and solar telescope is equal to 90 plus or minus the declination, 

 or the sun's polar distance. 



3rd step. Without disturbing this relative position of the tele- 

 scopes, open the hori/ontal axis and set off on the vertical circle the 

 co-latitude of the place, w hich gives the instrument a relative position 

 as indicated in Fig. 95. At the equator, the main telescope would 

 he vertical and at the poles it would be hori/ontal. 



The polar axis is now parallel with the earth's polar axis pro- 

 longed, and a hori/ontal angle between the two telescopes, equal to 

 Xl'S, is the one sought. 



4th step. Open the a/imuth axis of the transit and the polar 

 axis of the solar. If in the northern hemisphere, point the main 

 telescope in a southerly direction. It is evident now that the sun's 

 image cannot be centered between the "equatorial" and "time" 

 lines in the solar telescope until the main telescope lies in the 

 meridian, Zl*. Maneuver the instrument by use of the tangent 

 screws on the vertical and polar axes only until the sun can be con- 

 tinuously bisected while following him in his path with the tangent 

 screw of the polar axis. 



No great speed is required in this operation. Ten or fifteen 

 minutes can be consumed if necessary without affecting the accuracy 

 of the observation. If there was no hourly change in the sun's 

 declination, the sun could be followed all day through the solar 

 telescope if the main telescope and polar axis were in the meridian. 

 During the middle of June and September this condition can be very 

 nearly reali/ed, but in March and September declination settings 

 have to be corrected carefully for each set of observations. 



If the instrument is provided with a compass, the needle may 

 now be allowed to swing freely and the magnetic \ariation observed. 



Latitude Determinations 



Latitude may be variously determined by observing the transit 

 of a Mar, by a mean altitude of polaris or by a direct obser\ation on 

 the altitude of the sun at apparent noon. 



() ing to the earth's annual motion in its orbit, the sun changes 

 his position along the ecliptic with resj>ect to the stars at a not al- 

 together uniform rate; so that some solar days are either longer or 

 shorter than others. 



For the reason that a chronometer could not conveniently be 

 made to change its speed to suit this solar phenomenon, there has 

 been established a uniform system of time called "mean solar time." 



168 



