ON PRACTICAL ASTRONOMY. 433 



and setting ; and by means of a graduated circle, perpendicular to the hori- 

 zon, we may measure the altitude of the sun or planet at any other time, 

 and also its azimuth, or the distance of this circle from the north or south 

 point of the horizon. If we have a ring of any kind parallel to the horizon, 

 and 33 minutes below it, we may consider this ring as the apparent hori- 

 zon, allowing for the effects of refraction ; if it be still 15 or 16 minutes 

 lower, it will represent the rising or setting of the extreme margin of the 

 sun or moon : we might also have a circle about a degree above either of 

 these, which might represent the sensible or apparent horizon, with regard 

 to the moon, including the correction for her parallax ; and a similar ring, 

 placed still lower, would show the duratkm of twilight, on any supposition 

 that might be formed respecting the depression of the sun required for pro- 

 ducing total darkness. By means of the celestial globe, the apparent 

 motions of the fixed stars may be represented in a manner nearly similar, 

 proper attention being paid to the situation of the sun in the ecliptic, as 

 determining the time corresponding. 



Many of these operations may also be performed with equal convenience 

 with a planisphere, which is a stereographical projection of the globe on a 

 plane surface. Professor Bode's planisphere comprehends in one view all 

 the stars that are ever visible at Berlin : he has added to it a moveable cir- 

 cle, representing the horizon of that place, carrying with it the circles of 

 altitude and azimuth, delineated on a transparent paper, which is adjusted, 

 by graduations at the margin of the chart, to the day and hour for which 

 we wish to ascertain the apparent places of the heavenly bodies. Any 

 other chart of the stars, having the pole in its centre, may be applied to a 

 similar use, by cutting out a circle, or a part of a circle, to represent the 

 horizon of a place of which the latitude is given ; and if the stars are pro- 

 jected, as is usual, on two equal charts, they must have two equal arcs to 

 represent the respective parts of the horizon belonging to them. A simple 

 construction may also often be made to serve for solving many problems of 

 a similar nature. (Plate XXXV. Fig. 515, 516. Plate XXXVI. Fig. 517. 

 Plate XXXVII. Fig. 518.) 



For representing the real as well as the apparent motions of the different 

 parts of the solar system, planetariums or orreries have sometimes been 

 employed, in which the comparative periods of the revolutions have been 

 expressed by various combinations of wheelwork. Of these instruments 

 Archimedes was the original inventor, and Huygens revived them, with 

 many improvements, in modern times. The construction of the large pla- 

 netarium, which has been made in the house of the Royal Institution, was 

 principally directed by Mr. Pearson. I suggested to him, that the instru- 

 ment might be placed in a vertical position, and that the eccentricities of 

 the planetary orbits might be shown by the revolution of short arms, retained 

 in their situation by weights, and their deviation from the plane of the 

 ecliptic by inclining the axes of these arms, in a proper angle, to the plane 

 of the instrument. The other parts of the arrangement, which have any 

 claim to novelty, were entirely of Mr. Pearson's invention, and he appears 

 to have rendered the instrument in many respects more accurate than any 

 other planetarium that has ever been constructed.* 



* On this subject see the article Planetarium, by Pearson, in Rees's Cyclopaedia. 



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