172 ASTRONOMY. 
rods, with their proportional velocities, and at their proportional distances— 
Mercury, H; Venus, G; Earth, F, with the moon, c; Mars, I; Jupiter, M, 
with four moons, e,e,e,e; Saturn, N, with the ring, and the seven satellites, f, f, 
LAS Ff; as also Uranus, K,with the six moons, d,d,d,d,d,d. Fig. 39 repre- 
sents a contrivance, which, attached to this planetarium, serves to give the 
earth’s axis a parallel motion, and to exhibit the cause of the seasons and their 
succession. It consists of the small globe, F’, to whose equator the hollow brass 
tube, E, is fastened, which carries the weight, C, and the carrier, D. If now 
the planetarium be hung to the wall by the ring E (/fig.38), and set in 
motion, C being held fast, the earth will revolve, her axis remaining 
constantly parallel, and thus representing the courses of the seasons, which 
are indicated on the fixed disk, A. , 
The best Planetaria and Telluria are those of Riedig and Schulze in 
Leipzig, and Seifert of Hohenstein near Chemnitz in Saxony. 
101. It remains to mention, in conclusion, that there are still other 
pieces of apparatus, some of them ancient, and others more modern, which 
are used with excellent results both by the practical astronomer and the 
teacher. Among these may be mentioned the circle micrometer, as also the 
differential micrometer (invented by Boguslawski) for determining the 
difference of right ascension and declination of two stars: the dipleidoscope 
(invented by Dent of London), an apparatus which replaces the transit 
instrument ; the beautiful model by Mobius of Leipzig, for representing the 
orbits of the asteroids, Ceres, Pallas, Juno, and Vesta, with respect to their 
magnitudes, inclinations, and eccentricities; finally, the mercurial clock of 
Kater, and the astrograph of Steinheil. 
Observatories. 
102. The place where astronomical observations are conducted and the 
necessary apparatus erected, is called an observatory. The choice of such a 
place is sometimes very much restricted ; where this is not the case, it should 
be established in a dry locality, where, remote from all motion which might 
produce vibrations, the foundations and lower stones of the buildings and the 
instruments may be protected from the influences of weather and temperature. 
The building itself must be constructed in the most solid manner, and, if possi- 
ble, facing the four quarters of the heavens. It is desirable to have an elevated 
station from which the horizon can be surveyed in every direction; where 
this is impossible, the parts of the building in which are placed the principal 
instruments must be much elevated. The foundations of the edifice must 
be very solid, and each principal instrument must have an isolated base, or 
must be connected with the ground by special foundations, not touching any 
part of the edifice, in order that all shaking of the instrument may be 
avoided. A free view in all directions must be had, and for the meridian 
there must be a vertical slit passing through the whole height of the building. 
The place also where the refractor, or other instrument supplying its place, 
stands, must be so arranged that the whole of a vertical plane can be seen 
172 
