^ 



120 



M A T H E M A T 1 C A L and 



r 



way, as by a fore obfervation, by one central Ipeculum 

 and noted; let the index be pufhed dill farther along the 

 arch and the image of the Sun will again be brought down 

 to the horizon by the other central fpeculum, which affords 

 another obfervation of the altitude to be noted alfo; count- 

 ing from the end of the arch next to the obferver in the 

 firft cafe, and from the middle of it in the latter. Then 

 let the arch of the inftrument be held upwards, and the 

 center downwards, and the index be moved the contrary 

 way; this will give two other altitudes. The mean of any 

 two of thefe obfervations that depend on the fame glaffes 

 fives the true altitude free from the abovementioned errors. 

 The fame may be faid of taking any other angles. 



The inverting of the inftrument is not neceffary in tak- 

 ing angles, when it is indifferent which of the objeds is 

 brought to the other by refledion ; as in meafuring the dif- 

 tancc between two ftars. But when one of the objects is 

 brighter than the other, it is neceffary to bring the bright- 

 er to the other by reflexion, in that cafe it is neceffary to 

 invert the double fextant. In other cafes it will be found 

 more convenient to make all the obfervations, by only mov- 

 ing the index both ways. 



When the diftance of two objeds is continually chang- 

 ing, and expedition is neceffary in the obfervation; two 

 or more pieces of brafs fliould be made to Hide on the arch 

 of the inftrument, that the degrees noted by the index: 

 may be marked, by bringing one of them up to the index 

 and fcrewing it faft to the arch, where it mufl remain, un- 

 til all the obfervations are made. In the fame manner 

 may all the obfervations but the laft be marked; that no 

 tune may be loft in reading oft^ the degrees and minutes 



and writing them clown. 



Wh 



pleted, they may be read off, by bringing the index dole . 

 up to the abovementioned pieces, and written down at 



leifure. 



That the moving the index backwards, will give the alti- 

 tude of the Sun or ftar above the horizon, when the arcl 



1 



hangs 



