240 Mr Robison's Account of the Failure of the 



a mineral not even found in England, or by some unknown 

 species.* 



If we admit this probability, which is inferred from the ab- 

 sence of cleavage in Haytorite, then we are entitled to suspect 

 that many of the old species, particularly those which have 

 only one locality, and which are destitute of cleavage, such as 

 Wagnerite and others, may also be pseudomorphous crystals, 

 deriving their form from that of an unknown species. When 

 the forms of cleavage correspond with the external forms of 

 the mineral, all perplexity is removed ; but if there should be 

 no such correspondence, or no cleavage, the crystallographer 

 is left in utter darkness. That which is merely external, and 

 which, therefore, may be impressed by external causes, can 

 never of itself be regarded as a secure ground for discrimina- 

 ting mineral species. Those properties which are internal, 

 which have their origin in the nature of the molecules of the 

 body itself, or in their mode of aggregation, and which are, 

 therefore, incommunicable, and independent of all external 

 circumstances, form the only secure basis of mineralogical clas- 

 sification. 



Art. XI. — Account of the Failure of the Suspension Bridge 



at Paris. By John Robison, Esq. F. R. S. Edin. In a 



Letter to the Editor. 



Dkae Sir, 

 I regret that I am unable to give more than a very superficial 

 reply to your inquiries relative to the failure of the Suspension 

 Bridge at Paris, as I did not visit it after that event. On 

 viewing it a few days before the accident happened, I saw 

 enough to make me fear that it could not stand long ; the 

 greatest resistance which the supports could oppose to the pull 

 of the chains having already begun to yield to the load of the 

 unfinished roadway. To explain this it is necessary to describe 

 the mode of attachment adopted in this bridge, as it differs 

 essentially from any other which I have seen or heard of. 



The supporting chains in this case, instead of proceeding in 

 the direction DE, Plate IV. Fig 4. to be inserted in a mass of 



* This curious subject will be treated in a subsequent article in this 

 Number. 



