23i 



Royal Society :• 



with inclinometers of the later English pattern with that of the 

 instruments of earlier construction, four of the latter were selected, 

 viz. a 9-inch circle by Robinson, a 9-inch circle by Barrow, and two 

 G-inch circles by Robinson, all in good order. Each circle was 

 furnished with two needles of the same length as the diameter of the 

 circle, and read by a lens in lieu of verniers and microscopes. Table IV. 

 contains the particulars of 20 determinations made with these instru- 

 ments in 1860 by Messrs. Stewart and Chambers. Their mean 

 result is 68° 20'*04 reduced to the epoch 1860.5, and corrected for 

 annual variation. The mean result of the 71 determinations at the 

 same epoch in Table III. is 68° 1 9' # 8. There is therefore no notable 

 difference in the mean results obtained by the two classes of instru- 

 ments ; but there is a considerable difference in the probable error ; 

 as from the. 20 determinations in Table IV. we obtain +3''65 as the 

 probable error of a single determination with the instruments of the 

 earlier pattern, whilst + 1'*5 has been shown to be the probable error 

 when inclinometers of the more recent pattern were employed. 



Table IV. — Observations of the Magnetic Dip, at the Kew Observatory, in 

 I860, with 9- and 6-inch Circles (Robinson's and Barrow's), without Verniers. 



6 

 



Circle. 



0) 



u 



D 

 > 



s 



to 



O 



si 



S 







O 





 N 







IS 



o^ 1 



i860. 









/ 





/ 



/ 



Mar. 16 



Robinson's 9-inch 



1 



Mr. Stewart. 



68 29-4 



-o-8 



o'o 



68 28-6 



17 



» 



1 



>» 



24-7 



o-8 



O'O 



23-9 



17 



»» 



1 



?> 



i8'o 



o-8 



O'O 



l 7 '2 



19 



it 



2 



» 



14*2 



o*8 



O'O 



13*4 



19 



>> 



2 



>> 



13-5 



o'8 



O'O 



127 



*9 



»> 



2 



» 



22*7 



o-8 



O'O 



21'9 



21 



Barrow's 9-inch 



1 



» 



n*o 



o-8 



O'O 



IO*2 



21 



>> 



1 



JJ 



i6- 5 



o-8 



O'O 



I 5 '7 



21 



>> 



2 



»» 



24* 



o-8 



O'O 



23'2 



21 



Robinson's 9-inch 



1 



Mr. Chambers. 



i6'6 



o'8 



O'O 



15-8 



21 



» 



2 



>> 



21-8 



o-8 



O'O 



21'0 



22 



Barrow's 9 -inch 



2 



Mr. Stewart. 



26*5 



o-8 



O'O 



257 



22 



» 



2 



Mr. Chambers. 



23'2 



o-8 



o-o 



22*4 



22 



„ 



1 



„ 



18-0 



o-8 



O'O 



17*2 



May 2 



Robinson's 6-inch, No. 1 



1 



„ 



26*3 



°'5 



+o'8 



26-6 



4 



» 



2 



>» 



3°7 



°'S 



+o-8 



31*0 



4 



Robinson's 6-inch, No. 2 



1 



>> 



18-1 



0-5 



+o-8 



18-4 



4 



» 



2 



?' 



17-5 



-0-5 



-fo'8 



i 7 '8 



Nov. 26 



» 



1 



Mr. Stewart. 



19-0 



+n 



-o-8 



19-3 



26 



» 



2 



>» 



68 18-5 



-f-i-i 



-o'8 



68 i8*8 



July 



1, i860, mean of 20 obs 



srvati 



ons 









68 20*04 











The observations were all made in the plane of the magnetic meridian. 



The values obtained for the Dip at the epochs of 1821 and 1854, 

 having been derived from observations made at the close of August 

 and beginning of September in those years, require a small cor- 



