SHIFTING OF THE EARTH'S AXIS 



425 



The four stations first established, Mizusawa, Carloforte, Gaithers- 

 burg and Ukiah, are provided with zenith telescopes of exactly the 

 same pattern, and constructed especially for this work of observing 

 latitudes by the Talcott method. 3 



These instruments, illustrated in Fig. 4, were made by Wanschaff, 

 of Berlin, and have objectives of 4^ inches aperture and focal lengths 

 of fifty-one inches. The instru- 

 ments at Tschardjui and Cincin- 

 nati are of similar design by the 

 same maker, but smaller. From 

 the figure it may be seen that 

 the telescope is fixed perpendicu- 

 lar to the end of a horizontal 

 axis. By placing this axis in an 

 east and west direction the tele- 

 scope will move only in the plane 

 of the meridian as the horizontal 

 axis is rotated in its supports. 

 The whole instrument may be re- 

 volved about the vertical axis, 

 m, and by properly adjusted stops 

 on the base-piece the amount of 

 rotation may be limited to 180°, 

 thus giving two east and west posi- 

 tions for the horizontal axis, 

 one, telescope east, the other, tele- 

 scope west. It is readily seen that 

 if the telescope is set to point 

 say 10° north of the zenith when 

 east of the vertical axis, then, 

 without disturbing the setting of 

 the telescope, if the whole instru- 

 ment be revolved about the ver- 

 tical axis, the telescope will, 

 when it comes into the posi- 

 tion west of the axis, be pointed 10° south of the zenith. It is 

 thus possible to measure the difference of zenith distance of two stars, 



3 Descriptions of this method may be found in any work on practical astron- 

 omy. The following statements concerning the method may be of help to those 

 who are not familiar with its details. In order to make a determination of the 

 latitude by this method it is necessary to measure, by means of an eye-pieec 

 micrometer attached to the zenith-telescope, the difference of zenith distance of 

 two stars of known declination which culminate at nearly equal zenith-distances, 

 one north of and the other south of the zenith. The telescope is set at the mean 

 of the zenith-distances of the two stars and the first to culminate will pass a 

 little above or below the middle of the field of view. The distance from the 



Fig. 4. 



