122 
FIFTH REPORT— 1835. 
barometer was noted, as w T ell as the temperature; the hour of 
observation was,, nearly, 10 a.m. 
Time of 200 Vibrations of a Standard Needle. 
Date. 
Barom. 
Therm. 
Time. 
inch 
o 
s. 
June 15. 
30-260 
66-0 
1139-60 
16. 
30-268 
64-5 
1138-20 
17. 
30-188 
62-0 
1138-27 
18. 
30-188 
61-0 
1135-87 
19. 
30-200 
59-0 
1136-67 
20. 
30-080 
62-0 
1135-93 
21. 
62-0 
1137*67 
22. 
29-640 
61-0 
1138-53 
23. 
29-682 
59-0 
1138-40 
24. 
29-360 
57-5 
1137-80 
25. 
29-850 
57-0 
1136-87 
26. 
29-580 
57-0 
1136-70 
27. 
30-165 
57-0 
1137-13 
29. 
30-120 
59-0 
1137-80 
30. 
29-820 
58-0 
1137-73 
July 2. 
29-900 
62-0 
1137-30 
4. 
29-850 
60-5 
1138-73 
5. 
29-650 
60-0 
1139-53 
6. 
30-000 
58-5 
1136-60 
7. 
29-850 
59-0 
1137-87 
The mean time of 200 vibrations, deduced from these results, 
is 1137^*66 at the temperature 60 0, 6. But the time observed 
on the 18th of June is 1135 f,, 87 at Ol^O; so that on this day 
the rate of the needle was less than the mean by a differ¬ 
ence which corresponds to an increase of *003 in the horizontal 
force. The observation of the 5th of July exhibits a difference 
somewhat greater on the other side. 
6. The last source of error which requires to be noticed under 
this head, is the change of the magnetic condition of the needles 
employed. Independently of the derangements of magnetic 
equilibrium induced by the presence of iron, or other disturbing 
causes, it is well known that most needles lose something of 
their original force. This loss is greatest at first; and the needle, 
if originally well tempered and then magnetized to saturation, is 
usually found to arrive at a nearly settled state in about a year. 
Most of the cylinders employed in the following observations 
seem to have reached that condition; and the changes of mag¬ 
netic state which they have exhibited are, except in the case of 
Cyl. S (Z>), unimportant. In order to detect any such changes, 
and to correct for them if they arise, it is necessary to observe at 
