VOL. XLI.J PHILOSOPHICAL TRANSACTIONS, 511 



and some smaller circles concentric with it ; we have then in one view those 

 places where the sun is hid by the moon, or any part of it concealed from our 

 sight. 



But such an image is momentary ; and though it shows well enough what 

 happens at any precise moment of time, as for instance when the centre of the 

 lunar penumbra first enters the earth's disk, yet it cannot exhibit the other 

 phenomena, which depend partly on the earth's rotation, and partly on the 

 moon's motion. 



While the earth turns round, the circles of latitude indeed, and consequently 

 their projection, remain the same ; but the meridians, or circles of longitude, 

 are continually changed, and consequently their projection, and the situation 

 of places on the earth, as far as depends on them. 



But the artificial globe of the earth shows the illuminated hemisphere at any 

 point of time, with very little trouble. For the pole being elevated above the 

 horizon, or depressed below it, so that the elevation or depression may be equal 

 to the sun's declination at that given time ; or, which comes to the same thing, 

 the sun's place in the ecliptic of the globe being brought to the zenith, the 

 artificial -horizon becomes the boundary of light and shade ; and nothing re- 

 mains, to exhibit plainly the illuminated hemisphere, but to turn the globe on 

 its axis, till it obtains the position adapted to the hour of the day. 



Thus, what is very difficult in projections, is easily performed by the globe, 

 and also more conformably to nature. Considering this, it occurred that we 

 still wanted, for representing all the phenomena of a solar eclipse, to project 

 the lunar penumbra on the globe, and to make an instrument to represent its 

 situation at any time, and to refer it to those places on the earth which are 

 marked on the globe. Hence Mr. Segner devised such an instrument, which 

 is as follows. 



Fig. 4, pi. 11, represents a common terrestrial globe, furnished with its 

 horizon, meridian, and hour circle. To the horizon are connected 2 wooden 

 arms, ab, ab, of a length a little exceeding the semidiameter of the globe ; and 

 one end of each arm is made to clasp the horizon, to any part of which it may 

 be fastened by screws, a§ shown at d. 



On the opposite extremities, b, b, are wooden columns, be, be, erected per- 

 pendicular to the horizon, of the height of the semidiameter of the globe, and 

 of the breadth of the brazen meridian ; so that a right line being drawn through 

 the tops of the columns, cannot touch the meridian. 



On the top of each column is a small brazen ball, each perforated by an iron 

 axis, projecting on both sides, and firmly joined to the ball, the lower parts of 



