2 



ASTRONOMY, 



[Sect. I 



10'^ 



ct. M 



'S 



without the means of verification, is worse than no ob> 

 servation at all. In order to make the observations good, 

 the following cautions must be observed :~ 



The index-error of the sextant must be carefully ascer- 

 tained. If it has not been found a short time before the 

 observations, it must be found as soon as possible after 



them. 



The distance of the comet from three conspicuous stai 



in different directions must be measured with the sextant. 

 The point of the comet which is observed with the sextant 

 ihould be precisely described. It is desirable that the 

 navio-ator should be possessed of some star-maps or star- 

 charts, bv means of which he will be able at once to give 

 the proper names to the stars, and much confusion and 



loss of time will be avoided. 



If the time at the ship and the latitude are very well 



will be no occasion to make further observa- 



known, thei 



•e 



tions; but if these are not w^ell known, some attempt 

 must be made (by the use of Becher's horizon, or by any 

 equivalent method) to ascertain the altitude of the stars 

 and the comet. The lower these objects are, the greater 

 must be the care in the determination of their altitude. 

 For affording means of verification, these rules should 



be followed : 



The observations of distance with the sextant should he 

 entered in the book precisely in the manner in which they 





re made. The reading of the 



uncor 



reded. 



should be written down : in a column by the side of this 

 should be written the correction for index-error, with a 

 statement whether it is to be added to, or to be subtracted 

 from, the sextant-reading : in the next column should he 



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