432 



Mr. J. Nixon on the Triftonometrical Height 



Observed Refractions, 



Trigonometrical Differences of Level. 

 Explanation, — The angles of elevation {El.) and depression 

 (Z)/7.) are given corrected for the cylindrical error of the in- 

 strument ( = 20"). At every station the height of the eye above 

 the point there selected for bisection is stated, and from these 

 data the differences of level are computed with a constant re- 

 fraction of ^i^ ; but when any other point has been bisected, 

 its height above ( + )or below ( — ), the standard datum, is 

 given within brackets. The distances prefixed to the angles 

 are those from the sector to the point observed*. The devia- 



♦ Call S the signal at the station, P the adjacent place of the sector, and 

 O the distant signal observed; then add the log. cosine of the angle SPO 



