24 REPORT—1857. 
reduced to a planisphere. The same planisphere, when properly projected, was 
made to serve for both the northern and southern hemispheres, by colouring the 
projecting lines which marked radially on the outer circle the longitudes of the cul- 
minating points and of the perihelia for the northern hemisphere blue, and for the 
southern black; and on each of these radial lines was marked the number assigned 
to the comet in the catalogue of the 263 discussed by the author. If, then, we con- 
ceive these two lines to be produced to the centre, and caused to revolve towards the 
northern hemisphere if marked with the sign plus, to the southern if marked minus, 
until they take the position of the inclination of the orbit marked as belonging to 
each, the position of the two lines will present to the mind a picture of the position 
which the orbit will hold as well in space as in its own plane. At the end of each 
of the eight tables, corresponding to the eight zones, were specified the total num- 
ber of orbits found in that zone, as well as the number of those having their peri- 
helia in the northern or southern hemisphere, and their motion direct or retrograde; 
combining the data thence given, the author drew up the summary of the whole. 
He found the orbits to have a tendency to approach in prevailing numters the polar 
regions of the ecliptic. The minimum occurs in the fifth zone of each hemisphere. 
Those whose perihelia are in the northern hemisphere exceed those whose perihelia 
are in the southern in the proportion of 3 to 2; the number of those having a 
direct motion to those retrograde as 5 to 6, or nearly equal. The author calls the 
Great Circle, which passes so as to divide the Milky Way pretty equally, the Galaxy 
Circle. In the centre of this the sun and earth may be considered to be placed ; 
it cuts the ecliptic towards the solstitial points, and is inclined to it at about 60°. 
He then finds that the planes of the orbits of the comets are, for the most part, 
little, if at all, inclined to the plane of the Galaxy Circle, and that they go on de- 
creasing in number as that inclination increases ; and therefore he concludes that 
some cosmical cause must have led to such a result. Also, the perihelia of by far 
the greater number of those he has discussed are found near the Galaxy Circle, 
showing that when they are passing most closely under the influence of the sun 
they are both near the Galaxy Circle, and their proper motion is nearly parallel to 
its plane. Hence the greater number of comets come to us from the region of the 
Galaxy itself. 
On a Moveable Horizontal Sun-dial, which shows correct Solar Time within 
a Fraction of a Minute. By M. Donovan, M.R.LA. 
The author first pointed out the inaccuracies incidental to or inseparable from the 
ordinary horizontal sun-dial, even when executed with the greatest care. He then 
adverted to the peculiarities of his own dial, showing how it can be placed with the 
greatest precision in the meridian of the place. After alluding to the defects of the 
ordinary style, he showed the advantages of substituting a human hair, which, cast- 
ing a shadow as slender as itself for several inches of its length, affords a line of 
direction for another hair springing from the same source, which, when stretched 
through the centre of the most slender part of the shadow, marks the precise time to 
a few seconds on a large divided circle. One of the peculiarities of this dial is, that 
it may be placed in any spot illuminated by the sun; an advantage, from which the 
common horizontal dial is precluded by its being a fixture, and without means of 
exactly placing its meridian in the meridian of the place. 
Tables to simplify and render more general the Method of finding the Time, 
by observing Circumpolar Stars in the same Vertical. By C. Tomson. 
(Communicated by Sir W. R. Hamirton.) . 
The author described the tables, and exhibited to the Section a little apparatus 
constructed by his ingenious assistant, Mr. Thomson, which illustrated the method 
of observing circumpolar stars for this purpose. 
On the Direction of Gravity at the Earth's Surface. 
By Professor Henvessy, MR.LA. 
If the earth’s surface be considered to coincide with that of the liquid which 
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