248 NICOLAUS STENO 



a way indeed that the striae in opposite planes ran along in the 

 same direction, while planes adjacent to each other showed a 

 different direction of the striee. From the direction of the 

 striae it follows that the surrounding fluid was directed about 

 every cube by a threefold movement.^ Of these movements 

 p. 61. one was perpendicular to the horizon ; the remaining two, 

 parallel to the horizon, were perpendicular in relation to each 

 other. And it is not difficult to account for this threefold 

 movement; for while the fluid is trying to withdraw from the 

 earth's centre, that direct movement is checked by the base of 

 the cube, with the result that the fluid is deflected toward the 

 narrower sides, inasmuch as the force of the ascending fluid is 

 stronger along the wider sides and so allows no approach in 

 that quarter; and in this way two pairs of planes are marked 

 out by the traces of the striae. The third pair of planes re- 

 ceives its striae from that part of the fluid which passes between 

 the cube and the fluid rebounding from the base of the cube. 



4. In perfection of form ; for in crystals scarcely a single one 

 is found in whose form something is not lacking. Cubes of 

 marcasites, however, rarely have a missing part ; and the ex- 

 planation is not difficult. For, since all the solid angles in the 

 crystal, except the terminal angles, are obtuse, and the crystal- 

 line matter is added little by little to their separate planes, any 

 given plane remains imperfect, if the adjacent planes change 

 their shape, in just the degree that more substance is added to 

 that one alone. Since in cubes of marcasites, however, all the 

 solid angles are right angles, even if new matter be added to 

 one plane only, that same plane always retains the same dimen- 

 sion, provided the adjacent planes do not change their form. 



Various other things may be noted in the cubes of marca- 

 sites, such as cubes enclosed in cubes; the transparent matter ^ 

 p. 62. enveloped in the substance of the marcasite which encloses 

 another marcasite; and other matters of this kind, which I 

 keep for the Dissertation itself. 



There are also angular bodies which are broken up into 



^ Steno was probably the first to observe that the cube surfaces of pyrite are commonly 

 striated parallel to three intersecting edges. 

 ^ Not clear, because pyrite is opaque. 



