ON THE MAGNETIC INTENSITY OF THE EARTH. 81 
lutely the highest. If we suppose the minimum to reach 
0°74, (one of M. Erman’s sea observations is 0°743) and the 
every station with its face to the face of the instrument, and that at some of the 
- stations it was also vibrated in the reverse position. Where this has been done 
there often appears a considerable difference between the times of vibration 
at the same place in the two positions, which must be ascribed to instrumental 
defect. It does not appear to have been of the nature of a constant error in 
either position of the needle, as sometimes one position gives the highest inten- 
, Sityand sometimes theother. I have taken the twelfth column just as it stands, — 
that is, the times of vibration in the position which was everywhere observed, 
as there can be no question of the comparability of those with each other ; and 
T haye reduced the times of vibration to an uniform temperature by the coefli- 
cient which Mr. Christie found for that needle; but I have introduced no other 
corrections, either for loss of magnetism or on any other acconnt. I have grouped 
the results by taking the mean latitude, longitude, and intensity of the neigh- 
bouring stations, connected by brackets. 
If the intensities are taken from a mean of all the observations at each of the 
stations, including those in the reversed, as well as in the direct position of the 
needle, the inferences drawn above are somewhat strengthened, as is shown in 
the following table :— 
Station. Lat. Long. Time of |Ther.'| Intensity. 
North. East. Vib. 
New York ......-..] 40 42 | 285 59 | 1:2857 | 69 | 1-000 
Group (a) .........| 53 20 | 257 47 | 1:2644 | 69 | 1-033 
: Group (2) ......4 .| 59 56 | 247 28 12607 | 80 1:045 
bo Fort Reliance..... | 62 46 | 250 59 | 1:2758 | 40 1002 
Pm Group (c) .........| 66 51 | 261 41 | 1:2857 | 70 0°999 
‘* 
_ Mr. Christie, in combining the observations at different stations and in differ- 
ba ent positions of the needle, has followed a somewhat different course, and has 
"arrived at somewhat different conclusions. With more perfect instruments,— 
with observations alike complete at all the stations,—and repeated at New 
York as well as in London, to test the permanency of the needle’s magnetism,— 
_ there would not have been room for any difference of view. The only result 
absolutely deducible from the observations, and in which all persons must 
_ agree, is the comparability of the intensities at the different stations of the 
© northern group with each other, and with Fort Reliance ; as the observa- 
_ tions of May and October, 1834, show by their agreement that during 
_ that interval the needle underwent no change. The conclusion to be drawn 
from this portion of the observations, which are as strictly comparable as 
_ the imperfection of the instrument permits, is, that in the district which it 
comprises no consistent alteration takes place in the intensity. If any small 
alteration does take place, it would require a more delicate instrument than’ 
| Capt. Back was furnished with to determine it. 
It is in these countries that the statical method of Professor Lloyd would be 
_ of the greatest advantage. I have already had occasion to speak of the disad- 
vantage to which the method by horizontal vibrations is exposed in countries 
of very high dip, where every error in the dip is magnified to a high degree in 
__ its effect on the intensity deduced; and of the preference due in such cases to 
_ the vibrations of a dipping-needle. But it is well known that this latter method, 
_ though a trust-worthy, is far from being a delicate test of differences of mag- 
me VOL. VI. 18d7. G 
