A PAPER OF GREAT INTEREST ON ATMOSPHERIC TIDES. 
221 
to each other like the faces of a wedge, whose 
sharp edge is the summit of the crystal. The 
broad faces are inclined upon the adjacent 
faces at an angle of 64°. The cx-ystal is ter- 
minated by four minute planes, equally in- 
clined to the broad faces and the axis of the 
prism, but two of the faces often disappear. 
The crystals are generally opaque, and at 
thickness not much above the ^5 tli of an inch ; 
they are quite impervious to the sun’s rays. 
Their colour by reflected light is then nearly 
black ; but their powder, in daylight, is green, 
and French gray by candle light. In the smaller 
crystals the colour is both by reflected 
and transmitted light. The refractive index is 
about 1.605 and 1.506. Atacertain smallthick- 
ness the least refracted image is bright blue, and 
the most refracted image bright green in day- 
light, or in candlelight. The blue, 
when analyzed, consists of a mixture of 
green, and the green an admixture of red. 
At greater thicknesses the blue becomes 
purer and fainter, and the green passes into 
red ; and at a certain thickness of the least 
refracted blue image disappears altogether, 
and the most refracted image is olive-green. 
At still greater thicknesses this image disap- 
pears also, and absolute opacity ensues. 
With polarized light, when the axis of the 
crystals is in the plane of polarization, the 
transmitted light is green, but when perpen- 
dicular it is blue. In solution the double 
refraction disappears, but the other appear- 
ances are observed as in the solid state. 
The crystal excites a specific action on the 
red ray between A & B of Fraunhofer ; a 
sharp and narrow black band being formed, 
which constitutes a fixed line in ail artificial 
lights. The relations of this salt to common 
and polarized light may be examined by 
placing upon a plate of glass a few drops of 
a saturated solution of it in water. If the 
crystals are slowly formed they will be found 
of various thicknesses, each thickness exhi- 
biting a different colour, varying from per- 
fect transparency through all shades of pale- 
yellow, green, and blue, indaylight, and through 
all shades of pale-yellow, pale-orange, red, and 
bluein candle-light. 
SECOND ESSAY ON A GENERAL 
METHOD IN DYNAMICS. BY W. R. 
HAMILTON. 
In his first Essay, the author observed, 
that many eliminations required by this me- 
thod, in its first conception, might be avoided 
by a general transformation, introducing the 
time explicitly into a part S of the whole cha- 
racteristic function V. In the present Essay 
he fixes his attention chiefly on this part S, 
and to call it the principal function. Its 
properties ate more fully developed, espe- 
cially in application to questions of pertur- 
bation, in which it enables us to express 
accurately the disturbed configuration of 
a system by the rules of undisturbed motion, 
if only the mutual components of velocities 
be changed in a suitable manner. 
CONTINUATION OF A PAPER ON 
THE TWENTY-FIVE FEET ZENITH 
TELESCOPE, LATELY ERECTED 
AT THE 5 ROYAL OBSERVATORY. 
BY JOHN POND. 
The observations made by means of this 
instrument have confirmed the accuracy, of 
the results obtained in determining the place of 
any star passing the meridian near the zenith. 
We have now three methods of observing 
this: 1 st, By the mural circles : 2 nd, By the 
zenith telescope, used alternately east and 
west, and 3rd, by means of a small subsidiary 
star. 
GEOMETRICAL INVESTIGATION 
CONCERNING THE PHENOMENA 
OF lERRESTRlAL MAGNETISM. 
BY T. S. DAVIES. 
The present series of papers is chiefly inten- 
ded to deduce the mathematical consequences 
of the theory of two poles situated arbitrarily 
within the earth, and especially to investigate 
the singular points and lines which result 
from the intersection of the earth’s surface 
with other surfaces related to the magnetic 
poles, especially the points at which the nee- 
dle is vertical, the lines of equal dip, the 
Halleyan lines, the isodynamic lines, and the 
Hansteen poles. He investigates at length 
the hypothesis of the duality of the terrestrial 
magnetic poles, and shews that the ques- 
tion cannot be determined definitely until 
the dipping needle is brought to a greater 
state of perfection hnd the influence of 
geological and meteorological sources of 
disturbance can be accurately appreciated. 
RESEARCHES 10 WARDS ESTA- 
BLISHING A THEORY OF THE 
DISPERSION OF LIGHT. BY THE 
REV BADEN POWELL. 
An abstract of this paper has already 
appeared. 
METEOROLOGY. 
ON THE ATMOSPHERIC TIDES AND 
METEOROLOGY OF DUKHUN, 
(DECCAN). EAST INDIES. BY 
LIEUT-COLONEL W. H. SYKES. 
This is a paper of great interest, as it 
contains a mass of facts accumulated with 
great labour and care, in a portion of the 
world where science, with the exception of 
botany, has hitherto been almost unknown. 
The author, in the first instance, proceeds 
to describe his instruments and his mode of 
proceeding to observation. These are impor- 
tant points, and deserve an attentive con- 
sideration. The proper mode of mounting 
meteorological instruments for observations 
in tropical climates is particularly adverted 
to. Ivory scales and reservoirs are proved 
to be useless ; the substitution of metals 
being absolutely necessary. The conclu- 
sions to which the observations lead are 
principally as follow : In the Dukhun four 
atmospheric tides exist in the 24 hours ; 
two diurnal and two nocturnal, each consist- 
ing of a maximum and minimum tide. These, 
