66 



BULLETIN OF THE UNIVERSITY OF WISCONSIN 



We now helve for the refraction in declination : 



d = 100 

 n 



The altitude of the sun, 7i, should be noted on the verti- 

 cal circle of the instrument to the nearest half degree at 

 the time of determining n. 



The tabulated values of N correspond to a temperature 

 of 50 F. and a barometric pressure of 30 inches. They 

 may be adapted to any other temperature by diminishing 

 d by one per cent for each 5 by which the temperature 

 exceeds 50, or by increasing one per cent for each 5 be- 

 low 50, but this correction and the correction for varia- 

 tion of the barometer can usually be neglected. At great 

 elevations the barometric pressure becomes so much re- 

 duced that its variation must be taken account of, and this 

 may be done by diminishing d by one per cent for each 

 300 feet of elevation above the sea. 



The following examples will serve to illustrate the ap- 

 plication of the formulae above developed. On the after- 

 noon of May 12, 1894, at a place in latitude 43 5' N., lon- 

 gitude approximately 90 west of Greenwich, I took the 

 following observations with an engineer's transit: 



