18 



A MANUAL OF TOPOGRAPHIC METHODS. 



measuring - the differences of zenith distance at culmination of two stars 

 which culminate on opposite sides of the zenith. 



Longitude is determined by telegraphic comparison of local time at 

 two stations, the longitude of one of which is known. This involves the 

 determination of the errors of the clocks or chronometers used, which is 

 done by observation of transits of stars across the meridians of the places of 

 observation. 



ASTRONOMICAL TRANSIT AND ZENITH TELESCOPE. 



A single instrument is used for the determination both of latitude and 



time. This is a combination of 

 the transit and zenith telescope. 

 The instruments in use upon the 

 Geological Survey were made by 

 Saegmuller and embody the latest 

 improvements in these combined 

 instruments, (hie of them is fig- 

 ured herewith. The circular base 

 rests upon three leveling screws. 

 Upon this circular base the whole 

 instrument can be made to re- 

 volve when using it as a zenith 

 telescope A circle is graduated 

 around the base, having a microm- 

 eter screw for slow motion, for 

 making settings and adjusting the 

 instrument in azimuth. The frame 

 of the instrument is cast in one 

 piece, and the standards are hol- 

 low in order to reduce the weight 

 of the upper part of the instrument. 

 The telescope lias a focal distance 

 of 27 inches and a clear aperture 

 of 2.5 inches. Its magnifying 

 power with diagonal eyepiece is 74 diameters. The length of the axis of 



Fin. 1.— Astronomical transit and zenith telescope. 



