28 SEVENTH REPORT—1837. 
ably in their rates of vibration are so kept together, it does not 
unfrequently happen that the weaker needle acquires magnetism, 
and the stronger loses it; and such appears to have been 
the case in this instance. It was not until 1829 that Nos. 3 
and 4 were put together, having been previously paired in a 
similar manner with other needles, whose magnetic strength in 
both cases very nearly coincided with their own. It is pro- 
bable, therefore, that the one began to lose and the other to 
gain from that time forth ; and that the whole gain or loss took 
place in the earlier portion, rather than equably throughout 
the interval from 1829 to 1836. 
When needles are so kept together in pairs, the two should 
be employed on every occasion, and their combined result 
should be regarded as one determination. Mr. Douglas never 
employed them singly. If in such cases the gain of the one 
needle were exactly proportioned to the loss of the other, the 
results of the two needles taken separately would differ, but 
combined would furnish a mutual compensation. In the pre- 
sent case the gain and loss, though not identical, were so 
nearly equal, that by taking a mean between the London 
rates of each needle in 1829 and 1836, and combining at Lon- 
don and at the other stations the results of the two needles into 
one determination, we obtain the values of the intensity as they 
would have been given by a single needle whose magnetism 
had undergone little or no change. 
The intensities thus calculated by needles 3 and 4, for the 
Sandwich Islands and the stations in North America, are al- 
most identical with those computed from Nos. 5 and 6, taken 
jointly in the same manner, using the London rates which they 
had before they left England. These needles have been 
sought for in vain amongst Mr. Douglas’s effects sent to En- 
gland ; their steadiness, therefore, can only be judged of from 
a comparison of their results with those of Nos. 3 and 4. 
The special objects of Mr. Douglas’s mission leading him 
in excursions on foot into the interior of the country, in Cali- 
fornia, and on the rivers tributary to the Columbia, the use of 
the horizontal needles was the only service he could there ren- 
der to magnetism, as the dip circle was not sufficiently port- 
able to be taken with him. There are 18 stations at which 
he used the horizontal needles alone, between 34}° and 543° 
N. lat., and all nearly on the same meridian, viz., between 119° 
and 124° W. from Greenwich. The only absolute deduction 
in these cases is that of the horizontal intensity. In deducing 
the total force from its horizontal component, the dip employed ~ 
must necessarily be computed from the dips observed at other 
