6 Mr Barlow on the Effect of' Rotation of solid 



Hence writing all the faces with their proper designations, 

 the symbol of the crystalline form will stand thus. 

 I R', R, 2 Rv f R', 05 R, R3, \ R 3 

 a c e d g f b 



We have thus entirely determined the form proposed, with- 

 out any information, except what is obvious to the eye on con- 

 sidering the figure. And in by far the majority of cases, si- 

 milar considerations, combined with a few theorems in addi- 

 tion to those which we have referred to (A, B, &c-) will suffice 

 for a similar solution of the problem. 



The laws of the derivation of the faces being known, and the 

 angles of the primary form (R,) the angles which any other 

 faces make may be determined by formulae depending upon 

 the primary angles combined with the indices of the derived 

 forms. And this is another important use of the system of 

 notation of which we here speak. 



The mathematical science of crystalline forms, has there- 

 fore for its object, 1st, To show the kind of forms produced 

 in each system by the laws of derivation above-mentioned : %d, 

 To discover and demonstrate such theorems, with respect to 

 these forms, as may be useful : 3d, To classify the combina- 

 tions of these forms, and to show how, from the properties of 

 these combinations, we may, by means of the theorems just 

 mentioned, form certain rules for obtaining their indices of 

 derivation : 4>th, To establish f or mulce, by which we may find 

 the angles made by any planes of a crystalline form, the in- 

 dices being known. These branches of the science being pro- 

 perly developed, we shall be able to apply to all crystals of 

 which the faces are sufficiently exhibited, a process of analysis 

 similar to that explained in the above instance. 



Art. II. — Comparative Effect ofRotationqfa Solid IronBall, 

 and Hollow Iron Shell, in producing the deflection of a 

 Magnetic Needle. By Peter Barlow, Esq. F. R. S., 

 Mem. Imp. Ac. Petrop. &c. Communicated by the Author. 



It appears from the theory of M. Poisson, * relative to the 

 phenomena which have been observed in the rapid rotation 



• An account of this theory is given in our last Number, p. 328. 



