244 



MINERALOGY 



[Sect, VIIL 



lit. Right rectangular pristn 



The 



-.1 



prism, the rectangular octahedron, the rhom 



right rhomboid 

 boidal octahedron, &c. 



With oblique axes : 



IV. Rhomhohedron. — Tncluding equiaxial rhcmbohedrons, scalene tri- 

 ano-ular dodecahedrons, two regular prisms with six faces, aud 

 isosceles triangular dodecahedrons. 



V. Ohlique rhomboidal prism.—Wiih its modifications. 



Ncn 



With its modifications. 



Instruineiits, named goniometers^ have been invented 

 to measure the angles made by the different planes of the 

 crvstals, as well those considered primary as their modi- 

 fications. The most simple is that of ITaiiy, consisting of 

 a graduated semicircle, %yith metallic adjustments, so that 

 by applying these adjustments the angle sought is read 



-1 



off upon the graduated semicircle. This was found a 

 somewhat rough process in many cases, and Wullaston 

 invented his reflective goniometer, by means of which 

 adjustments are applied so that the bright surfaces of 

 the crysta^Uine planes are made to coincide with them. 



and the angles 



are 



thus closelv read off 



This go- 



niometer being inapplicable to crystals, the surfaces of 



which are not 



:ht 



constructed by M, Adelmann, by which both methods 



employed, as the cases for their use in preference 





mav arise- WoUaston s goniometer is a very valuable 



instrument, and 



« 



A 



^ There are boxes containing the models of crystals, one of which 

 (for thej occupy little space, and could readily be put away in a berth or 

 cabin^ might be found very convenient for the voyager desirous of avail- 

 ing himself of the external characters of minerals for their determina- 

 tion. The number of models in these boxes varies, but most of them 



con tarn 



the crystalline types according to some system, their chief 



modifications not uufrequently having the names of some characteristic 

 minerals upon them. 



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