122 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
We have now to notice the two needles Nos. 6. and 8, which have not been in- 
cluded in the intercom parison which the others have undergone. 
By means of the horizontal intensity derived for each station from the mean of 
Nos. 5. 7- 9- 11. 12. and 13, and the observed times of vibration of No. 8. at the several 
stations, the corresponding times of vibration which that needle would have had at 
the respective periods at Woolwich have been computed, and are as follows : — 
Mazatlan, November 1839 . 677‘7 
San Bias, December 1839 . . 678*5 
Martin’s Island, January 1840 678*8 
Bow Island, March 1840 . . 679*6 
Tahiti, May 1840 678*8 
Nuknlau, June 1840 . . . 679*6 
New Ireland, July 1840 . . 680*7 
Jobie Island, August 1840. . 680*2 
Amboyna, September 1840 . 680*7 
Macassar, September 1840 . 681*8 
Singapore, October 1840 . . 68 1*6 
Singapore, December 1841 . 682*9 
Penang, December 1841 . . 683*2 
Point de Galle, January 1842. 683*0 
Seychelles, February 1 842 . . 683*0 
Madagascar, March 1842 . . 684*6 
Cape of Good Hope, April 1 842 684*3 
Ascension, May 1842 . . . 685*1 
Woolwich, October, December 686* 1 
The progressive increase in the times of vibration indicates that No. 8. was con- 
tinually parting with small portions of its magnetism, a condition, which, when it 
occurs in one needle amongst many, renders that particular needle of less value in 
general deductions than those which have the character of general steadiness with 
only occasional loss. Corrections might be assigned for No. 8, derived either from its 
comparison with the other needles, or on a supposition of uniform loss in reference 
to time or to occasions of employment, but the latter could only be regarded as 
approximate, and the former would add no independent value to the general con- 
clusions. The observations with No. 8. therefore are given in the Tables, but no de- 
ductions have been made from them. 
The derangement in the magnetic state of No. 6, which took place in a journey to 
the summit of the volcano of Conchagua in December 1838, and its subsequent un- 
steadiness, have been already noticed in No. II. of these Contributions. The mag- 
netism of this needle continued unsettled for the whole remainder of the voyage, 
affording an instructive example of the extent of injury which an exposure to unfa- 
vourable circumstances may produce in a long-tried and valuable needle ; the one in 
question having been in use during the twenty-one previous months without under- 
going apparently the slightest change in its magnetism. It unfortunately happened 
that No. 6. is one of two needles, No. 5. being the other, which Sir Edward Belcher 
employed throughout his voyage at stations where time did not permit him to use 
more than two; and it is desirable therefore to draw from the observations with 
No. 6. at such stations, all the evidence they are capable of affording. For this pur- 
pose I have made the stations at which several needles were used base stations, for 
the deduction of the force with No. 6. at any intermediate place where Nos. 5. and 
