108 W. H. HOBBS METEORITE FROM ALGOMA, WISCONSIN 



face markings, ofifers many analogies with the Algoma iron. It is, how- 

 ever, less symmetrical on both the broad and the narrow sections, and 

 its weight is thus concentrated to one side of its center of form, but it is 

 distinctly a thin oblate or discoid meteorite of the same general type as 

 the Algoma iron. Cohen was led, from the study of its surface mark- 

 ings, to the conclusion that it moved forward with its thin edge inclined 

 at a sharp angle to its path, but from personal correspondence I learn 

 that he does not now attach special weight to this. 



On the convex side of N'Goureyma the metal has been fused and has 

 flowed in radial directions, and its surface is also locally furrowed in 

 these directions. On the flatter and more concave side the photo- 

 gravures published with the article and the beautiful photographs from 

 which they were made (kindly submitted to the writer for examination) 

 show only the faintest of grooves or lines, and these are found only near 

 the periphery, where they are radial. What to the writer seems the 

 strongest evidence for a broadside attitude during its flight is the pres- 

 ence on the concave side of very marked groovings on the outer side of 

 two openings through the stone from front to back. These furrows are 

 as perfect as any which are found upon the front, but they are restricted 

 to the area pheripheral to the two openings. They indicate, it would seem,, 

 that the compressed air, against whose enormous pressure the meteorite 

 was opposing its convex surface, moved off" that surface in radial direc- 

 tions, and escaping in part through the orifices, eroded those portions of 

 the back which lay between the orifices and the circumference of the disc. 



It is not impossible that at some period of its flight the now concave 

 side of the N'Goureyma meteorite may have been the front, and the faint 

 radial markings upon the back have been then induced. At all events, 

 the removal of material from its now convex side may have altered its 

 shape materially, as Cohen indeed assumes. 



Professor A. G. Greenhill, the distinguished mathematician of the 

 Ordnance College at Woolwich, was sent a photograph of the front of 

 the Algoma iron. He has very kindly written to the author upon the 

 subject, and says of the meteorite : " It illustrates very clearly the sta- 

 bility, without rotation, of an oblate body moving in fluid in the direc- 

 tion of its shorter axis. An elongated prolate body requires rotation for 

 stability, as we know in the artillery of rifled guns." It should be men- 

 tioned that the slight curvature of the furrows upon the front of the 

 meteorite is hardly perceptible in the photograph, owing to the inclined 

 position in which the object was photographed. 



The Arlington iron from Minnesota is also of the flat type. This iron 

 has, according to Winchell,* dimensions in its plane of greatest extension 



*Am. Geologist, vol. 18 (1896), pp. 267-271. 



