FIFTH REPORT-1835. 
142 
When the observations of dip made at the same station with 
different needles are compared together, it will be found that 
they are by no means in accordance. Thus the dip at Limerick 
in November, 1833, deduced from four observations with a needle 
on Meyer’s principle, was 71° ll'*7j while the mean of five ob¬ 
servations with needle L (1) at the same place and in the fol¬ 
lowing year was only 70° 59'*5, differing from the former by 12'. 
When from this difference the amount of the annual change is 
deducted, the remainder appears to be greater than can be fairly 
ascribed to the errors of observation. But these discrepancies 
in the results given by different needles have been placed in the 
strongest light by the recent observations of Captain James 
Ross in London. In these observations, which were undertaken 
with the view of determining the amount of the annual decrease 
of dip at London, eight different needles were employed, and 
from eight to ten observations were made with each, the result 
of each separate observation being a mean of eighty readings. 
The results were as follow : 
Needle B (1), Admiralty . . 
(10 obs.) . . . 
dip = 
o 
= 69 
1*5 
Needle L (1). 
(9 obs.) . . . 
- 69 
6-3 
Needle S (1). 
(8 obs.) . . . 
- 69 
11*3 
Needle J .......... 
(10 obs.) . . . 
- 69 
16-1 
Needle R. 
(10 obs.) . . . 
• 69 
18-9 
Meyer’s needle. 
(8 obs.) . . . 
- 69 
19*6 
Needle B (2), Admiralty . . 
(10 obs.) . . . 
- 69 
21-8 
Needle P ......... , 
(8 obs.) . . . 
- 69 
42-6 
Mean 
dip = 
= 69 
17*3 
Thus it appears that there is a difference amounting to 41' in 
the results of two of the needles used; and that this difference 
is very far beyond the limits of the errors of observation will 
appear from the fact that the extreme difference in the partial 
results with one of these needles (B (0) does not amount to four 
minutes and a half, while with the other (P) the extreme differ¬ 
ence is only two minutes. In fact, it so happens that these 
very needles which differ most widely in their mean results, are 
those in which the accordance of the partial results is most 
complete. Of the eight results obtained with needle (P), there 
is one only which differs from the mean of the eight by a single 
minute; and yet the mean of all the observations with this 
needle differs by more than 20' from the mean of any of the 
others, while its excess above the mean of the entire series 
amounts to 25'. 
These differences cannot be ascribed to any partial magnetism 
in the apparatus, for three of the needles (J, P, and R) were of 
