VOL. II.] PHILOSOPHICAL TRANSACTIONS. 15$ 



The Directions for Seamen explained. N" 24, p. 433. 



The particulars themselves follow : 



1 . To observe the variation of the compass or needle. 



At land, where by the help of good fixed dials and other fit instruments, the 

 precise meridian of the place may be known, it is easy to find the variation of 

 the needle divers ways : As by applying the needle, &c. to the shadow of a 

 thread hanging perpendicular, when the sun is in the meridian ; or to the meri- 

 dian line or to the side of a fixed horizontal dial, &c. 



But at sea, as the meridian is not so easy to be found to any tolerable exact- 

 ness, to know the variation of the needle is much more laborious and difficult. 

 The height of the pole and the sun's declination being known, a large ring- 

 dial, having a compass or needle fixed to its meridian below, may go near to 

 show the variation required. For when it is set to the exact hour and minute of 

 the day, its meridian stands just in its due place ; and so shows how far the 

 needle varies from it. But because these dials are rarely just, they are not to 

 be relied on. The following method may therefore be employed. Find the' 

 sun's azimuthal distance from the meridian, some hours before or after noon ; 

 and then its magnetical azimuth or distance from the meridian pointed at by the 

 needle there, the difference of these two distances is the variation of the 

 needle. 



To find the sun's true azimuth, or by how many degrees, &c. of the horizon 

 it is distant from the meridian : its declination, its altitude, and the elevation of 

 the pole must all three be known. 



To do which accurately, constitute a spherical oblique angled triangle of the 

 three complements of the sun's declination of his altitude, and of the height of 

 the pole ; the measures of all the sides whereof are known ; one from the 

 zenith to the pole; another from the pole to the point of the sun's altitude; 

 and the third from that point to the zenith. From these find the angle at the 

 zenith, which subtract from 1 80, and the remainder is the sun's true azimuth 

 or distance from the meridian of the place. 



The true azimuth of the sun being thus found, and the magnetical azimuth, 

 according to your needle observed, subtract the less number from the greater,' 

 and the remainder is the variation of the needle. If the magnetical azimuth be" 

 less than the other, then the variation is on the same side of the meridian 

 with the sun ; if greater, on the other side. 



To find this variation by the stars is no more requisite than to find out the 

 true north, that is the meridian, and compare the needle's position with it. 

 By this means the variation may be had well enough to degrees, half degrees, 



VOL. I. U 



