324 On the Use of the Globe 



face containing the angle coming first. Thus angles (o I, 2), 

 (2 i, 3) mean the plane angle on face No. o made by faces 

 Nos. i and 2 and that formed on face No. 2 by faces Nos. I 

 and 3. The great circle corresponding to the fundamental 

 face of reference will always be called No. o, and the funda- 

 mental edge (o, i) is the diameter made by the intersection 

 of the planes of great circles Nos. o and i. One of the nodes 

 of these two circles, which will be called node o, is chosen 

 as the zero of arcs of azimuth, which are measured along 

 great circle No. o, from o to 360. Points lying outside of 

 great circle No. o, which is the equator of our system, are 

 further fixed by their altitude above or below it. The original 

 position of the metrosphere means the one which it had when 

 great circle No. o was drawn and node o was marked : then 

 the equator of the metrosphere coincided with the equator 

 of the diagram on the globe, and the zero of azimuths on the 

 metrosphere corresponded with that on the globe. 



In central representation, positions on the sphere are ex- 

 pressed by the azimuth from node (o) in which a great circle 

 passing through the pole of circle No. o and the point cuts 

 circle No. o, and the arc on this circle contained between its 

 intersection with circle No. o and the point. This is the 

 altitude of the point and it is + or according as it is to 

 the right hand or the left of the equator, when moving in the 

 positive direction of the measurement of azimuths. The 

 coordinates of a point will be expressed shortly in the 

 form (6, </>), where is the azimuth and </> the altitude. 

 The azimuth will always come first. In central representation 

 a face is specified by giving the angle which the plane of 

 the great circle which represents it makes with the. plane 

 of the equator, or great circle No. o, and the azimuth of one 

 of the nodes of intersection of the two circles. An edge is 

 specified by azimuth and altitude of the node marking its 

 extremity in the positive hemisphere. As it is a diameter of 

 the sphere its direction is fixed. 



As the measurements with the metrosphere used may vary 

 to the extent of half a degree, angles and arcs whether 

 observed or calculated are stated in degrees, and half or 



