754 ALBERT J OH ANN SEN 



rately turned on the lathe and has a diameter of 21.8 cm., which 

 leaves, when inserted in the recess prepared for it, a very slight 

 margin for expansion. In the exact center a 2 cm. hole was turned 

 and into this the plug P-G was glued. P is a copper rivet set in 

 the top of the plug to serve as a compass center. K is a metal 

 washer and L-L a wooden knob held on the plug G only by friction. 

 This permits its removal in case the dial should ever bind and need 

 trimming down. M-M are knobs, one of which is glued to each 

 corner of the board. They serve as feet to keep the button L from 

 touching when the board is placed on a flat surface. The plug P-G 

 fits snugly into the drawing-board and the dial will readily remain 

 in any position to which it is turned. 



The net B-C in the accompanying figure is shown divided only 

 into parts of 10 degrees each. Actually the section B was made by 

 gluing half a Wulff 1 net to the top of the dial disk. The mathe- 

 matical center is located by a very small pit in the top of the plug 

 P. A further guide to centering the net is a scratch circle described 

 upon the wooden disk and having a diameter of 2 mm. more than 

 the Wulff net. The section C of the dial is used to measure dis- 

 tances on horizontal small circles and vertical great circles. It was 

 made on a sheet of cardboard by drawing circles from the stereo- 

 graphically projected lines of the Wulff net. These also, as well 

 as the projected great circles which appear as radii, are drawn 2 

 degrees apart although they are shown 10 degrees apart in the figure. 

 Upon the drawing-board itself the N-S and E-W lines were drawn 

 parallel to the sides of the board. 



Cutting the net in half occasionally makes it necessary to com- 

 plete a vertical small circle in two lines. The curves of the right- 

 hand net (C) might have been drawn, say in red, over a complete 

 Wulff net, but the confusion resulting would probably cause more 

 inconvenience than the present necessity of occasionally drawing a 

 vertical small circle in two operations. In most cases where such 

 circles are used, the degree divisions on the equator are a sufficient 

 substitute for the half-net cut off. Perhaps if the lines of every 

 fifth vertical small circle were extended over the upper half of the 

 right-hand net, it would be a convenience. 



"Zeitschr.f. Kryst., XXXVI (1902), 14-18. 



