'25'2 rHlLOSOPHICAL TRANSACTIONS. FaNNO J 738. 



the rim of the box, which the observer judges to be nearly equal to the height 

 of the sun or star whose azimuth is sought; for by this means the object will 

 be more readily found. 



3. Turn the quadrant round towards the sun or star, till it appear on the 

 vertical hair within the telescope, to an eye looking through the small hole or 

 sight; and then slide the index a little upward or downward on the arch, till 

 the object by this means be brought to coincide or touch the visible horizon. 



Lastly, The degrees and minutes then marked by the index on the arch of 

 the quadrant, will show the altitude of the object, which will always be the 

 same, whether the instrument be in motion or at rest; at the same time the 

 degree cut by the index on the horizontal rim, or circumference of the compass 

 box, will give the magnetical azimuth of the sun or star. 



How the variation of the needle is found by means of magnetical azimuth 

 and altitude thus obtained, is taught in every treatise of navigation. But as 

 the resolution of this problem is somewhat troublesome, and requires such a 

 knowledge of the doctrine of the sphere, as every seaman has not attained, an 

 easy method is here exhibited of discovering the variation of the compass with- 

 out any manner of calculation. 



1 . Let the magnetic azimuth of the sun, or any star, when it is near the 

 prime vertical, and considerably elevated above the horizon, be found according 

 to the directions already given, before it arrive at the nieridian, and note well 

 the altitude, or let the index remain fixed at the same point on the arch. 



2. Find the magnetic azimuth of the sun or star in like manner as before, 

 when it is exactly at the same degree of altitude, after it has passed the meri- 

 dian: and, 



3. If these two magnetical azimuths be equal, the needle has no variation; 

 if urrequal, add them together, and half their sum will be the true azimuth; 

 or subtract the less from the greater, and half the difference will be the varia- 

 tion required. The circumstances of the observation will the more readily dis- 

 cover whether the declination is easterly or westerly. 



N. B. Though it would be very commendable in gentlemen who use the sea, 

 to learn the names of most of the principal fixed stars, yet even that knowledge 

 is not necessary in the use of this instrument: neither is it needful in this case 

 to know exactly the latitude of the place of observation, provided the difference 

 of latitude between the observations be not very great; it is sufficient, that 

 care be taken to observe the self-same star, before it comes to the meridian, 

 and after it has passed it; and for the sake of greater exactness, the caution 

 before given should be regarded, viz. that the star be at some considerable 

 height above the horizon, and also near the prime vertical. 



