MAGAZINE OP SCIENCE AND ART. 
205 
tlie resistance being supposed to bear a sensible propor¬ 
tion to the gravitating action. The constitution of the 
central body of our own system presents a nearer and 
more interesting subject of speculation. Towards the 
close of the last century many hypotheses were advanced 
regarding the nature and constitution of the sun, all of 
which agreed in considering it to be an opaque body, 
surrounded at some distance by a luminous envelope. 
But the only certain fact which has been added to 
science in this department is the proof given by Arago 
that the light of the sun emanated (not from an incan¬ 
descent solid, but) from a gaseous atmosphere, the light 
of incandescent solid bodies being polarized by refroc- 
tioii , while the light of. the sun and that omitted by 
gaseous bodies, is unpolarkcd. According to the ob¬ 
servations of Sclvwabe, which have been continued 
without intermission for more than thirty years, the 
magnitude of the solar surface, obscixred by spots, 
increases and decreases periodically the length of the 
period being 11 years and 40 days. This remarkable 
fact, and the relation which it appears to hear to cer¬ 
tain phenomena of terrestrial magnetism, have attracted 
fresh interest to the study of the solar surface; and, 
upon the suggestion of Sit John Hcrschcl, a photohe¬ 
liographic apparatus lias lately been established at 
Kew, for the purpose of depicting the actual macular 
state of the sun’s surface from time to time. It is 
well known that Sir William Herschcl accounted 
for the solar spots bv currents of an clastio fluid 
descending from the bodv of the sun, and pene¬ 
trating the exterior luminous envelope. A some¬ 
what different speculation of the same kind has 
been recently advanced by 3f0ssoti, who has endeavou¬ 
red to connect the phenomena of the solar spots with 
with those of the red protuberances which appear to 
issue from the sun in a total eclipse, qnd which so 
much interested astronomers in the remarkable eclipse 
of 1842. . Next to the sun, our own satellite has al¬ 
ways claimed the attention of astronomers, while the 
comparative smallne3 of its distance inspired the hope 
that some knowledge of its physical structure could be 
attained with the large instrumental means now avail¬ 
able. Accordingly, at the Meeting of the Association 
held at Belfast in 1852, it was proposed that the Earl 
of Rosso, Dr. Robinson, and Prof. Phillips he requested 
to draw up a Report on the physical character of the 
moon’s surface,, as compared with that of the earth, 
That the attention of these eminent observers has been 
directed.to the subject, may be inferred from the com¬ 
munication lately made ’ by Prof. Phillips to the 
Royal Society on the mountain Gassendi, and the 
surrounding region. But I am not aware that the 
subject is yet ripe for a Report. I need not remind 
you that the moon possesses neither sea nor atmos¬ 
phere of appreciable extent. Still, as a negative, in 
such case, is relative only to the capabilities of the 
instruments employed, the search for the indications 
of a lunar atmosphere has been renewed with every 
fresh augmentation of telescopic power. Or such 
indications, the most delicate, perhaps, are those 
afforded by the occnltation of a planet by the moon. 
The ocultation of Jupiter, which took place on the 
2nd of January last, was observed with this refer¬ 
ence, and is said to have exhibited no hesitation, or 
change of form or brightness, such as would be pro¬ 
duced by the refraction or absorption of an atmos¬ 
phere. I As respects the sea, the mode of examination 
long since suggested by Sir David Brewster is 
is probably the most effective. If water existed on the 
moon’s surface, the snn’s light reflected from it should 
be completely polarised at a certain elongation of the 
moon from the sun. No traces of such light have been 
observed; but l am not aware that the observations 
have been repeated recently with any of the larger 
telescopes. It is now well understood that tho 
path of astronomical discovery is obstructed much 
more by the earth’s atmosphere than by the limita¬ 
tion of telescopic powers. Impressed with this con¬ 
viction, the Association has, for some time past,, urged 
upon Her Majesty’s Government the scientific impor¬ 
tance of estabhsing a large reflector at some elevated 
station, in the Southern Hemisphere. In the mean 
time, and to gain (as it were) a sample of the results 
which might be expected from a more systematic 
search, Prof. Piazri Smyth undertook, last summer, 
the task of transporting a large collection of instru¬ 
ments—meteorological and raagnetical, as welL as 
astronomical—to a high point on the Peak of Tone rifle. 
His stations were two in number, at the altitudes 
above the sea of 8,840 and 10,700 feet respectively; 
and thq astonomieal advantages gained may be inferred 
from the fact that the heat radiated from, the moon 
which has been so often sought for in vain in a lower 
region, was distinctly perceptible, even at the lower of 
the two stations. 
The researches relative to tho Figure of the Earth 
and tho Tides are intimately connected with astronomy 
and next claim our attention. The results of tho 
Ordnance Survey of Britain, so far as they relate to 
the earth’s figure and mean density, have been lately 
laid before the Society by Col. James, the superin¬ 
tendent of the Survey* The cllipticity deduced is 
1-299'S3 . The mean specific gravity of the earth, as 
obtained from tiio attraction of Arthur’s Seat, near 
Edinburgh, is 5*31 G; a result which accords satis¬ 
factorily with tho mean of tho results obtained by tliB 
torsion balance. Of tho accurracy of this important 
work, it is sufficient to observe, that when the length of 
each of the measured bases (in Salisbury Plain and on 
tho shores of Lough Foyle)‘was computed from the 
other, through tho whole series of intermediate tri¬ 
angles, the difference from tho measured, length was 
only 5 inches in length of from 5 to 7 miles. Our 
knowledge of tho laws of the Tides lias, received an 
important accession in the results of the tidal observa¬ 
tions made around the Irish coasts in 1851, under tho 
direction of the Royal Irish Academy. The discus¬ 
sion of these observations was undertaken by Prof. 
Haughton, and that portion of it which relates to the 
diurnal tides has been already completed and pub¬ 
lished. ' The most important result of this discussion 
is the separation of the effects of the sun and moon in 
the diurnal tide—a problem which was proposed by tho 
Academy as one of the objects to be attained by the con¬ 
templated observations, and which has been now for the 
first rime accomplished. From the comparison of these 
effects Prof. Haughton has drawn some remarkable con¬ 
clusions relative to the mean depth of the sea in the 
Atlantic. In the dynamical theory of the sides, the 
ratio of the solar to tho lunar effect depends not only 
on the masses, distances, and periodic times, of the two 
luminaries, but also on the depth of the sea; and this 
accordingly, may be computed when tbo other quan¬ 
tities are known. In this manner Prof. Haughton has 
deduced, from the*solar and lunav co-efficients, of the 
diurnal ride, a mean depth of 5*12 miles—a result 
which accords in a remarkable manner with that 
inferred from the ratio of tlie semi-diurnal co-efficients, 
as obtained by Laplace from the Brest observations. 
The subject, however, is far from being exhausted. 
Tlie depth of the sea, deduced from tho solar and lunar 
tidal intervals , and from the age of tho lunar diurnal 
tide, is somewhat more than double of the foregoing; 
and the consistency of tho individual results is such as 
to indicate that their wide difference from the funner is 
not attributable to errors of observation. Prof. Hang- 
ton throw's out the conjecture that the depth deduced 
from the tidal inter mis and ages , corresponds to a differ - 
ent part of the ocean from that inferred from the 
heights. 
Tho phenomena of terrestrial magnetism present 
many close analogies with those of the tides; and 
their study lias boon, in a peculiar manner, connected 
with the labours of this Association. To this body, and 
by the hands of its present general secretary,' were 
presented those reports on the distribution of the ter- 
