PHYSICAL ASTRONOMY. 173 
in any direction, for which reason there is generally a roof attached which 
turns on a railroad, or can be entirely removed. In the first case, the roof 
is divided by a vertical section of four feet in breadth, into two halves; the 
aperture, however, can be closed by trap-doors. Such an arrangement, for 
example, is to be found in the building for the great refractor at Dorpat, and 
in the turning cupola of the observatory at Washington. 
The best observatories are those at Altona, Berlin, Dorpat, Gottingen, 
Greenwich, Kénigsburg, Mailand, Munich, Ofen, Pulkowa near St. Peters- 
burg, Seeberg near Gotha, Vienna, &c. In the other parts of the world, 
those at the Cape of Good Hope, Paramatta in New South Wales, and in 
the United States, are the best known. In the United States, the principal 
observatories and instruments are at Cambridge, Philadelphia, Washington, 
Cincinnati, Hudson (O.), &c. 
Practical Astrognosy. 
103. The finding of particular stars and constellations is effected by means 
of the celestial globe and star maps; as also by the method of alignments 
already mentioned (sec. 26). This latter method will now be detailed a 
little more at length. It was there seen that from the position of the Great 
Wain, or the Great Bear, the polar star could be determined ; and in the same 
manner other stars are identified. For this the star maps (pl. 12) are employ- 
ed, on which the alignments of the principal fixed stars are given. Produce 
the direction of the stars ¢ and 1 towards the wain, it will strike a star of the 
_ first magnitude, Arcturus in Bootes, which, with the polar star and Vega in 
Lyra, forms an isosceles triangle, Arcturus being at the vertex. The polar 
star, which, by its almost unchangeable position, is very well calculated for 
the purpose, serves as a point of departure for the rest of the heavens ; the 
altitude of the polar star above the horizon being nearly equal to the geo- 
graphical latitude of the place. Twice the length of a straight line from 
Vega to Arcturus strikes Spica, a star of the first magnitude in Virgo. 
Spica forms, with Denebola (in Leo) and Arcturus, and also with Arcturus 
and the star a Libre, triangles nearly isosceles. Spica forms the vertex of 
the first of these; Arcturus of the second. The star a Libre lies almost 
in the continuation of the connecting line between the polar star and Arctu- 
rus. Furthermore, the alignments of Vega, the polar star, Capella, and 
Aldebaran, form a large flat arc. Aldebaran, a star of reddish light, is one of 
five stars lying near to each other. which form a V, and are called Hyades. 
Aldebaran and Capella form an almost right-angled triangle with Castor, a 
starin Gemini. A line drawn from Denebola to the polar star, and produced 
some distance beyond, strikes a bright star, which, with three others, forms a 
large almost regular quadrilateral, the greater part of Pegasus. A line from 
Perseus to Aldebaran, and sufficiently prolonged, strikes three bright stars, 
the belt of Orion. Produce the line indicated by this belt to the left, and it 
will meet the brightest star in the sky, Sirius (the Dog star). In this man- 
ner, straight lines may in succession be drawn from two known stars to 
others, and the triangles thus formed, constructed in the heavens. 
173 
