 respectively, 

 the degree of humidity being as low as 42. The air then became somewhat 

 more moist, and the degree of humidity increased to 53 at 5400 feet ; at 

 5000 feet, on descending, the temperature of the air was 30°, and that of the 



