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 

 8i^ years. 



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 is a 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. II, 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 : k 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 n is a winch by 

 which the machinery is moved. The earth is surrftunded by a 

 little brass ring s, which is set upon four pillars 1 1, 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. 

 12 3 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 after, I believed myself to be the original inventor of this 

 mode of preserving the parallelism of the earth's axis, but I was at length unde- 

 ceived by a perusal of Ferguson's book. 



