INTRODUCTION. 45 



fully. In this difficult observation, time must 

 be taken to give the body as firm a position 

 as possible. It is generally advised, in taking 

 the distances from the sun, to look directly 

 at the moon, and to let the sun be reflected ; but, 

 with a good sextant, provided with proper tele- 

 scopes, it is better not to make any particular rule, 

 but only to consult the convenient position of the 

 body, and never to neglect the indication of the 

 index correction, on the sextant : the observer must 

 take particular care to bring the edges of the sun 

 and moon in contact, without allowing either an 

 interval or a coincidence ; for which purpose the 

 powerful magnifiers are very serviceable. It is a 

 considerable aid to the observer, if the solar and 

 lunar altitudes are measured at the same time with 

 the distances. For making the calculations, Men- 

 doza's tables are to be preferred to all other aids 

 and methods. 



Next to the determination of the latitude and 

 longitude, the variation of the magnetic needle 

 calls for the attention of the navigator. Without 

 going into the methods of observing and calcu- 

 lating the azimuth, we merely observe that the 

 local influence of the larger or smaller masses of 

 iron on board the ship cannot be avoided, but by 

 the great number of observations in different parts 

 of the ship ; that the point of the pin which carries 

 the needle, must be now and then examined and 

 sharpened j and sometimes, to increase the magnet- 

 ism of the needle, it must be rubbed on a powerful 



