VARrATION OF THE COMPASS. 295 



tween the said mark and the true meridian, was found by com- 

 putation to be 3 1 ° 8', 8 as above. 



2. For the purpose of ascertair.ing what error there might Method of as 

 be, from a want of parallelism between the line joining the ^,!u^/''i-"| of 

 indices and the magnetism of the needle, and thereby to de- magnetic di- 

 termine whether, in the usual method of observing, the in- '^^.[|°"' |'^c^^* 

 dices shew the true angle which the direction of magnetism needle 

 makes with the first division or zero, a great many observa- 

 tions were made "on both ends of the needle, and with both 



sides of the needle uppermost (the cap of the needle being 

 made to fit on readily on either face for this purpose), viz. 

 north end and south end in its upright position, and north 

 end and south end with the needle inverted, and the mean of 

 the four giving the angle greater by 2', than that shewn by 

 the north end in the upright position of the needle (which 

 was the end always used in these observations), two minutes 

 have been added to all the observations read from the instru- 

 ment, as the correction for this error to angles on the east 

 side of zero, and subtracted from angles on the west side, to 

 obtain the true angle; which error to angles on the west 

 side, however, only occurred when the instrument was taken 

 out of doors, to determine the effect of the iron work of the 

 building. 



3. The variation compass being placed in the house for ob ^and ofascer 



servation, could not be supposed to be entirely out of the in- fl^'^^^ce oTiron 

 flnence or iron; I was therefore desirous to a^certainhow far In the building; 

 that influence might extend: for the determination of which, 



the following method was adopted. 



Having caused to be sunk into the earth, to some depth, a 

 strong post, in the wood-yard of Somerset-House, at a j consi- 

 derable distance from the influence of any iron, on which the 

 compass might be placed, and from which station there was 

 a convenient mark, at a proper ditance, to which its telescope 

 might be adjusted : I took the compass there at those times 

 of the day v/hen the needle was stationary, viz. morning and 

 afternoon. Before the compass was carried out of doors, ob- 

 servations were made in the room ; then it was taken out of 

 <Joorr, to the above-mentioned station, for observation there ; 

 and the observations were again repeated, after the compass 

 had been restored to its situation in the room; so that had any 

 sUeration taken place in the interval, such alteration would 

 2 A 4 hav« 



