296 VARIATION' OF THE COMPASS. 



have been delected ; but during the whole series, no material 

 difference occurred between the observations made in the 

 hou>e belbre, and after those 'aken in the yard. 

 —by obscrva- The observations therefore made in the yard, compared 

 tions made out with those taken in the house both before and after those taken 

 ■of doors, and .r-r ii • r i • ■ , i 



compared with ^"'^ ^" '^» tormed the comparison tor obtaining the error, or the 

 those within, effect of the iron-work of the room on the needle in the hotise, 

 and there is reason to believe that considerable accuracy has 

 been obtained. They arc as follow : 



By a mean of 20 sets, or 200 obsefvations taken wnth the 

 compass in the yard, compared with twice that number taken 

 in the house, before and after those taken in the yard, ths 

 variation observed in the house was found to be greater than 

 that observed in the yard by 5', 4. The mean of nine sets of 

 observations taken in the morning giving for the error 5'f5 ; 

 and the mean of eleven taken in the afternoon giving for the 

 error 5', 3. 7'he variation in those tables have therefore been 

 lessened by the above-mentioned quantity 5',+, as the error 

 for the effect of the iron-work of the room on tha needle in the 

 house. 

 Coroborating I must not omit to mention, that of these 20 sets of obier- 

 racv^^ ^° 2CCU- yjjtjQ,-,^ mentioned above, nine only were made with the com- 

 pass in the same situation, and eleven in that of a different one; 

 for, after nine sets had been taken, a pile of boards was put up 

 between the compass and the mark to which it had been ad- 

 justed, which made it necessary to remove the post on which 

 the compass had been placed, a few feet to the westward of its 

 former situation, to clear it from the said pile of boards; and 

 eleven sets of observations were made from this new station, 

 with the compass adjusted to the same mark it had been ad- 

 justed to before, and the angles that this mark made with the 

 true meridian from each of these stations, were ascertained by 

 placing a transit-instrument precisely where the compass had 

 been placed, and observing the transits of the sun and stars, 

 in the same manner as has been described in finding the angle 

 of the mark that the compass was adjusted to in the house. 

 And it Is conceived that this accidental circumstance adds some 

 Weight to the accuracy with which these operations were per- 

 formed, as the error from the two results of nine, and eleven, 

 does not differ so much as 0,5 from each other. 



Dipping 



