304 MAdM.TIvM 



is l) r The needle is now turned oxer and hung : 



telesc-ope is moved round >o that again tip of 8 appeal- on 



the cross wire, and the telescope xernicrs are again i 



say, D. 2 . The magnetic axis bi>. ct> the two directions given bx the 



telescope readings, and corresponds to a reading J ( l)j -f l) t ). 



For simplicity we shall suppose that by prcxion- (ions 



on the sun >omc object on the hori/on at a d: 



on as the geographical South point. The t< is turned so 



that this point is on it* CTottirirt and thevern M K a 



mean reading S, say. ANY have then the declinati. 



1 - S. The instrument is proxidid x\ith apparatus h\ 



which a sun ohservat ion ni.ix I) taken and the reading of the South 

 point on the circle r may be directly determined, l>nt into tin- ue 

 shall not enter. The determination of the declination bx diffi 

 instruments* may differ bx -< \c ial minutes. 



To find the dip or'inclination. The ideal is for 



the- determination of the dip \\ould consist of a needle with 



axis of figure- coinciding with the magnetic a\ 



freelv round an axis through its , ,\it\. this .-n 



.supported so as to be hori/ontal and perpmdicular to the plai 



the magnetic meridian. A \ertical d would be placed 



in the magnetic meridian with its rent re in tin 'ion 



of the mcdle and its /ero leading in the hori/ontal thm 



Kis, the ends of the needle King just in trout of th< 



The reading of cither end of tl, would then gixc the dip. 



In practice, of com il conditions ar more 



or less nearly approach. d. Th, 1\ of a 



vertical circle contained in a rectangular box which is gla, 

 and back, and it is mount, d -<> as to turn round an axis which 

 be adjusted to be vertical by a level and lexelliug screws. There 

 is a hori/ontal dix idcd circle round the axis \s Inch gives the a/imuth 

 of the vertical circle. The \ ne'e, shown without the box 



in Fig. !W(>, i> divided from 0at the ends ot th 



KIO. 225. 



to 90 at the ends of the vertical diam. g. 225, is 



a flat plate of steel from 3 in. to (> in. long, with sharply poiir 

 and an axis is ti\ed as nearly through the centre nfgrax it\ as p<>- 

 and perpendicular to the plate. Tin- thin cyhndiical steel 



rod projecting on both sides of the plate, and is a> truly circular in 

 section as it can be made. Almost level with the centre of the 

 vertical circle are two hori/ontal and parallel agate knife edges, on 

 which the axis of the needle itself can roll, the central line of that 



* Thorpe and Rttcker, Phil Tro**., A, vol. clxxxriii. (1896), p. 1. 



