276 M. Bertrand de Doue's Memoir on the 



If this screw be applied to wheel-work, its accuracy may be 

 tried in a way analogous to that resorted to for proving the 

 correctness of the screw for drawing parallel lines. 



Make two round brass plates, one smaller than the other ; 

 sink the smaller into the larger plate till the surfaces of both 

 are in the same plane. Fix these plates on the machine moved 

 by the screw G ; set the index and draw a line on both the 

 plates ; move the screw a given number of degrees ; draw an- 

 other line, and continue this operation till you are satisfied that 

 a sufficient number of lines has been drawn. Otte of the plates 

 is now made to revolve, and any two of the lines on the ex- 

 terior and interior plates brought together; if the screw be 

 correct, all the other lines will coincide. 



It is needless to describe the construction of the parts ne- 

 cessary to the accomplishment of this proof. 



Art. XV. — Memoir on the Fossil Bones of SainUPrivat- 

 d'^AUier, (m iJie province of Velay, France,) and upon the 

 basaltic district in which they have been discovered. By 

 M. J.-M. Bertrand de Doue, Member of the Society of 

 Agriculture, Science, Arts and Commerce of Le Puy, of 

 the Geological Society of London, &c. * 



It may not be amiss to premise, that the discovery of the fos- 

 sil bones of St Privat had its origin in the following circum- 

 stance: — Dr Hibbert, in an excursion from the Cantal to Le 

 Puy en Velay, crossed the granitic mountains of La Margeride, 

 and took the unfrequented route, little known to geologists, of 

 Monistrol d'AUier and St Privat. At the latter place, Mrs 

 Hibbert, who accompanied him, drew his attention to some 

 small specks of a whitish substance interspersed in a bed of 

 volcanic tufa and cinders, conceiving it at first to be fossil 

 wood, similar to what she had often assisted him in discovering 

 in the volcanic district of the Lower Rhine. But upon an 

 examination of this substance, he found it, to his surprise, to 

 be osseous instead of ligneous matter. His hammer, as well 

 as the pickaxe of a labourer from an adjoining house, were 



• From the Annals of the Society of Agriculture j Sciences, S^c. of Le Puy. 



