THE SOLAR ATTACHMENT 



N ORDINARY compass surveys, the elimination of 

 theeffectoflocalattractionisan elementary considera- 

 tion, but in the azimuth method an arbitrary assump- 

 tion that the first course is free of such inflt: 

 must govern the entire calculation; so that we have 

 an undetermined error applied to every deduction 

 unless the True Mi ri Jinn is previously established 

 at the first station by means of a solar attachment, 

 of which the Saegmuller type is the best known.* 



Unless such a meridian is first established, the sum of the 

 interior angles is no proof of the accuracy of the work, even though 

 it should equal the proper theoretic aggregate. Empirical formulae 

 for the declination of the needle at any given place are not always 

 reliable, and it is better that the needle's declination should be 

 determined by comparison with the Solar Meridian, which is readily 

 and accurately obtained as hereinafter set forth. 



The Solar Attach mentis a telescopic device mounted equatorially 

 at the top of the Surveyor's Transit for the more accurate determina- 

 tion of the meridian by an observation on the sun. The cut, 

 p. 158, shows the latest improved model with a 12-power, interior 

 focusing telescope mounted in standards that are a part of a > 

 tinuous metallic frame. The arch above is of sufficient height to 

 permit inclinations of the solar telescope of somewhat more than 

 23 30' to the axis of the main telescope. 



The solar telescope is mounted so as to revolve in a/imuth 

 about its "polar axis", which is adjusted to perpendicularity \\ith 

 the sight line of the main telescope as well as with the horizontal 

 axis of the transit, as hereinafter described. 



At the top of the telescope tube is mounted a sensitive level 

 vial which dispenses with the necessity for a specially contrived 

 declination arc. T- -t sights mounted at the ends of the 



bubble tube are used to direct the solar telescope toward the sun. 

 When the shadow of one is cast upon the other, the sun is in the 

 field of view. The "hour circle" furnished with the earlier models 

 of this attachment is therefore another needless elaboration. 



SteS*rvtyt*f Manual, Ptnct & Kttckmm, JM, *., 1909, p. 127. mh,> 

 Amer. Civ, En*. P<xket Book 1st Ed., 1911, p 



157 



