Desquamation of certain Rocks. 257 



weather on the one side of this rock, where the desquamation 

 takes place, contributes to prove, still more clearly than in many 

 of the instances formerly enumerated, that the whole of this 

 process is caused by the action of the atmosphere and the rains. 

 However mysterious it may at present appear, it is the result of 

 some chemical actions which cannot for ever be concealed. 

 How far it may be connected with any other circumstances of 

 the same nature more generally interesting, it is impossible to 

 foresee. But, like all new facts in an obscure science, it is 

 worthy of record. In multiplying the examples of difficulties 

 and obscurities, they become gradually removed from the list 

 of exceptions; while the varieties which are discovered in them 

 on the comparison of many examples, sometimes point out the 

 causes which have influenced the whole. 



Art. II. Account of an Instrument for detei'minittg the 



Specif c Gravities of Solid Bodies. By H. Creighton, 



Esq. 



[In a Letter to the Editor.] 

 Sir, 



Some time ago, I constructed an instrument for deter- 

 mining the specific gravities of solid bodies, of which the 

 following is a description: I presume the same may be found 

 acceptable to some of the readers of the Quarterly Journal. 



The theory of this instrument depends upon the known pro- 

 perty of springs, undergoing degrees of extension proportional 

 to the weight or force applied to them, and the screw form 

 usually adopted for weighing of bodies is best adapted to the 

 purpose now in view. 



In the accompanying sketch A. B. Plate IV. represents one of 

 these springs suspended at A, and carrying an index at B. its 

 lower extremity : C. D. two cages of curved wires, with joints to 

 fold into a small compass, (or scales similar to those usually 

 attached to the ends of a balance-beam may be used instead) ; 

 the lower one D. is immersed in a vessel of water (which must 

 be considerably deeper than represented) and both are sus- 



