164 Mr. J. Y. Buchanan on the Use of the Globe 



elastic band. If the tapes or threads are of different colours, 

 the constructions made with them are very plain and easily 

 followed. A point may be marked on a varnished globe by 

 attaching a small piece of gummed paper near its position, 

 and marking the exact position on the paper with a pencil. 

 Small circles, if they are wanted for themselves, are difficult 

 to deal with ; but it is usually only intersections of pairs of 

 them that are wanted and they are dealt with as points. If 

 the globe is kept fixed with circle No. in the plane of the 

 equator, points on its upper hemisphere can be laid down in 

 altitude and azimuth by means of the flexible quadrant. It 

 will be seen, therefore, that an ordinary globe, even though 

 its surface be not prepared for drawing on, can be used for 

 the purposes of crystallography. The terrestrial globe is to be 

 preferred to the celestial, because on the latter the meridians 

 are those of celestial longitude which meet in the pole of the 

 ecliptic, while the axis of the globe passes through the poles 

 of the equatorial. 



As a first example let us consider the cube, because its 

 details are so simple and so familiar that they can be easily 

 followed without drawings and models. If we place it on 

 the goniometer and measure the inclination of its contiguous 

 faces, we remark that they are all right angles, and we mark 

 the poles on the globe in the way already described. Let us 

 now draw small circles round each pole, the radius of the 

 small circles being sufficiently great for neighbouring circles 

 to intersect. If we then draw great circle arcs through each 

 pair of intersections, we shall, after obliterating the small 

 circles and other superfluous lines, have not only a correct 

 projection of the cube on the sphere, but also a striking 

 bodily presentation of it. All the geometrical details can be 

 studied on it. No face is represented by a great circle, 

 therefore, in specifying positions we have to take a great 

 circle in a plane parallel to one of the faces for our equator of 

 reference, and any point on it as the intersection with another 

 face for the zero of reckoning. We find, on examining our 

 projection, that there are only three independent faces, and 

 we can at once construct the central representation of the 

 crystal by drawing great circles round the poles of the faces. 



Place the metrosphere on the globe and draw the complete 

 equatorial circle representing face No. of the crystal. 

 Mark on it the nodes of the meridian. The one correspond- 

 ing to 0° on the equator of the metrosphere is node No. 0, 

 from which azimuths are measured. The diameter connecting 

 nodes No. and No. (0, 1') represents edge No. (0, 1) which 

 face No. 1 makes with No. 0. From the specification of 



