ATTRACTION. 



141 



"One surface (C) of the cube, in each figure, is represented as if 

 no addition were made to it, in order that the situation of the nucleus, 

 relatively to the pyramids raised upon it, may be understood. It 

 must be evident that each rhombus, R R R R, in fig. 12, and penta- 

 gon, PPPPP, in fig. 13, is made up of the surfaces of two adjoin- 

 ing pyramids, built upon a cubic nucleus." 



; ' The decrements may proceed only on two sides, or a diminution 

 of two, three, or more rows may take place on all the sides ; yet in 

 either case, secondary crystalline forms may be built upon the com- 

 mon nucleus, or primitive form." 



FIG. 14. Of tJiP. Goniometer, or instrument for measuring the an- 

 gles of crystals. 



" The goniometer is founded upon the 1 5th proposition of Euclid^ 

 which demonstrates that the opposite angles, made by any two lines 

 in crossing each other, are equal. Hence it follows that the angles 

 made by the legs BB, BCB, of this instrument, fig. 14, above and 

 below the pivot on which they revolve, are equal to each other. 

 Consequently, if they be made to close upon any solid crystalline 

 angle, presented to them at C, they will comprise a similar angle on 

 the other side of the centre about which they turn. This angle is 

 evidently equivalent to that of the crystal, and is ascertained by in- 

 specting the semicircle A, graduated into 180 degrees precisely in the 

 same manner as a protractor." 



"The construction of goniometers is usually such as to allow the 

 legs to be detached from the arch, in order to facilitate their appli- 

 cation to crystalline angles ; and yet, so that they may be reapplied 

 to the semicircle, without deranging them from the angle to whicla 

 they may have been adjusted." 



