PHYSICAL ASTRONOMY. 171 
and brass, or platina plates, being so combined that their changes through 
temperature counterbalance those which, for the same reason, take place in 
the balance. , | 
99. Clocks for astronomical purposes must be very carefully constructed, 
and every tendency to inaccuracy must be specially counteracted. They 
may be pendulum clocks, as used in observatories, or balance clocks— 
chronometers—as employed at sea to assist in the astronomical determi- 
nations there necessary. The English, to whom the perfection of chrono- 
meters is due, set great'value upon the best of them, and Harrison furnished 
instruments which, in a voyage round the world, did not vary three seconds. 
Such chronometers are little different in construction from the best watches, 
except in having a peculiar escapement ; all their parts are, however, very 
carefully constructed, many precautions taken against accidental injury, 
and throughout, compensations for the effects of temperature and other 
physical agents (as magnetism) introduced. Longitude or marine time 
keepers (box chronometers) for nautical and astronomical purposes, are 
constructed just like the pocket chronometers ; they are, however, larger. 
and inclosed im a special box. 
The Planetarium. 
100. Among the numerous helps to the study of astronomy and mathe- 
matical geography, must be mentioned those artificial models and contri- 
vances known under the names of Lunarium, Tellurium, and Planetarium. 
The lunarium is an apparatus by which the motions of the moon about the 
earth, her phases, &c., can be readily illustrated and explained. It is 
usually combined with the tellurium. The tellurium, called sometimes 
geocyclic machine, is a particular form of planetarium, which exhibits the 
motion of the earth round the sun, the course of the moon about the earth, 
and with her about the sun, as also all attendant phenomena, such as the 
seasons, quarters of the moon, &c. Finally, the planetarium is a model, 
intended to render perceptible to the senses the motions of the planets and 
all resulting phenomena, on which account it has received various con- 
structions. Common planetaria are. moved by hand; the better and more 
complicated have a wheelwork, which, like a watch, is set in motion by a 
spiral spring, and causes the planets to revolve with their respective velo- 
cities around a globe or lamp placed in the centre. As Lord Orrery was 
the first to construct a planetarium of this character, they are sometimes 
known as Orreries. 
Henderson, formerly director of the Edinburgh observatory, has published 
the description of a simplified planetarium (pl. 15, fig. 38), whose construc- 
tion will be here briefly mentioned. In a circular box standing on four feet, 
the clockwork is set in motion by the handle D. Upon the upper surface of 
the box are marked the ecliptic, the perpetual calendar, and other items 
relating to the planets. In the centre is a large sphere representing the sun, 
about which the planets revolve on vertical posts fastened to horizontal 
171 
