telescope inclined to the declination, level the solar telescope and 

 clamp it. Follow the sun in his upward path, through the solar 



telescope with the tangent 

 screws of the transit, by keep- 

 ing his image centered between 

 the two outside horizontal cross 



\ / ^lill^"'' J wires. This will be most con- 



XJ^J^J.X ^^SSg;^ veniently accomplished by set- 



Fig. 97 ting up the transit to begin 



with so that the slow motion 



of the telescope can be maneuvered with the right hand and 

 the lower slow motion of the vertical axis with the left. 



At the instant when the sun has apparently ceased to rise, take 

 the reading on the vertical circle and subtract the correction for 

 refraction due to altitude as given in the table referred to below. 

 The result is the co-latitude or the angular distance of the equator 

 from the horizon as will be noted by reference to the diagram, p. 

 160. This is the value required in determining the meridian with 

 the solar attachment but if the latitude of the place is also desired, 

 it can be found by subtracting this net result from 90. 



In his original experiments with the Saegmuller Solar Attach- 

 ment in 1885, the late lamented Prof. ]. B. Johnson said: "It is 

 evident that if the instrument is out of adjustment, the latitude 

 found by a meridian observation will be in error; but if this 

 observed latitude be used in setting off the co-latitude, the instru- 

 mental error is eliminated. Therefore, in a meridian observation, 

 always use for the co-latitude that given by the instrument itself." 



A Table of Mean Refraction is given on p. XXI. It is 



the one compiled by Hayford from data furnished by Laplace, Bessel 

 and Doolittle. The table is calculated for the barometer at 29.'' 

 in. (760 mm) and the thermometer at 50 F or 10 C. When the 

 barometer rises to 30.9 in., the factor 1.033 should be used and 

 when it drops to 28.9 in., use 0.966; or for 27.9 in., use 0.933. 

 Other positions will be nearly in proportion. When the thermometer 

 rises to 60 For 15.6C, use 0.981; for 70 F, or 21. 1 C, use 0.962; 

 for 40 F or 4. 4 C, use 1.020, and other situations proportionately. 

 Refraction is minus if a deduction is to be made from an observa- 

 tion as in leveling, but plus if a setting is to be made for locating a 

 celestial body as in solar work. It has no effect on azimuth. 



170 



