BAROMETRICAL MEASUREMENT OF HEIGHTS. 



433 



XXII. TABLE TO REDUCE, BY INTERPOLATION, 



THE OBSERVATIONS TO THE SAME ABSOLUTE TIME. 



DECIMALS OF AN HOUR. 



TABLE FOR CORRECTION OF CURVATURE AND REFRACTION. 



From a mountain, when furnished with a barometer, or with an apparatus for de- 

 termining the temperature of boiling water, and a pocket level, an observer can 

 find the elevations of distant points, which are in sight, but lower than the mountain 

 itself on which he stands. He has only to seek, with the level, the point on the 

 slope of the mountain which corresponds to the point at a distance that he wishes to 

 determine, and to take there a barometrical, or a boiling point observation. This 

 observation is to be calculated in the usual way, but the result must be corrected for 

 the curvature of the surface of the globe, and for the atmospheric refraction, by 

 means of the following Table. 



This method, which furnishes the means of multiplying, without much trouble, 

 the measurements of heights, gives approximations which are sufficient for most 

 of the purposes of Physical Geography. It may even seem preferable to direct 

 measurements for determining the mean elevation of certain physical lines, which 

 are best estimated when seen from a distance ; such as the upper limit of the 

 growth of trees, the limits of different kinds of vegetation, that of permanent snow, 

 that of the mean elevation of the crest of a mountain range, &c. 



Table XXTII. is taken from Captain LEE'S Collection of Tables and Formula, 

 2d edit., page 81. 



D 93 



