ASTRONOMICAL PAPERS. 



129 



X 



vens 



Kangs downwards, will appear very evident, by confide- 

 ring, that the image of the Sun is brought down to the 

 horizon, by puftiing the index from the obferver, and con- 

 fequently the image of the horizon is alfo funk as much 

 belowthetrue horizon; therefore, when the indexismoved 

 in a contrary direction or towards the obferver, the image 

 of the horizon is thereby raifed up to the Sun in the Hea- 



and their diftance is fhewn on the arch. But avS it 

 is requifite to bring the image of the Sun to the horizon, 

 by moving the index both ways, this is effe£led by invert- 



: inftrument; holding the arch downwards, while 

 one obfervation is made, and upwards while the other is 



made. 



The above illuftration is fufficient to anfwer all the piu^- 



pofes of ademonftration to fuch as are acquainted with the 

 theory and principles of this inftrument; as it (hews, that 

 the demonftriition is nearly the fame for the obfervations^ 

 made both with the arch hanging down, and with it invert- 

 ed. But as it may be defired by fome, 1 fhali infert the 

 demionftralion for the obfervation with the inverted double 



fextant, which will fhew more clearly the reafon of gra- 



in t^th 



duating the arch both ways from the middle. 



QAl 



QAR 



QR 



£> 



proved, tliat while the index moves from the polition 

 .QCA, to that of AFD, the folar image will move twice as 

 far from S, down to the horizontal line IDG, and will be 

 ken by the eye at I, in the horizontal linelG, parallel to 

 HO ; fo that the angle QAD Ihall be half of the ai 

 SFH, which is tlie Sun's altitude. 



Let SF be a ray of light from the Sun at S, falling on 

 the fpeculum at F, and from thence reflcded to the fpecu- 

 lum at G, and from thence reliedcd again to the eye at I, 

 where the folar imai^e will be ken In the horizontal line 

 IG; the fpeculum at G, being fct parallel to the line AQ^ 

 or to the larger fpeculum at F, when the index is at Q^or 



tl 



ic 



V. 



