^66 LECTURE XLV. 



yr 



that its axis may form the same angle with its horizon as the axis of the eartU 

 does with the real horizon of the place; then finding a point on its surface 

 corresponding to the place of the sun or planet, we may represent its apparent 

 motion by the motion of this point, and the time occupied by that motion 

 will be shown by the index of the globe; thus we may find the length of the 

 day and night, and the time and place of rising and setting; and by means of 

 a graduated circle, perpendicular to tlfS horizon, we may measure the al- 

 titude of the sun or planet at any other time, and also itsazimuthi or th« 

 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 horizon, allowing for the effects of re- 

 fraction; 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 duration of 

 twilight, on any supposition that might be formed respecting the depression 

 of the sun required for producing total darkness. By means of the celestial 

 globe, the apparent motions of the fixed stars may be represented in a man- 

 ner nearly similar, proper attention being paid to the situation of the sun ia 

 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 Bodc's planisphere comprehends in one view all tlie 

 stars that are ever visible at Berlin : he has added to it a moveable circle, re- 

 presenting the horizon of that place, carrying with it the circles of altitude 

 and azimuth, delineated on a transparent paper, which is adjusted, by gradu- 

 ations 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 projected, 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 



