March 17, 1870] 
NATURE 
517 
SOCIETIES AND ACADEMIES 
LONDON 
Royal Society, March 3.—The following papers were read : 
“Results of the monthly observations of dip and_ horizontal 
force made at the Kew Observatory from April 1863 to 
March 1866, inclusive,’ by Dr. B. Stewart. The author 
exhibited tabular statements of dip observations during six 
years, from which was deduced the existence of a semi- 
annual inequality, in virtue of which the dip is on an average 
o'27 lower in the six months from April to September, and 
0:27 higher in the six months from October to March than 
is due to its mean value. This result is in the same direction as 
that found by Sir E. Sabine for the six years ending March 
1863, but is less in amount than the latter, that determined from 
the first six years exhibiting a range of 1'-31, while that deter- 
mined from last six years only exhibits a range of 054. From 
the first six years we deduce a mean dip equal to 68° 20''07, cor- 
responding to middle epoch April 1, 1860, and from the latter 
six, a mean dip equal to 68° 6°62, corresponding to middle epoch 
April 1, 1866, while the secular change deduced from the first 
series is 2’00, and that deduced from the last series is 1’-92, the 
mean of these two values being 1'°96. If we apply this mean 
value of the secular change to the mean result corresponding to 
the epoch April 1, 1860, in order to bring it to the epoch 
April r, 1866, we obtain— 
68° 20''07 — 11°76 =68° §'°31, 
whereas that deduced from the second series corresponding to 
this epoch is 68° 6°82. The former of these is 1'-69 higher than 
the latter. As regards the reason of this difference the author 
does not think it due to any personal equation in the observer. 
It would appear that the Kew observations present a peculiarity 
similar to those at Toronto, so that the difference of 1'*69 between 
the two sets of observations may probably be accounted for by 
this cause. The probable error of a single monthly determina- 
tion of the dip, derived from the seventy-two monthly determi- 
nations, and after the application of the correction for secular 
change and annual variation, as derived from the results of 
these observations, has been made, is +0''96. There is, 
however, reason to believe that this probable error is 
increased to some extent by periods of disturbance, some 
of these of considerable duration. In 
of horizontal force during the first six years, mean 
value of the horizontal force is equal to 3°8034, cor- 
responding to the middle epoch April 1, 1860, and from the 
latter six years’ observations we have a mean value of horizontal 
force equal to 3°8360, corresponding to epoch April 1, 1866 ; 
also the secular change deduced from the first six years is 
+0053, while that deduced for the second six is + ‘0055, the 
mean of the two being +'0054. If we apply this mean value of 
the secular change to the mean result corresponding to epoch 
April 1, 1860, in order to bring it to epoch April 1, 1866, we 
obtain 3°8034 + 0°0324 =3°8358, a value which agrees as nearly 
as possible with that deduced from the second series, and corre- 
sponding to the same epoch which was 3°8360. The coincidence 
of these two values naturally leads us to imagine that the secular 
shange of the horizontal force does not present the same peculi- 
arity as that observed in the dip. In the observations of total 
force the mean of the April to September values of the horizontal 
component of the force in the last six years is 3°8346, correspond- 
ing in epoch to January 1, 1866; and the mean of the April to 
September values of the dip in the same six years is 68° 6''83. 
The mean of the October to March values are for the horizontal 
force 3°8372, and for the dip 68° 6'-41, corresponding to epoch 
July 1, 1866. We may reduce these to a common epoch by 
applying to the former dip the correction —o'96, this being the 
proportional secular change (as shown by these six years) neces- 
sary to reduce the former epoch to the latter. The former dip. 
will therefgre become 68°6'°83—0''96=68° 5'°87. Reducing in 
the same way the horizontal force, we have 
3°8346 + 000275 =3'83735. 
