An Astronomical Machine, the Tellurium. 339 
being made in a sidereal day, or 23h. 56m. 4s. The moon moves 
eastward round the earth and completes a sidereal revolution in 
27d. 7h. 43m. ; its nodes shift round contrary to the order of the 
signs, and its apogee has its direct motion eastward, the former 
completing a sidereal revolution in 18.6 years, and the latter in 
In contriving this machine I have availed myself somewhat of 
the inventions of other artists. To effect the unequable motion 
of the earth in its orbit, I have had recourse to a combination of 
elliptical wheels similar to that used by Dr. Desaguliers in his 
Cometarium. There isa little Planetarium described in Fergu- 
son’s Astronomy, in which the parallelism of the earth’s axis is 
preserved in the same manner as in this. But this machine, in- 
dependently of the elliptical orbit and unequable motion of the 
earth, is very different from that, as will be apparent to any one 
who may compare them.* (See Brewster’s Ed. Ferg. Astron. 
Vol. If, p.6.) Although these particular parts are the inventions 
of preceding artists, still I think I may venture to assert, that this 
machine, considered as a whole, constitutes a new combination 
in mechanics. 
In Plate IV, this machine is represented as it would appear to 
an eye situated directly above it. Plate V exhibits a lateral view 
of the wheelwork. In either plate the ball W represents the 
sun, the ball U the earth, and V the moon: # is an index for 
showing the place of the moon’s ascending node, e is another 
index for showing the place of its apogee, and m is a winch by 
which the machinery is moved. The earth is surrounded by a 
little brass ring s; which is set upon four pillars ¢ ¢, and has the 
Signs of the zodiac marked upon it. Upon this ring, which moves 
With the earth and keeps its parallelism, the geocentric places of 
the sun, moon, its ascending node and apogee, can be seen. 
123 4, Plate V, is a wooden frame, in the top of which are two 
equal elliptical grooves similar to the earth’s orbit, and which 
have their foci all situated in one straight line. Within the 
frame are two elliptical wheels, K and L, which are of the same 
Size and eccentricity as these grooves, each wheel having its axis 
* About fourteen years ago I made the first machine of this kind. At that time 
and for several years afier, L believed myself to be the original inventor of this 
Mode of preserving the parallelism of the — 8 axis, but 7 was at length unde- 
eeived by a perusal of Ferguson's book. et i ea 