The values thus become as follows :— 
From the April to September obser- Hor. orce. Dip. 
vations (reduced to epoch July 1, 3°83735 68° 5/87 
HGS), <6 eta OO 6b 
And from the October to March ob- 
servations (corresponding to the 3783720 68° 6-41 
Snead, oO Go co oo 
the observations 
The total force derived from the first series will therefore be 
10°28717, and that derived from the second series 10°29080, 
showing thus a difference of 0°00363 in British units as the mea- 
sure of the greater intensity of the terrestrial magnetic force in 
the October to March period, than in the April to September 
period. This is in the same direction, and very nearly of the 
same amount, as that determined by Sir E. Sabine from the first 
six years, which exhibited a similar difference of 0°00317 in Bri- 
tish units. Thus we find that the two series agree in showing 
nearly the same semi-annual variation for the total force, while 
the first period exhibits the greatest semi-annual variation of the 
dip. It ought, however, to be borne in mind that the two series _ 
bear a different relation to the disturbance period, the maximum 
of disturbances occurring about the middle of the first series, and 
the minimum near the middle of the second. 
‘« Spectroscopic observations with the great Melbourne Tele- 
scope.” By A. Le Sueur. The author stated that the 
spectroscopic observations of the nebula of Orion show distinctly 
that considerable nebulosity exists within and about the 
trapezium. The author’s ‘e/escofic observations reveal a posi- 
tive though comparatively faint nebulosity within and about the 
trapezium ; the spectroscope, however, shows with much force 
that this nebulosity not only exists, but is comparable in bright- 
ness to that surrounding the trapezium at some distance,—the 
brightest part ofthe nebvle in fact; that, in ordinary observa- 
tions, therefore, the faintness or apparent complete absence ot 
nebula is mainly due to the disturbing brightness of the four stars, 
not to any intrinsic extreme faintness or absolute vacuity. 
In the examination of Jupiter, the large size of image is brought 
into prominent play; with the original Cassegrain image the light 
is barely sufficient, but with the image condensed (at pleasure 
within certain limits) fair work becomes possible, the spectrum 
being considerably bright. The lines G, F, 4, C, D are seen 
without the slightest difficulty, and many other lines with atten- 
tion. A marked feature isa nebulous band between C, D ; from 
measures this turns out to be one of the bands examined by Mr. 
Huggins, 882 of his scale (Cy of Brewster ?). With the slit per- 
pendicular to Jupiter’s equator and theadvantage of a large image, 
an admirable opportunity is afforded of noting the behaviour ot 
the lines as they cross the different parts of the surface, a spectro- 
scopic picture of the planet, as it were, being presented beauti- 
fully totheeye. The nebulous line C, was specially and narrowly 
watched, but without any satisfactory evidence being elicited ; 
It is found that the spectrum of 7 Argo is crossed by bright 
lines. The mere fact of a bright line spectrum is not very diffi- 
cult to ascertain on a good night; for although from faintness 
of the light the phenomenon is necessarily delicate, yet the bright 
lines occasionally flash out so sharply that the character of the 
spectrum cannot be mistaken, The most marked lines were 
made out to be, if not coincident with, very near to C, D, 4, F, 
and the principal green nitrogen line. It cannot be determined 
whether the coincidence is with the magnesium group or the air- 
band; nothing more definite can be said than that the star line 
lies within the limits of the group. The comparison spectrum 
employed does not show F, but the position of the previously 
adjusted pointer, with reference to air lines in the neighbour- 
hood, leaves little doubt as to the identity of the blue star line 
with F, due regard being had to the collateral evidence (when 
such close limits are reached) that C coincides with a red star 
line. The yellow (or orange?) line in the star has not yet 
received sufficient attention, it is, however, very near D. With 
the dispersion employed, D and the bright air line on the less re- 
frangible side of D are well separated; so that, notwithstanding 
the delicacy of the star line, the author hopes if not to get satis- 
factory evidence of coincidence with a particular line, at least to 
eliminate one of the competitors ; at present it cannot even be 
said whether the line may not be slightly more refrangible than 
D; the limits are, however, very small, placing the bright air 
group about 1,180 of Mr, Huggins’s scale completely outside 
the possible range. The very faint nebulosity (if any) in the 
immediate neighbourhood of the star 7 is incompetent to give a 
trace of spectral lines with even a wide slit; for a considerable 
space S and / of 7 no lines at all are visible ; the nearest nebula 
bright enough to show a line (the three usual lines are now easily 
seen on a good night over the brighter parts) is reached in the 
direction about 45° N p from 7, and even then the distances from 7, 
as judged bythe appearance in the spectroscope with threaded on 
the thus directed slit, is little less than one minute. This remark is 
of some importance in connection with the ordinary telescopic 
observations of the nebula, but is mentioned at this point to 
